# Habitat of early stegocephalians (Chordata, Vertebrata, Sarcopterygii): a little saltier than most paleontologists like?

Michel Laurin

► To cite this version:

Michel Laurin. Habitat of early stegocephalians (Chordata, Vertebrata, Sarcopterygii): a little saltier than most paleontologists like?. Fossil Record, 2024, 27 (3), pp.299-332. 10.3897/fr.27.123291. hal-04737263

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1

2 Habitat of early stegocephalians (*Chordata, Vertebrata, Sarcopterygii*): a little saltier than

3 most paleontologists like?

4

5 Michel Laurin

6 CR2P (Centre de Recherches en Paléontologie – Paris)

7 CNRS/MNHN/Sorbonne Université

8 Muséum national d'Histoire naturelle

9 75231 Paris cedex 05, France.

10 michel.laurin@mnhn.fr

11
12 **Abstract**

A controversy on the degree of marine influence in the paleoenvironments represented by
many Paleozoic stegocephalian-bearing fossiliferous localities has persisted for decades.

Many authors have equated the absence of a typical stenohaline marine fauna with freshwater
environments, but this ignores continental salt lakes and the many transitional environments
(deltaic, estuarine, lagoonal, and some epicontinental seas that receive much freshwater
influx, like the Baltic Sea) that separate typical marine environments from freshwater
environments. This is problematic because it seems plausible that many of the late Paleozoic
sediments that have been preserved were deposited on coasts in deltas and estuaries. The
author had compiled a dataset of paleoenvironmental interpretations of Devonian to Early
Permian stegocephalian (“tetrapod”)-bearing fossiliferous localities in 2010. How have these
interpretations withstood the test of time, especially in the face of new results from different
kinds of evidence? An updated dataset and a new literature review show that the case for a
marine origin of stegocephalians has strengthened, especially through additional discoveries
or reinterpretations of fossils that suggest marine influence in various classical vertebrate-
bearing Permo-Carboniferous localities, and a recent analysis of stable isotopes in Late
Devonian localities. In addition, many Permo-Carboniferous localities traditionally
interpreted as freshwater show evidence of marine influence.

30

31 **Keywords**

32 Paleoenvironments, tetrapods, amphibians, brackish, freshwater, marine environment,
33 epicontinental seas, Devonian, Carboniferous, Permian

34

35

36
## 37 Introduction

Most early studies on Paleozoic stegocephalians have assumed that these taxa normally inhabited freshwater or dry land, unless their remains were clearly associated with typically marine fossils. This is exemplified by this quote from Milner (1987: 497):

> "Most of the British and North American tetrapod localities represent water bodies within non-marine swamps and have generally been assumed to be in the freshwater regions of fluviodeltaic systems, not least because of the presence of amphibians as presumed freshwater indicators together with the absence of unambiguously marine organisms." (Emphasis mine in all quotes unless stated otherwise.)

Thus, the fact that most of the Devonian and Carboniferous stegocephalians were called "amphibians" (in the paraphyletic sense of "anamniotic limbed vertebrate") probably played a role in this, even though parsimony does not validate the inference that these were freshwater taxa (Laurin and Soler-Gijón 2010).

This quote highlights the need for a precise nomenclature. Indeed, if the word "amphibians" had been consistently used in the sense that is now established under the *PhyloCode* (Cantino and de Queiroz 2020), as the lissamphibian total-group (Laurin et al. 2020), it is possible that the paleoenvironment of many Permo-Carboniferous localities would have been interpreted more cautiously. Similarly, it may be useful to mention here the difference between Tetrapoda (or "tetrapods"), and Stegocephali (or "stegocephalians") as used in this report. Both terms are here used as established under the *PhyloCode*. Namely, Stegocephali is defined as "The largest clade that includes *Eryops megacephalus* Cope 1877 (Temnospondyli) but not *Tiktaalik roseae* Daeschler et al. 2006, *Panderichthys rhombolepis* Gross 1930 (Panderichthyidae), and *Eusthenopteron foordi* Whiteaves 1881 (Osteolepiformes)." As such, it includes all known limbed vertebrates, and may conceivably contain some finned tetrapodomorphs, given that the fragmentary remains of the basalmost members of that clade, like *Elginerpeton* and *Ventastega*, leave doubt on the nature of their paired appendages (Laurin 2010). Tetrapoda is defined as the crown-group of limbed vertebrates, or the smallest clade that includes lissamphibians and amniotes, to simplify the definition provided by Laurin (2020a). As such, Tetrapoda is less inclusive than Stegocephali; it minimally excludes Devonian limbed vertebrates like *Acanthostega*, *Ichthyostega* and *Tulerpeton*, and under some phylogenies (e.g., Marjanović and Laurin 2019: fig. 14), it also excludes temnospondyls, embolomeres and seymouriamorphs. This review focuses on all stegocephalians, not only on tetrapods.

Many late Paleozoic fossiliferous localities lacking fossils of stenohaline, strictly marine organisms (such as echinoderms, cephalopods and coral reefs) have been interpreted as freshwater habitats, or the marine influence on such habitats has been minimized. However, factors other than low and fluctuating salinity may explain the absence of many stenohaline marine taxa; these include high sedimentation rates and turbidity (Feldman et al. 1993: 494), which are deleterious to filter-feeding, sessile organisms, as well as photosynthetic ones. Most contemporary estuaries and possibly tide-dominated deltas are precisely zones of maximal turbidity, because of tidal circulation and coagulation of clays resulting from mixing of fresh and salt water (Feldman et al. 1993: 494). This makes such environments difficult to recognize in the fossil record; the absence of typical marine taxa therein may well lead to erroneous interpretations of a freshwater environment.

Taxa found in these localities were often assumed to have been stenohaline, freshwater forms. Carpenter et al. (2011: 639) summarized this potential source of bias thus: "Traditionally, Pennsylvanian fishes have been classified either as marine or nonmarine (Calder, 1998), the
latter often taken to mean freshwater (Dick, 1998)." Worse, they pointed out that "In very many cases, the mere absence of an associated stenohaline fauna and the co-occurrence of plant fossils have been cited as sure evidence for a freshwater habitat". This comment could apply to the classical Texas Cisuralian redbeds, which include many of the most fossiliferous stegocephalian-bearing localities (Fig. 1). Yet, brackish water is not uncommon, especially along the coasts in deltas, estuaries, lagoons, and large epicontinental seas, such as the Baltic sea, and it seems that brackish habitats were widespread in the past too (Buatois et al. 1998, 2005; Falcon-Lang 2005). Furthermore, most sediments ultimately end up in the seas and oceans, with the continents providing only temporary storage (Hay 1998). Some rivers flow toward internal basins without connection to the seas, but these basins represent a small proportion of the emerged lands, and their main water bodies (e.g., Great Salt Lake in the USA, Dead Sea in the Middle East, etc.) typically contain saltwater (Moscatello and Belmonte 2009; Pagaling et al. 2009: 5751), which implies that the deltas and estuaries of rivers that flow into these salt lakes are brackish. Given that most sedimentation occurs in deltas and estuaries along the coasts (Peters and Husson 2017: 323) and that these environments are among the most favorable for fossilization, it seems plausible that most vertebrate fossils might come from such environments. However, the literature suggests that most Permo-Carboniferous stegocephalians that left a fossil record inhabited freshwater bodies, which may seem anomalous given where most of the long-term sedimentation occurs.

<figure><img src="image_2.png" /><figcaption>Figure 1. A *Dimetrodon grandis* chases an *Eryops megacephalus* and an *Edaphosaurus pogonias* through a *Sigillaria* forest. In the foreground, a *Meganeuropsis* flies near strobili of *Equisetum*. Reconstruction of the Artinskian (Early Permian) in what is now Texas, USA. Drawing by Rafael Albo (Corumbá, Brazil), Jacek Major (Starachowice, Poland), and Amin Khaleghparast (Tehran, Iran); coloring by Ruben Koops (Haarlem, Netherlands). Advisors for *Dimetrodon*: Tracy Lee Ford (San Diego, California, USA) and Russell J. Hawley (Casper, Wyoming, USA).</figcaption></figure>

110

Determining the habitat of long-extinct taxa is difficult because sedimentation and regression/transgression cycles cause shorelines to move quickly in geological terms (Carpenter et al. 2011 and references therein), and carcasses can be transported before being buried and
fossilized. Thus, progress on this front will ultimately require detailed inventories of taxa occurring in fairly narrow strata and with good geographic data. The literature is not as rich as we would like in such detailed studies, but the review below tries to summarize a representative sample of the data currently available, especially in the Famennian and Carboniferous and, to a lesser extent, in the Cisuralian.

Fortunately, some studies considered brackish habitats in their assessment of paleohabitats, but often still seem to have minimized the marine influence. This can be illustrated by the “Birthday Bonebed” of the Permo-Carboniferous Halgaito Formation (Utah), which was studied recently by Huttenlocker et al. (2018), who suggested (p. 87) that

Within the bonebed, microconchids, xenacanths, actinopterygians, and the lungfish Sagenodus suggest an assemblage that was to some degree dependent on permanent standing water. We interpret these stream systems, particularly those associated with the major tiered channel bodies, as primarily freshwater with little marine influence, though the microconchids and xenacanths potentially leave open the possibility of proximity to marine-influenced channel reaches (Carpenter et al., 2015; Ó Gogáin et al., 2016).

However, Huttenlocker et al. (2018: 87) concluded that the aquatic fauna preserved in the bonebed included “freshwater xenacanths, actinopterygians, and lungfish” (in addition to the amphibious temnospondyl *Eryops* and some presumably more terrestrial eupelycosaur amniotes). As this quote shows, a primarily freshwater interpretation is favored, despite the presence of microconchids, xenacanths, and dipnoans, three clades of marine origin. Of these, only diponans are still extant, and may thus have played a greater role, historically, in inferring paleoenvironments. Extant dipnoans inhabit freshwater and by the Carboniferous, some had already invaded freshwater environments, but other remained marine (Schultze and Soler-Gijón 2004: 341); thus, their presence in a Carboniferous locality does not resolve the degree of marine influence. Microconchids and xenacanths are frequently associated with early stegocephalians, which justifies a short review of their probable paleoenvironment (in dedicated sections below). The predominantly freshwater interpretation of the Halgaito Formation by Huttenlocker et al. (2018) is surprising given that they indicated (pp. 73–74) interfingering with a marine facies in the lower half of the formation. Given that the “Birthday Bonebed” is precisely in the lower half of this formation, the open sea (preserved in the marine facies) was presumably not far. Similarly, Huttenlocker et al. (2018: 86) reported results of their strontium isotopic analyses, and concluded that “These results are consistent with the hypothesis that tooth formation occurred in an environment with some freshwater-influence, supporting that the xenacanths associated with the bonebed were either occasional or permanent residents of these freshwater systems.” Note how the indication of freshwater influence was used to infer that the xenacanths lived in freshwater. Why could the “Birthday Bonebed” not represent a marginal-marine, brackish-water environment? As we will see below, interpreting the isotopic signatures is not always straightforward.

I studied the problematic paleoenvironments inhabited by Permo-Carboniferous stegocephalians before (Laurin and Soler-Gijón 2010), but some important empirical studies were published since then, and my earlier study did not cover (or only superficially) some relevant topics, such as the probable habitat of some taxa frequently associated with Paleozoic stegocephalians; this new review attempts to fill some of these gaps, especially the probable habitat of various taxa frequently associated with stegocephalians, in the same temporal interval (Famennian to Kungurian). This paper first presents (below) evidence that marine influence has been minimized in many earlier studies, as shown for three examples; it then surveys briefly
162 the main isotopic methods that have been used to assess paleosalinity. This is followed by
163 sections on the paleoenvironmental significance of several taxa that are often associated with
164 early stegocephalians. All of these data are then used to review the paleoenvironments of a few
165 Famennian and Carboniferous localities that have yielded stegocephalians; Cisuralian localities
166 would be relevant too, but could not be included for lack of time and space.

167 Biased interpretation in favor of freshwater?

168 *Unrecognized tidal deposits?*

169 There are reasons to believe that the extent of marine influence in the habitat of early
170 stegocephalians has been significantly under-estimated in the literature (see below). A similar
171 bias against marginal-marine environments has recently been argued to be present in the
172 paleobotanical literature, at least in the Carboniferous. DiMichele et al. (2023: 14) stated:

173 "Tidal environments have been identified in other areas, including in
174 European basinal settings (e.g. Fossil Grove: Gastaldo 1986, reinterpreted as
175 a tidal setting). Such deposits are likely much more widespread than
176 recognized because of the difficulty of identifying tidalites in their nascent,
177 very finely laminated stages, particularly in mudrocks (most likely to be
178 encountered in the basal coal-roof transition strata found in mines close to
179 channel environments, see Fig. 10a). However, the limited recognition of
180 tidal-flat settings also may reflect that most palaeobotanists are generally
181 unfamiliar with these kinds of strata"

182 This is hardly a new claim, though it remains highly relevant. Kuecher et al. (1990: 211) had
183 already concluded, in their study of the Francis Creek Shale (in which the Mazon Creek biota
184 is preserved) that “evidence of Westphalian tides may have been misinterpreted or overlooked
185 in field studies elsewhere.” One of the reasons is that weathering can obscure the fine
186 laminations; Kuecher et al. (1990: 212) indicated that cores were better for such studies, but
187 most paleontological prospection or excavation does not entail coring. Kuecher et al. (1990:
188 219) observed that many Late Carboniferous coal deposits from Europe and North America
189 were associated with fluvio-delatic sediments interbedded with marine sediments, and that tidal
190 rythmites should be common in such deposits, and conjectured that many tidal rythmites had
191 been misinterpreted by previous authors as fluviatile. The fact that the tidal rythmites of Mazon
192 Creek went unnoticed for about 30 years despite intensive geological field work there supports
193 this suggestion (Kuecher et al. 1990: 219). Indeed, tidal rythmites are now known from other
194 Eastern Interior Coal Basin localities that were initially interpreted as non-marine (Archer et al.
195 1995: 408).

196 Schultze (2009:127) had similarly pointed out that features typically interpreted as fluvial “can
197 be produced by the extension of rivers into the intertidal area”, and indeed, recent research has
198 shown previously unrecognized similarities between sedimentary structures left by tidal
199 channels and meanders formed by rivers (Finotello et al. 2018). More generally, Feldman et al.
200 (1993: 485) stated that “Alternative interpretations are possible because nearly identical, varve-
201 like, normally graded laminations can be produced in environments ranging from deep marine
202 to shallow lagoons and freshwater lakes.” What characterizes tidal deposits is the cycles of
203 thickness variation linked to tides and neap-spring cycles (Feldman et al. 1993: 494), but
204 without thorough analysis, this cyclicity may go unnoticed. However, tidal currents can
205 propagate far upstream of deltas, so that the presence of tidal rythmites alone does not
206 guarantee that water was brackish or of regular marine salinity (Feldman et al. 1993: 495).
207 Old Red Sandstone

208 This bias in favor of a freshwater interpretation is not restricted to formations and sites that have
209 yielded early stegocephalians. Even older strata have arguably been affected by a similar
210 interpration bias. A good example is provided by the Old Red Sandstone (ORS from here on;
211 this ranges from late Silurian to Early Carboniferous in age), which yielded many early
212 vertebrates. Janvier (2007: 41) explained why the ORS was initially interpreted as a freshwater
213 deposit:

214 "The first Silurian and Devonian fish remains described in the beginning of
215 the nineteenth century were preserved in sandstones (e.g., the "Old Red
216 Sandstone" of Britain and the Baltic States) and generally associated with
217 plant remains, but rarely with marine invertebrates. In addition, these
218 heavily armored fishes were regarded as "ganoids," a group which
219 classically included living bichirs, gars, and catfishes, all reputedly
220 freshwater. Progressively, the received wisdom became that all these early
221 fishes lived in freshwater and occasionally passed into the sea, when found
222 in marine sediments."

223 This may explain why Boucot and Janis (1983: 262) interpreted the "Vertebrate-eurypterid
224 fauna" of the "Lyne Water Fish Beds" (Wenlock/Ludlow, Silurian) as non-marine (brackish or
225 freshwater).

226 However, subsequent findings showed that some ORS was almost certainly deposited in a
227 marginal marine environment as had been suspected already by some authors in the 1970s (e.g.,
228 Goujet and Blieck 1977). For some deposits, marine influence seems certain. Thus, Goujet and
229 Emig (1985) described *Lingula* fossils from the top of the Wood Bay Formation (Emsian, Early
230 Devonian) of Spitzbergen, in levels above those that yielded a diversified fauna of
231 gnathostomes and jawless vertebrates. These deposits had previously been interpreted, based
232 on sedimentological data and the absence of typically marine metazoans, as freshwater
233 sediments (Friend and Moody-Stuart 1970), but Goujet and Emig (1985) interpreted them as
234 marine and suggested that the *Lingula* fossils represented individuals killed by a catastrophic
235 event, such as a massive influx of freshwater following a storm or sudden short-term increase
236 of water discharged by one of the nearby rivers. They also noted that the absence of brachiopods
237 from the lower, vertebrate-bearing levels was not anomalous because shells of *Lingula* are
238 fragile and are seldom preserved (Emig 1981). One of the earliest Sr isotopic studies of the
239 ORS also concluded that at least some of its deposits seemed to be marine (Schmitz et al. 1991:
240 793).

241 Janvier (2007: 42–43) remained cautious about the paleoenvironment in which the ORS was
242 deposited; he mentioned that some paleontologists considered that it had been deposited in
243 marginal marine environments, whereas others viewed it as composed of freshwater deposits.
244 Subsequently, isotopic analyses documented additional instances (in addition to the case
245 reported by Goujet and Emig 1985) of ORS strata deposited in marginal-marine environments
246 (Goedert et al. 2018; see section “Devonian localities”).

247 *Escuminac Formation*

248 The bias against marine influence is not restricted to vertebrate paleontologists. In her
249 monograph on freshwater ecosystems in the fossil record, Gray (1988: 24) sharply criticized
250 previous suggestions that the Escuminac Formation was deposited in a marginal marine
251 (probably estuarine and brackish) environment:
"Similar in its defective logic and willingness to ignore the impressive data gathered by others concerning both depositional environment and characteristics of the biota, is Schultze's (1972; Schultze and Arsenault, 1985) conclusion that the Late Devonian, freshwater-lacustrine, vertebrate faunas of the Escuminac Formation, Miguasha, Quebec, Canada are "coastal marine, based on the fauna present" merely because some of the genera are also found in undoubted marine deposits elsewhere in the world. Unwillingness to consider that any Devonian vertebrate might have been able to flourish in both marine and freshwater environments, as is the case with many taxa today, is biologically, as well as geologically, so unrealistic as scarcely to merit serious attention. With regard to the locality at Miguasha, the nearest known marine Upper Devonian beds are no closer than the Hudson Bay region, Canada, central New York, U.S.A., and the south of England."

Yet, shortly after, an early isotopic study based on isotopes of several chemical elements concluded that the *Bothriolepis canadensis* sample that they had included yielded "a strong marine signal" (Schmitz et al. 1991: 793). Similarly, Matton et al. (2012) showed that isotopic data from specimens of several taxa (in addition to *B. canadensis*, these include the finned tetrapodomorph *Eusthenopteron foordi*, the dipnoan *Scaumenacia curta*, the actinopterygian *Cheirolepis canadensis*, and the acanthodian *Homalacanthus concinnus*) indicate significant marine influence, and this formation is now interpreted as a brackish estuarine environment, an interpretation supported by other types of data, notably the fossil record (Cloutier 2013).

These developments illustrate the danger of relying on previous reconstructions of ancient coastlines to assess local paleoenvironments (Schultze 1995, 2009: 129); if previous paleoenvironmental interpretations were erroneous, subsequent ones risk reinforcing previous misinterpretations, which hampers scientific progress. Another spectacular case of a marine-influenced paleoenvironment recognized far from the closest reconstructed paleo-coast is the Early Carboniferous Ducabrook Formation, where Parker and Webb (2008: 525) documented estuarine conditions about 400 km from the nearest open marine sediments.

## *El Cobre Canyon*

A possible bias against even the presence of a modest marine influence may be visible in previous works on the Permo-Carboniferous El Cobre Canyon in New Mexico, which is typically considered to be a freshwater (fluvial) environment (Fracasso 1980; Lucas and Lerner, 2010). Contrary to the few cases examined above, possible marine influence in El Cobre has not been evoked in the recent literature as far as I know. Yet, in one of the earliest studies of this locality, Williston and Case (1912) reported the presence of a brachiopod (initially called *Spirifer*, but later re-named *Anthracospirifer*), a typically marine taxon. This was dismissed by Langston (1953:412), who stated that "It is unfortunate that the material of which Case's specimen was composed was not recorded. Had it been limestone the question might have been resolved quickly since the only limestone in El Cobre Canyon is of Jurassic age (Todilto)." Because of this, Langston (1953) considered this brachiopod fossil "as intrusive and therefore of no stratigraphic [or paleoenvironmental] significance" for the vertebrate fauna preserved in El Cobre. There are several problems with this reasoning. First, the brachiopod fossil suggests a Pennsylvanian age, not Jurassic, so Langston's (1953) assumption about the origin of the fossil does not seem plausible. Second, brachiopods need not come from limestone; they also occur in sandstone. In fact, some brachiopods, such as orthoids, are typically associated with siliciclastic environments, even though these environments are negatively correlated with
299 brachiopod diversity (e.g., Jakobsen et al. 2014: 196). Third, Langston summarily dismissed a
300 finding to which Williston and Case (1912) seemed to attach great importance; it is worth
301 quoting the short passage on this topic (Williston and Case 1912: 6–7):

302 "At the south side of the cañon, the junior author found a **perfect cast** of a
303 Spirifer, identified by Professor Schuchert as S. rockymontanus Marcou, a
304 form occurring in Colorado in the Pennsylvanian. Though the specimen was
305 found free, so that its exact horizon could not be determined, its **excellent
306 preservation proves conclusively that it had not been carried far from its
307 original bed**, and inasmuch as vertebrate fossils are found in the deepest
308 strata of the cañon **it seems quite certain that the specimen came from an
309 intercalated bed among those yielding so-called Permian vertebrates. No
310 other explanation seems possible."

311 One might tend to prefer the opinion of more recent studies that benefited from additional
312 decades of research, but the statements by Langston (1953: 412) suggest that he did not see the
313 brachiopod fossil; how should this be weighted against the fact that Williston and Case (1912)
314 found and saw the fossil?

315 The presence of this sole brachiopod fossil does not imply that all sediments of levels of the El
316 Cobre Canyon were deposited in a marine-influenced environment; some data suggest
317 otherwise. For instance, Utting and Lucas (2010: 73) reported that "No evidence was found of
318 any marine palynomorphs such as acanthomorph acritarchs or scolecodonts." However, their
319 sample was restricted to "a 2 m shale bed in the middle part of the El Cobre Canyon Formation",
320 so their findings represent a small proportion of the deposits found in the canyon and are not
321 incompatible with marine influence in other levels of the locality.

322 Other evidence suggests that there might be marine influence in El Cobre Canyon. Lucas and
323 Lerner (2010) thus reported the presence of ichnofossils attributed to cf. *Paleohelcura*
324 *tridactyla*, which may have been produced by scorpions, other arachnids, or eurypterids. In their
325 discussion of the eurypterid prosoma assigned to *Adelophthalmus* from El Cobre canyon, Lucas
326 and Lerner (2010: 40–41) stated: “These eurypterids inhabited shallow ponds or enclosed
327 lagoons on the deltaic plain adjacent to the ancestral Cañon del Cobre area”. Lagoons and deltas
328 may be brackish environments, and indeed, *Adelophthalmus* from the Red Tanks Member of
329 the Madera Formation, a few tens of km away (Kues and Kietzke 1981; Braddy et al. 2021), is
330 clearly from a marginal-marine, brackish environment (see below, section on eurypterids).
331 While Lucas and Lerner (2010) clearly preferred to interpret the ichnofauna that they described
332 as a freshwater assemblage, their reference to lagoons and a deltaic plain suggests that the sea
333 was not far. Another quote from Lucas and Lerner (2010: 41) reinforces this impression: “There
334 is a single prior Pennsylvanian record [of cf. *Paleohelcura tridactyla*], which comes from a
335 **tidal flat** setting in the McAlester Formation of Oklahoma (Lucas et al., 2004). There are no
336 previous North American records of *Paleohelcura* from lacustrine settings.” One might add that
337 the McAlester Formation of Oklahoma in which *Paleohelcura tridactyla* was reported also
338 yielded “a few impressions of the shells of marine gastropods” (Lucas et al. 2004: 45). Could
339 at least some levels of El Cobre Canyon document brackish water assemblages?

340 **Autochtonous or allochtonous stegocephalian remains?**

341 An important consideration in paleoenvironmental studies is the autochtonous or allochtonous
342 nature of the fossils preserved in a given locality. Only autochtonous fossils are informative in
343 this respect. Allochtonous fossils provide information about more distant environments. In most
344 cases, only fairly long-distance transport (several km) can be easily detected, through wear
345 marks (erosion) on the surface of fossils, disarticulation, and the fragmentary nature of
346 specimens (for instance, only a few isolated bones, rather than a nearly complete, articulated
347 skeleton). Abundance of material can also be used, to an extent, in combination with quality of
348 preservation (Bennett et al. 2021: 15), because erratics are likely to be allochtonous. However,
349 even for localities that have been subjected to detailed taphonomic studies, such as Jarrow, it is
350 difficult to know if the stegocephalians are preserved in their environment or if they were
351 transported over relatively short distances (Ó Gogáin et al. 2022: 15). In other cases, it looks
352 like some fossils are allochtonous, whereas others (including vertebrates) seem to be
353 autoctonous (Bennett et al. 2021: 18). When there is evidence of transportation, it is generally
354 assumed that carcasses have been carried by rivers from a more continental environment,
355 although the possibility that it was washed in by tides or storms from a more marine
356 environment, while less likely, cannot be ruled out either, as emphasized by Gierlowski-
357 Kordesch and Cassle (2015: 210-211).

358 In a few cases, transportation can be ruled out, even for fairly short distances. Some fossils of
359 sessile organisms are obviously preserved in situ; this may occur for brachiopods, coral-forming
360 organisms (especially cnidarians), some echinoderms and some mollusks, among others, but
361 not vertebrates, unless they are found in burrows (a few examples are known, notably for
362 dipnoans). Other compelling but rarer cases are found when evidence of predation is
363 encountered, which suggests, minimally, that predator and prey occupied the same habitat,
364 although this does not rule out transport of the bodies over a short distance. An example is
365 described below, of a shark (*Triodus sessilis*) that ate a temnospondyl that had eaten an
366 acanthoderm (Kriwet et al. 2008). Such fossils provide excellent information on the local
367 paleoenvironment, and such examples are described in a few localities below.

368 Another type of fossils that nearly always reflects local paleoenvironments are ichnofossils.
369 The identity of the trackmakers is often poorly constrained, but some ichnofossils are associated
370 with specific paleoenvironments and as such, they may be informative. They are even more
371 useful when the identity of the trackmaker is reasonably well-constrained. An example is
372 provided by the ichnofossils of Puertollano (Spain), which includes an interesting assemblage
373 of trackways left by a stegocephalian and traces left by a finned vertebrate that swam above the
374 substrate. The trackway, called *Puertollanopus microdactylus*, was left by a small
375 stegocephalian, tentatively identified as an amphibian (a “microsaur”) or, less probably, an
376 amniote. The traces left by a gnathostome that swam above the substrate, presumably in shallow
377 water, are called *Undichna Britannica* and *Undichna britannica* and were probably left by the
378 xenacanthid chondrichtyan *Orthacanthus*, which is also known by skeletal remains from the
379 locality (Soler-Gijón 1997). Several instances of superposition of both tracks is explained by
380 tides, with the stegocephalian being left at low tide, while the swimming traces were left at high
381 tide (Soler-Gijón and Moratalla 2001).

382 Isotopic signal and paleosalinity

383 *Basic principles and limits of isotopic methods to assess paleosalinity*

384 The isotopic ratios of various elements have been used to assess paleoenvironments, notably to
385 shed new light on the degree of marine influence and paleosalinity in various fossiliferous
386 localities. We saw above that Huttenlocker et al. (2018: 86) concluded, from their strontium
387 isotopic analyses, that the Halgaito Formation had formed in a freshwater environment. Earlier,
388 Fischer et al. (2013) had also concluded, based on isotopic analyses of oxygen and strontium,
389 that many vertebrate-bearing European Permo-Carboniferous localities (including, among
390 others, Buxières-les-Mines and Nýřany) represented freshwater environments lacking any
391 detectable marine influence. Similar conclusions, following similar reasonings but based on
392 isotopes of other elements (carbon, oxygen and sulfur), were drawn about the of the Bourbon-
393 l'Archambault Basin, which contains xenacanthiform chondrichthyans, by Luccisano et al.
394 (2023: 8).

395 As we will see below, some of these conclusions may rest on tenuous ground because the
396 isotopic signature of strontium reflects freshwater input and flux between a given water body
397 and the ocean, rather than salinity; a similar phenomenon has been documented for neodymium
398 by Woodard et al. (2013: 57) and (to a lesser extent) for carbon by Quinton and Rygel (2023).
399 However, conclusion based on recent analyses of isotopes of other elements (especially on
400 sulfur) might be more robust (Luccisano et al. 2023: 8). Strontium (Sr) has been the most
401 intensively studied element in this respect, so to an extent, the discussion below will emphasize
402 this, but other elements, such as carbon (C), oxygen (O) and sulfur (S), which were used in
403 some recent studies, are also discussed below.

404 The 87Sr/86Sr ratio (87Sr being the radiogenic isotope) has varied through time, but oceanic
405 mixing appears to have resulted in fairly homogeneous world-wide 87Sr/86Sr oceanic values at
406 any given time. By contrast, freshwater bodies have much more variable ratios at any given
407 time because these ratios depend on the 87Sr/86Sr signature of the soil and bedrock in the
408 drainage basins. Thus, if the 87Sr/86Sr ratio of a given sediment matches the contemporary
409 oceanic ratio, the sediments were probably deposited in oceans, although in a small minority of
410 cases, the similarity might be coincidental. Conversely, a significant departure from the coeval
411 87Sr/86Sr oceanic ratio (beyond measurement error and outside two standard deviations)
412 indicates that the Sr in the water body in which the sediments were deposited was not at
413 equilibrium with oceanic values. This indicates that little or no exchange with the ocean took
414 place. Several studies have interpreted such cases as representing freshwater, but this is only
415 one of several other possibilities; the others include brackish water, which may be purely
416 continental, far from the coasts, but also coastal (ponds, lagoons, estuaries and deltas), and even
417 some epicontinental seas, such as the Baltic sea.

418 Thus, interpreting the isotopic signature of Sr in terms of salinity is not straightforward. Purely
419 continental saltwater lakes have a Sr isotopic signature that reflects that of the rivers that flow
420 into it, and brackish coastal environments, such as the Baltic sea, show strong deviations from
421 oceanic signatures, especially where the freshwater influx is greatest; for instance, Andersson
422 et al. (1992) reported a value as high as 0.709718±41 in the most freshwater part of the Baltic
423 (Gulf of Bothnia, close to the deltas of the Kalix and Kemi rivers), which is about 0.000568
424 (568 parts per million) higher than their measured value for the Atlantic Ocean (0.709150±24).
425 Fairly strong departures from the marine Sr isotopic ratio (and presumably in isotopic ratios of
426 other elements) can occur in marginal marine basins when limited flux occurs between that
427 basin and the ocean, especially when much of the water outflow is through evaporation; in this
428 respect, the Baltic sea is probably not the most spectacular example. This occurred in the
429 Mediterranean, in the Messinian salinity crisis, and Flecker et al. (2002: 227) concluded that
430 "excluding net evaporation from the hydrologic budget of the Mediterranean could result in an
431 order of magnitude error in the palaeosalinities inferred from Sr isotope data." Furthermore,
432 many parts of the Mediterranean developed their own Sr isotopic signature shortly before the
433 Messinian salinity crisis (Flecker et al. (2002: 228). This is surprising because Sr concentration
434 in oceans is about 7.7 ppm (parts per million), which is over a hundred times more than in
435 freshwater (Andersson et al. 1994). This great disparity in abundance should make the 87Sr/86Sr
436 ratio efficient to detect even a small amount of marine influence, but as shown by Flecker et al.
437 (2002), under even moderate restriction of water flux and fairly strong evaporation, sharp
438 departures from oceanic values can be observed.

439 Epeiric seas may not reflect oceanic isotopic values, as exemplified by the extant Baltic sea
440 (Andersson et al. 1992, 1994; Carpenter et al. 2011: 650). Large inland seas may also form, as
441 shown by the fate of the Paratethys, which remains in a much-reduced state as the brackish
442 Black, Caspian, and Aral seas, and in which typically marine faunal components have fluctuated over time, depending on the opening and closing of connections with the ocean (Richards et al. 2018; Dewaele et al. 2022). Even basins located far from any coast and without any link to the sea may be brackish or even hypersaline, and in these cases, Sr isotopic values may not reflect paleosalinity. Salt lakes are not rare in deserts, and even occur in moderately dry environments (e.g., Moscatello and Belmonte 2009; Borzenko 2020). Hypersaline lakes may occur at high altitude, very far from the nearest sea; for instance, the hypersaline lake Salar Guayatayoc in Argentina is located at 3432 m above sea level (Pagaling et al. 2009: 5751). A large, perennial salt lake was present in what in now northern Germany in the Rotliegend, at least by the Guadalupian (Gast 1991), or perhaps toward the end of the Cisuralian (Verdier 1996), and smaller salt lakes may have occurred elsewhere in the Carboniferous and the Permian. The isotopic signatures of such water bodies may differ substantially from marine levels, but they are not freshwater environments.

455 Sulfur (S) has also been used as a paleosalinity indicator in some recent studies. As for Sr, S is highly variable in freshwater environments, with δ³⁴S values ranging from -20.0 to +20.0‰, whereas current oceans have a δ³⁴S value of about +21.0‰ (Goedert et al. 2020). δ³⁴S values are very useful paleosalinity indicators because S is much more abundant (about a hundred times) in the oceans and seas than in freshwater, so even a little oceanic water diluted in much freshwater should leave an isotopic signature. However, the same argument can be made for Sr, which does not prevent significant deviations from oceanic isotopic ratios in basins with limited (or no) exchange with the ocean, especially when much evaporation occurs (see above). Also, the δ³⁴S values of a water body also depend on the source of S, which can come from dissolution of evaporites in a drainage basin. In that case, the δ³⁴S values would reflect those of the oceans in which the evaporites precipitated, which may be much older that the salt or brackish lake, and this creates an additional difficulty given the fluctuation of the δ³⁴S oceanic values over time, which have varied between about 10 and 40‰ just in the Phanerozoic, and with uncertainties for some ages in of up to 10‰ (Wu et al. 2014). These phenomena complicate interpretation of δ³⁴S values because atypical cases of freshwater rivers with fairly high δ³⁴S values and saline springs with fairly low δ³⁴S values have been documented even in Canada (Hitchon and Krouse 1972). Thus, purely continental, but brackish or salty lakes, can have δ³⁴S non-marine values. A well-known example is the Great Salt Lake, which is strongly hypersaline and for which δ³⁴S values of +14.6 and +17.1‰ have been reported (for different dates) by Mayo and Klauk (1991: table 4). Numerous saltwater lakes (some of which are hypersaline) in Eastern Transkaibalia have been studied by Borzenko (2020), who showed that their δ³⁴S display a great range of values, from -8.4 to 27.4‰. Such environments, which must have existed in a distant past, might be challenging to model using δ³⁴S data. Thus, as for Sr, S isotopic data is more useful to assess the degree of marine influence than for paleosalinity.

479 A third element that has been used to assess paleosalinity is oxygen (O). Current oceanic seawater has a relatively uniform δ18O value of 0 ± 1‰, although it is lower at high latitudes, ranging from about -3 to -1‰ (Goedert et al. 2020), and this value has changed over time (Veizer et al. 1999). Current freshwater δ18O is more variable and ranges from -6 and -2‰ at low latitudes to about -15‰ at high latitudes. Hypersaline water (in lakes, lagoons and sabkhas) can have δ18O values higher than 2‰. Thus, comparing the δ18O inferred for an ancient water body to that inferred for the contemporary oceanic values, and considering the inferred paleolatitude of the water body, the δ18O can provide valuable data about whether that water body was part of a sea, an estuary, or a purely continental environment. However, contrary to Sr and S, there is as much O in freshwater as in marine water, so the δ18O should be less sensitive to detect a small proportion of seawater in a water body than methods based on Sr and S. Thus, the δ18O may be less useful to assess presumed brackish environments than δ³⁴S and
the <sup>87</sup>Sr/<sup>86</sup>Sr ratio. Also, the δ18O value of apatite (found in bone, dentin and enamel) differs from that of the ambient water, but there is a linear relationship between the δ18O of both substances (ambient water and apatite of animals living therein). Additional factors complicate further interpretation of apatite δ18O values because it is also affected by thermo-metabolism (thus differing between ectotherms and endotherms), evaporative transcutaneous water loss, and varies among taxa (Goedert et al. 2020: fig. 1).

### Paleosalinity assessed through isotopic methods: a few case studies

Isotopic analyses have been used to assess the paleoenvironment of many fossiliferous localities, but their relevance in this context can be analyzed through the example of the Joggins Formation, which has yielded a rich metazoan fauna, including many vertebrates, including some stegocephalians. On the basis of isotopic analyses, Brand (1994: 314) concluded that “The higher Sr ratio of the Joggins bivalves relative to its marine counterparts **unequivocally proves** the non-marine (continental) affinity of these fossils and their enclosing sediments.” His analyses were based on *Naiadites* bivalves, which are often interpreted as a freshwater taxon (see below, section on xiphosurans). This is not necessarily incongruent with more recent interpretations (see below), given that Brand (1994) sampled one of the most freshwater-adapted faunal elements from one of the levels with the least marine influence, but these results may not be as robust as initially suggested. For instance, in the graph of the δ<sup>13</sup>C vs. δ<sup>18</sup>O (Brand 1994: fig. 6), the *Naiadites* samples fall close to the marine (brachiopod) samples, but a «mineralogical adjustment» to correct for assumed diagenesis is invoked (especially for the δ<sup>13</sup>C) to interpret these bivalves as freshwater forms, which suggests that diagenetic effects may hamper straightforward interpretation of these results. In any case, many factors influence carbon isotopic ratios, and their effect is neither simple nor fully understood (Quinton and Rygel 2023), and various studies have shown that carbon isotopic ratios are not suitable to discriminate between marine and freshwater environments (Luccisano et al. 2023: 10). Thus, carbon isotopes are not discussed further in this review. Similarly, Brand (1994: 314) reported a <sup>87</sup>Sr/<sup>86</sup>Sr ratio of >0.7093 from “two well-preserved bivalves from the uppermost unit of the Joggins Formation”, and indicated that these “are at variance with that of coeval data (<0.7085) based on marine invertebrates.” However, his graph of the <sup>87</sup>Sr/<sup>86</sup>Sr ratio over time (Brand 1994: fig. 7) shows a spike in the late Westphalian A at >0.7091, which is close to the Joggins sample both temporally and in isotopic value (the difference is less than the observed range within the Baltic Sea; see above). Brand (1994) did not seriously consider the implications of this spike (which is briefly evoked in his text), probably because the spike was located, in Brand’s (1994) figure, slightly earlier than Joggins. However, considering stratigraphic uncertainties in such cases is critical, and indeed, Joggins is now considered a little older than previously thought (Carpenter et al. 2015; Rygel et al. 2015). Thus, the Sr isotopic signal found by Brand (1994) seems to give a weaker, less convincing continental signal than initially suggested, and as explained above, “continental” does not necessarily imply “freshwater”. Through comparisons with the Baltic Sea, Falcon-Lang (2005: 491) interpreted Brand’s (1994) results more categorically as “entirely consistent with brackish-water conditions.” Carpenter et al. (2015: 681) reached similar conclusions based on new analyses of two specimens.

Isotopic data are also directly relevant to assess the habitat of early stegocephalians and their associated fauna. Among the latter, xenacanthiform chondrichthyans are especially relevant and are discussed below. Some xenacanthids were found in marine sediments (Hampe and Ivanov 2007), but most were found in localities that have traditionally been interpreted as freshwater environments as suggested by Luccisano et al. (2023), who performed isotopic analyses by of
539 the localities of Buxières-les-Mines (Bourbon-l'Archambault Basin, Allier) and the Muse
540 (Autun Basin). These sites probably both date from the Asselian (Pellenard et al., 2017 ;
541 Luccisano et al., 2022 ; Mercuzot et al., 2022).

542 Luccisano et al. (2023: 8) reported that the δ³⁴S values of the xenacanthiforms and other
543 vertebrates from Buxières-les-Mines are “lower than 12‰ with variations from -6.0‰ to 9.6‰
544 for unaltered samples”, whereas those from the Muse “range from 1.2‰ to 6.9‰” and
545 concluded that “Those samples are depleted in heavy isotope by at least 3‰ and on average by
546 10‰ compared to seawater.” The samples that displayed the highest δ³⁴S values (the closest to
547 contemporary marine values) and might have inhabited brackish water for at least part of the
548 time (according Luccisano et al. 2023) belong to an actinopterygian and two temnospondyl
549 samples. Luccisano et al. (2023) based their estimate of a marine water δ³⁴S close to 12‰ near
550 the Carboniferous-Permian transition on Claypool et al. (1980), which is a fairly old study.
551 Nevertheless, the more recent isotopic curves by Wu et al. (2014) yield similar values, while
552 showing also substantial uncertainty on the estimates. The analyses of Fischer et al. (2013) also
553 suggested a fairly continental habitat for most of the European localities that they studied, and
554 for the chondrichthyans (hybodontiforms and xenacanthiforms) that inhabited them. To sum
555 up, these δ³⁴S results show deviations from the inferred coeval marine values, which implies
556 negligible water exchange with the marine environment and suggest freshwater environments,
557 but this does not necessarily rule out purely continental brackish environments.

558 The δ¹⁸O values obtained by Luccisano et al. (2023) from Buxières-les-Mines are less
559 convincing. The δ¹⁸O values from the Muse xenacanthiforms range from 17.3‰ to 25.1‰,
560 whereas vertebrates from Buxières-les-Mines yielded values around 17.0‰. Permian
561 conodonts, which are thought to be representative of contemporary seawater, have a δ¹⁸O is in
562 the range of 21‰–23‰, an interval that is encompassed by the δ¹⁸O values of Buxières-les-
563 Mines xenacanthiforms. Luccisano et al. (2023: 10) nevertheless concluded that their results
564 “may reflect ¹⁸O-enriched water due to large evaporation rates that commonly take place in arid
565 environments.” This is plausible, but these δ¹⁸O values (contrary to the δ³⁴S values) are not
566 independent evidence that the Buxières-les-Mines xenacanthiforms had not ventured in marine-
567 influenced environments, and large evaporation rates are compatible with continental brackish
568 lakes. These results are not entirely congruent with those of Fischer et al. (2013) on Buxières-
569 les-Mines, which appeared to be one of the most continental localities, with a δ¹⁸O of 16.6‰.
570 By contrast, shark teeth from Puertollano, which is probably a paralic, brackish environment
571 (Soler-Gijón and Moratalla 2001; Laurin and Soler-Gijón 2006; Soler-Gijón and López-
572 Martínez 2008), yielded an average δ¹⁸O of 18.5‰ (with a single value at 20.2‰), and shark
573 teeth from the marine locality of Kalinovskie Vyselki quarry of the Moscow region yielded only
574 a slightly higher δ¹⁸O of 19.6‰ (Fischer et al. 2013: table 1). The lack of discriminating power
575 in the δ¹⁸O values obtained by Luccisano et al. (2023) is unsurprising because extant taxa
576 display considerable variability in this respect, with some significant habitat-related differences,
577 but also some overlap between freshwater, marine and terrestrial taxa (Goedert et al. 2020: fig.
578 1). There is also some uncertainty about the value of oceanic δ¹⁸O over time; Luccisano et al.
579 (2023: 9) disagreed with Fischer et al. (2013) about the minimal δ¹⁸Oₚ (the lowerscript “p”
580 stands for “phosphate”) value (22‰ vs 18‰) that would suggest a truly marine signal.
581 However, the fact that Fischer et al. (2013:50) reported mean values of δ¹⁸Oₚ of 19.6‰ for shark
582 teeth and 21.1‰ for conodonts from the “unequivocal marine facies” of the Kalinovskie Vyselki
583 quarry near Moscow supports Fisher’s (2013) use of their lower threshold (18‰) for a marine
584 signal.

585 Luccisano et al. (2023: 8) concluded that “In light of the bioapatite δ³⁴S values, the environment
586 of Buxières-les-Mines and the Muse OSB do not seem to have been subjected to significant
587 marine influences contrary to what was assumed in previous studies”. While this seems
588 plausible, especially in light of the findings of Fischer et al. (2013), this only implies that
589 Buxières-les-Mines appears to represent a continental locality, plausibly freshwater, but
590 possibly also a brackish basin with little or no water flux with the ocean. It also suggests that
591 the vertebrates that lived there, including the xenacanthids, appeared not to have ventured into
592 the oceans.

593 A slightly different picture emerges for the xenacanthiforms from Joggins; the few Sr isotopic
594 analyses performed by Carpenter et al. (2015), on two specimens (a xenacanthid tooth and a
595 sarcopterygian scale) suggested that the 87Sr/86Sr ratio of 0.710338 ± 83 (cf. *Xenacanthus*) and
596 0.7097772 ± 300 (cf. *Rhizodopsis*) was slightly higher than that of the Pennsylvanian oceans,
597 which they considered to be typically ≤0.7083, and this implies at least a major freshwater
598 contribution to the environment. However, Carpenter et al. (2015: 680) noted that higher values
599 for Pennsylvanian epicontinental seas had also been reported. Indeed, Woodard et al. (2013)
600 reported values of up to 0.7105 for Patlanoaya (for a sample dated from about 306 Ma), but
601 suggested that these high values reflected alteration, as suggested by the “conodont color
602 alteration index”. Carpenter et al. (2015: 681) concluded that these results for a xenacanthid
603 from Joggins were “equivocal, merely indicating that the environment was unlikely to have
604 been fully marine”, which is consistent with a restricted, brackish sea similar to the Baltic Sea,
605 when taking into consideration other sources of information, such as the fossil community
606 found at Joggins (see section on Joggins below). Isotopic works on the habitat of the
607 xenacanthiforms from Buxières-les-Mines and the Muse suggests a much more continental and
608 much less salty environment, plausibly freshwater; hence, isotopic data suggest that
609 xenacanthiforms occupied various habitats, and other lines of evidence support this conclusion
610 (see next section).

611 **Taxa associated with Permo-Carboniferous stegocephalians and their paleoenvironmental
612 significance**

613 *Freshwater chondrichthyans?*

614 Today, the vast majority of extant chondrichthyans are strictly marine; only 43 species (about
615 5% of elasmobranch species currently recognized) venture into freshwater (Carpenter et al.
616 2011: 651 and references cited therein). Fewer still reproduce there (Feitosa et al. 2016, 2020);
617 all of these are viviparous, as extensively documented in the only extant strictly freshwater
618 chondrichthyans, the Potamotrygonidae, also known as freshwater stingrays (Charvet-Almeida
619 et al. 2005). This may not be a coincidence because only about 40% of the extant
620 chondrichthyan species are viviparous (Dulvy and Reynolds 1997). Thus, no extant
621 chondrichthyan lays eggs in freshwater environments. This raises doubts about the traditional
622 interpretation of various Permo-Carboniferous localities that have yielded fossilized
623 chondrichthyan eggs but were nevertheless interpreted as freshwater paleoenvironments. These
624 include the Mazon Creek Lagerstätte (Schultze 2009), in which a range of marine to freshwater
625 paleoenvironments were recognized until recently (Baird et al. 1985). However, the presence
626 of chondrichtyan eggs, among others, suggests that there was no freshwater fauna there
627 (Clements et al. 2019).
<figure><img src="image_3.png" /><figcaption>Figure 2. The temnospondyl *Eryops megacephalus* leaps to seize a small chondrichthyan (*Xenacanthus*). While the co-existence of both taxa is well established, the environment that they occupied (freshwater, brackish water, or even a marginal marine environment) remains enigmatic. Amin Khaleghparast (Tehran, Iran) drew the figure, which was colored by Dmitry Bogdanov (a cardiologist and paleo-artist from Chelyabinsk, Russia); anatomical advice was provided by Tracy Lee Ford (San Diego, California, USA) and Bryan Riolo (Ocala, Florida, USA).</figcaption></figure>

Xenacanthiforms are the chondrichthyans most frequently associated with Permo-Carboniferous stegocephalians (Fig. 2). They apparently lived in the same environments at least occasionally, as shown by a spectacular discovery of a *Triodus sessilis* specimen that had ingested two temnospondyls (*Cheliderpeton* [now *Glanochthon*] *latirostre* and *Archegosaurus decheni*); these temnospondyls were apparently not washed-in carcasses because the *Glanochthon* specimen had remains of *Acanthodes bronni* in its abdominal region, which suggests that all three taxa shared the same habitat (Kriwet et al. 2008). For the reassignment of *Cheliderpeton latirostre* to the more recent taxon *Glanochthon* that was erected partly for this purpose, see Schoch and Witzmann (2009). Xenacanthiforms have long been interpreted as freshwater sharks (Jain 1980; Kohring 1995: 263; Dick 1981, 1988; Kriwet et al. 2008), and would then be the first freshwater chondrichthyans (Compagno 1990). This once-widespread opinion may have resulted from their frequent occurrence in Pennsylvanian Coal Measures (Hampe 2003; Carpenter et al. 2011: 650) and with stegocephalians, which are often called (inappropriately) “amphibians”. The latter is not a good argument, but some isotopic data support this interpretation, as mentioned above (Fischer et al. 2013; Luccisano et al. 2023). However, occurrences in marine environments were subsequently documented by many studies. In fact, *Diplodoselache woodi*, which may well be one of the earliest and most basal xenacanthiform, was found in what Dick (1981: 111) interpreted as a large lagoon that was frequently isolated from the open sea by a sand bar, and *D. woodi* appears to have inhabited the lagoon (rather than representing an allochtonous element). This suggests a brackish environment. Hampe and Ivanov (2007) described xenacanthiform teeth for which they erected

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656 the nominal species *Triodus teberdaensis* in marine carbonates from the Pennsylvanian Tolstiy
657 Bugor Formation (Moscovian) of Karachay-Cherkess Republic (Russia), and geologically older
658 marine occurrences had been documented earlier, as in the Middle Devonian limestones of the
659 Rhenish Schiefergebirge (Stritzke 1986). Similarly, the basal conglomerate of the Hamilton
660 quarry has yielded teeth of *Xenacanthus*, *Orthacanthus* and a cochliodontid chondrichthyan, in
661 a clearly marine association that includes echinoderm ossicles, brachiopods (some of which are
662 in growth position), fusulinids and corals (Cunningham et al. 1993). Xenacanthiforms have also
663 been documented in coastal, probably brackish environments, such as the Kasimovian Cohn
664 Coal Member of the Mattoon Formation in Illinois (Carpenter et al. 2011). In Cohn Coal
665 Member, xenacanthiforms co-occur with microconchids, often called "Spirorbis" or "spirorbids"
666 in the older literature, which were probably euryhaline organisms living mostly in brackish
667 water (see below). Carpenter et al. (2011: 650) listed several other reports that showed that
668 xenacanthiforms are "intimately associated with brackish and marine fauna". In fact, many
669 xenacanthiforms seem to have disappeared from the British Coal Measures in the Bolsovian
670 substage (Hampe 2003), in which the last marine incursion in the British Pennsylvanian
671 occurred (Waters and Davies 2006). Carpenter et al. (2011) suggested that these events are
672 causally related; the extinctions would then imply that few xenacanthiforms could survive in
673 freshwater. Furthermore, growth patterns in *Orthacanthus* from Spain have been interpreted as
674 reflecting tidal cycles (Soler-Gijón 1999).

675 As we saw above (isotopic section), the δ34S results on xenacanthiform bioapatite obtained by
676 Luccisano et al. (2023) from the localities of Buxières-les-Mines (Bourbon-l'Archambault
677 Basin, Allier) and the Muse (Autun Basin) implied that these taxa did not venture into the open
678 ocean. This suggests that at least some xenacanthiform populations lived far from the open
679 ocean. However, note that other studies have not excluded the possibility of at least temporary
680 marine influence in the Bourbon-l'Archambault Basin. For instance, Steyer et al. (2012: 514)
681 speculated that the mass mortality of the seymouriamorph *Discosauriscus* in the Franchesse
682 locality (about 20 km North-North-East from Buxières) might have resulted from temporary
683 and sudden flooding, potentially of salty water from the North Sea, which is plausible according
684 to some paleogeographical reconstructions.

685 Collectively, xenacanthiforms appear to have inhabited both continental, plausibly freshwater
686 (at Buxière-les-Mines and the Muse) and marine environments (represented in other localities),
687 and most frequently environments transitional between these, but this does not imply that each
688 xenacanthiform species inhabited all these environments. Perhaps, as in extant teleosts, there
689 may have been freshwater, marine, and euryhaline taxa, but so far, we have a highly incomplete
690 picture of xenacanthiform environmental preferences.

691 *Freshwater microconchids?*

692 Microconchids (often called "Spirorbis" in the older literature) are often associated with Permo-
693 Carboniferous stegocephalians. Their paleoenvironmental significance is thus relevant to assess
694 the habitat of early stegocephalians. Note that microconchids have often called "Spirorbis" in
695 the older literature, but the latter is an extant marine annelid, and the coiled calcitic tubes
696 encountered in fossiliferous localities older than the Cretaceous were not formed by annelids,
697 but rather, by microconchids (Taylor and Vinn 2006). The fossil record of microconchids extend
698 from the Late Ordovician to the Middle Jurassic (Zatoń et al. 2012: 603). They originated in
699 the marine environment, but subsequently became adapted to other habitats (Zatoń et al. 2012:
700 604). What these habitats were is to an extent controversial. Microconchids undoubtedly
701 occurred in brackish, marine-influenced coastal environments, but their presence in truly
702 freshwater environments is contentious. Some authors seem certain that microconchids
703 inhabited freshwater for part of their evolutionary history, from the early Devonian to the Late
Triassic (Zatoń et al. 2012: 606), but others suggested that they are not found in freshwater, but only in brackish, marine-influenced environments, such as estuaries and deltas (Gierlowski-Kordesch and Cassle 2015). A vigorous exchange followed between the proponents of these opposing points of view (Gierlowski-Kordesch et al. 2016; Zatoń et al. 2016).

To assess the validity of these opposing claims, I checked the most relevant data presented by Zatoń et al. (2012: table S1, appendix A). I restricted my scrutiny to the interval ranging from the Middle Devonian to the Early Permian because this is the temporal interval relevant to the origin of stegocephalians and to their great evolutionary radiation that gave rise to the first amphibians (the total clade of lissamphibians; Laurin et al. 2020) and amniotes. In that interval, four occurrences were listed as fresh to brackish water (hence, uncertain salinity), but only two (both from the Late Carboniferous, one in Ohio and the other one from England) were indicated to be freshwater (rather than brackish). These two cases should thus provide the strongest evidence supporting the presence of microconchids in freshwater and will be examined critically.

The Ohio occurrence is justified by a conference abstract (Lewis and Dunagan 2000), which indicates that this occurrence is from the Conemaugh Group. However, in a more recent assessment of this occurrence, Martino (2016: 142) interpreted the significance of microconchids differently:

"Their presence indicates a marine connection and probable brackish influence (Schultz[e], 2009; Gierlowski-Kordesch and Cassle, 2015). Two occurrences of microconchid-bearing 'nonmarine' limestones in the Glenshaw Formation have been re-interpreted as brackish, clear water, nearshore facies (Morris, 1967; Busch and West, 1987)"

Indeed, Busch and West (1987) had previously indicated that the microconchids, which he called "spirorbid annelids", occurred during a major transgression, in "intertidal-supratidal ponds". Thus, this record, rather than supporting the occurrence of microconchids in freshwater, suggests that these records occurred in a marginal-marine, brackish environment.

To support the presence of "freshwater" microconchids in the Westphalian (Late Carboniferous) of England, Zatoń et al. (2012: table S1, appendix A) cited a book on British regional geology to which I have no access. However, Lomax et al. (2016) described Westphalian microconchids from Yorkshire, which I will consider representative of the record alluded to by Zatoń et al. (2012). In that locality, microconchids co-occur with xiphosurans and shark egg capsules, both of which (especially the latter) suggest a significant marine influence (see section "Freshwater chondrichthyans"). Thus, neither of these supposedly freshwater occurrences of microconchids stands up to scrutiny.

Before closing this section, it may be useful to discuss a recent study that clearly sides with the interpretation that some microconchids occurred in freshwater, even though this concerns taxa that occur after the temporal interval considered in this paper (namely, in the latest Permian and early Triassic). Shcherbakov et al. (2021) interpreted the Early Triassic Petropavlovka Lagerstätte as a freshwater ecosystem, but its fauna includes xiphosurans, which I demonstrate below (next section) suggest marine influence. They did not completely rule out the possibility that the Petropavlovka Lagerstätte represented a brackish water assemblage, but they seemed to prefer a freshwater interpretation (p. 1345) because several temnospondyls (which they call "amphibians") "have never been recorded in assemblages with marine fossils" and because dipnoans are also present in the Lagerstätte. About the latter, they stated that "the overwhelming majority of the Permian and later dipnoans were restricted to freshwater". This may not be entirely correct; the present study aims at demonstrating that many temnospondyls lived in
751 brackish, coastal environments (in addition to presumably occurring in freshwater), as I
752 previously suggested (Laurin and Soler-Gijón 2010), and Permian dipnoans frequently occurred
753 in the same environments as early stegocephalians and xenacanthiform chondrichtynans. For
754 instance, *Gnathorhiza* occurs in the Cisuralian Jemez Springs locality of the Cutler Group
755 (formerly Abo Formation), along with *Xenacanthus* and a great diversity of stegocephalians
756 (Berman 1993: 16). Furthermore, some Permian dipnoans appear to have been euryhaline
757 (McCahon and Miller 2015). A xiphosuran head shield is encrusted with microconchids, which
758 thus probably lived in the same environment. The Petropavlovka Lagerstätte may well represent
759 a brackish water assemblage.

760 The case for a freshwater community in the latest Permian Tunguska Basin, also described by
761 Shcherbakov et al. (2021), seems at first more convincing because it has yielded fossils of
762 aquatic insect larvae, namely mayfly nimphs of the taxon *Khungtukunia sibirica* (Sinitshenkova
763 2013), and extant mayfly nimphs require a salinity inferior to 8‰ (Chadwick et al. 2002),
764 though not necessarily freshwater. The Tunguska Basin also yielded lepisosteiforms and
765 amiiforms, which Shcherbakov et al. (2021:1344) interpreted as “relict freshwater” taxa, citing
766 for this Arratia (2004). However, Arratia (2004: 306) stated, on the contrary, that early
767 actinopterygians, including early stem- and crown-teleosts, were overwhelmingly marine, with
768 invasions of freshwater occurring in some lineages in the Middle and Late Jurassic. The
769 traditional interpretation of lepisosteiforms and amiiforms being adapted to freshwater is based
770 only on extant taxa, and it has been refuted by more recent research; Guinot and Cavin (2018:
771 580) concluded that “Our results indicate a marine ancestry for the entire actinopterygian clade.
772 Basal-most clades such as Cladistia, Chondrostei, Lepisosteiformes, Amiiformes, whose living
773 representatives are known from freshwater or mixed environments all arose from a marine
774 ancestry.” The slightly earlier study by Betancur-R et al. (2015) is congruent on this point. The
775 “relict freshwater” neopterygian taxa mentioned by Shcherbakov et al. (2021:1344) include
776 *Tungusichthys acentrophoroides*, *Arctosomus sibiricus*, *Evenkia eunotoptera* and *Eoperleidus
777 bergi*. Surprisingly, three of these taxa (*Tungusichthys*, *Arctosomus*, and *Evenkia*) are included
778 in Sepkoski’s (2002) monograph “A compendium of fossil marine animal genera”, and thus
779 presumably occur in marine environments. The distribution of *Eoperleidus* seems to be far more
780 limited and I could not find more information about its habitat (beyond its occurrence in the
781 Tunguska Basin), but the three other taxa hardly constitute convincing evidence of a freshwater
782 habitat.

783 Shcherbakov et al. (2021) also discussed other Permo-Triassic assemblages that contained what
784 they interpreted as possibly freshwater microconchids. The interpretation of these assemblages
785 is more complex than they suggest, and this will be exemplified by their discussion of the Grès
786 à Voltzia, in which they recognize some marine influence. They report that the assemblage
787 includes, among others, egg capsules (probably from chondricthyans), limnomedusae,
788 horseshoe crabs, euthycarcinoids, gilled mayfly, aquatic beetle larvae, aquatic insect egg
789 clutches, lingulids and temnospondyls, in addition to microconchids (Shcherbakov et al. 2021:
790 1347). It is possible that this assemblage reflects more than one community because many
791 (perhaps most) insect larvae normally develop in a salinity inferior to 8‰ (Chadwick et al.
792 2002) whereas lingulid brachiopods normally live in a much greater (at least 30 ‰) salinity
793 (Hammond 1983). However, even this surprising assemblage could possibly reflect a single
794 community because many insect larvae tolerate brackish water and occur in estuaries (Williams
795 and Williams 1998), and a few even live in hypersaline enviroments (Pallares et al. 2015). That
796 assemblage would then have to be coastal and brackish because there are no freshwater
797 brachiopods. To sum up, the case for the Permo-Triassic microconchids described by
798 Shcherbakov et al. (2021) as freshwater (rather than brackish water) taxa is not entirely
799 convincing.
Thus, in this paper, I will consider that the presence of microconchids implies marine influence. Bennett et al. (2021: 17), in their analysis of the palaeoenvironment of Mississippian coastal lakes of the Ballagan Formation, likewise accepted the conclusions of Gierlowski-Kordesch and Cassle (2015) that microconchids are marine organisms and concluded that larval spirorbids are “readily transported into non-marine environments by tidal currents or storm deposits”; this may explain the occurrence of a few scattered microconchids in assemblages that show little other evidence of marine influence, but that must nevertheless have formed near the coast.

### *Xiphosuran habitat through time*

The habitat of Permo-Carboniferous xiphosurans, which are sometimes associated with early stegocephalians, has proven particularly controversial. Extant xiphosurans (only four currently recognizes species) are basically marine, even though they frequently enter brackish estuaries and less frequently, rivers where the water is almost fresh; this is documented, for *Carcinoscorpius rotundicauda* (named *Limulus rotundicauda*, in the older literature), in the Hughli river at least as far as Calcutta (Annandale 1909, 1922). This is a bit less than 100 km (as the crow flies) from the coast, but only about 40 km from the proximal part of the Hughli estuary. Størmer (1952: 630) even stated that this species can “migrate up rivers into perfectly fresh water”, but provided no reference to support this statement. However, given the obviously extreme osmotic tolerance of *C. rotundicauda*, Størmer’s (1952: 630) statement may well be correct. The three other extant xiphosuran species seem to be less euryhaline and inhabit the seas, including coasts and estuaries.

Some paleontologists argued for an early invasion of freshwater habitats by xiphosurans. For instance, Gray (1988: 60) concluded that “The Rudstangen record suggests freshwater limuloids in the Late Silurian.” Similarly, Gray (1988: 58) indicated that their occurrence in the ORS led Størmer to infer that some xiphosurans invaded brackish or fresh waters, and that by the Carboniferous, most limuloids lived in these environments. Notably, Gray (1988: 57) concluded that the xiphosuran *Kodymirus* lived in freshwater, or perhaps brackish water, because of similarities between its depositional environment and the Old Red Sandstone facies. However, the ORS came to be accepted as a freshwater deposit (Janvier, 2007), partly because of marine fossils found in some strata (Goujet and Emig 1985) and because recent isotopic analyses of East Greenland ORS deposits show significant marine influence (Goedert et al., 2018). Thus, the occurrence of xiphosurans in ORS only supports presence of some xiphosurans in brackish (rather than fresh) water.

Xiphosurans occur in what was once called the Braidwood Mazon Creek fauna (Schultze 2009: 127), which was thought to include a freshwater fauna, whereas the Essex fauna from Mazon Creek was recognized to show strong marine influence. Gray (1988: 58) considered, as was common at the time, that the Braidwood was a freshwater “fauna”, and concluded, on the basis of such arguments, that “There seems **no doubt** that some of the most characteristic and common limuloids in Carboniferous coal swamp assemblages of Europe and North America”, namely, the euproopaceans which occur in Mazon Creek, were “associated with **freshwater** habitats”. However, Schultze and Maples (1992: 234) concluded that the “Braidwood fauna” inhabited a “tidally influenced coastal or estuarine” environment, and more recently, Schultze (2009: 127) mentioned the presence of the brachiopod *Lingula* in the Mazon Creek biota. Clements et al. (2019: 5) went further and argued that Mazon Creek preserves faunae located at various distances from the shore but that it lacks a discrete freshwater fauna, and that “Most of the major groups found in the Mazon Creek could tolerate varying degrees of salinity”. Among these, they listed both “horseshoe crabs” (xiphosurans) and eurypterids.
Lamsdell (2016: 185) also claimed that “Colonization of the freshwater realm is shown to have occurred at least five times” in xiphosurans, and this is summarized neatly in his figure 2, which optimizes xiphosuran habitat (binary, marine vs. non-marine) onto a timetree. However, a closer look at the supporting data leads to qualify these claims. The most speciose of these putative freshwater xiphosuran clades is Belinurina. The habitat of the terminal taxa shown in the tree (Lamsdell 2016: fig. 2) is indicated, along with supporting references, in Lamsdell’s (2016) tables 1–3, and these indicate that for all taxa in Belinurina, the habitat is freshwater/brackish. Thus, there is no firm evidence that any of these taxa were truly adapted to freshwater, rather than being euryhaline, like some extant xiphosurans (Annandale 1909). The second speciose clade of “non-marine” xiphosurans, Austrolimulidae, originated in the Paleozoic but radiated in the Mesozoic. For this review, I consider only Paleozoic taxa, and extant ones, for the information that they provide about habitat. The Paleozoic austrolimulids (solely represented by ‘*Paleolimus*’ *longispinus* in Lamsdell’s analysis), “are known from marginal marine, fluvially influenced environments”. Lamsdell (2016: 185) indicated that “Aside from these two clades [Belinurina and Austrolimulidae], xiphosurids are shown to have invaded the freshwater realm twice in the Palaeozoic (Schimper 1853; Anderson and Horrocks 1995) and once in the Mesozoic (Riek and Gill 1971).” These two additional “freshwater” invasions deserve additional scrutiny. Schimper (1853) reported the presence of “*Limmulites Bronnii*”, subsequently renamed *Limulites bronni* by Størmer (1952), in the “grès bigarré” near Wasselonne (about 20 km W-NW of Strasbourg), which is Triassic, as reported in Lamsdell (2016: fig. 2), rather than Paleozoic. Presumably, Lamsdell (2016: 185) considered that this represented a freshwater invasion because the ghost lineage of *Limulites bronni* extends into the Permian. Of all the Paleozoic xiphosurans listed by Lamsdell (2016) in his table 1, *L. bronni* is the only one listed as freshwater, rather than marine or freshwater/brackish. However, we know little about the paleoenvironmental conditions of the deposits. Schimper (1853) described no other fossil from the locality that yielded *L. bronni*, but he did report an ammonite, also from the “grès bigarré” from the quarry in Soultz-les-Bains, barely 10 km south of the *L. bronni* locality; this does not suggest a freshwater environment! The last putative Paleozoic freshwater xiphosuran mentioned by Lamsdell (2016) is *Valloisella lievinensis*. However, Anderson and Horrocks (1995) did not indicate that this was a freshwater taxon; they only indicated that “The arthropod was found associated with non-marine bivalves and the other, more commonly encountered, Coal Measures xiphosuran, *Bellinurus* [often spelled *Belinurus*] PICTET, 1846.” However, non-marine does not mean “freshwater”; brackish environments are also typically considered non-marine, and indeed, *Bellinurus* is listed in Lamsdell (2016: table 1) as freshwater/brackish. Furthermore, little is known about the fauna and stratigraphic context associated with *Valloisella lievinensis*. Anderson and Horrocks (1995) described two specimens of this taxon. For the first one, BU (University of Brimingham) 2466, Anderson and Horrocks (1995: 648) indicated:

"it was collected from a Coal Measures site near Dudley, Worcs. The Coal Measures strata in this area are Westphalian B in age (Upper Carboniferous) but unfortunately, this is the only stratigraphic detail available. The nearby site of Coseley (Westphalian B) has yielded Bellinurus koenigianus WOODWARD, 1872, Bellinurus bellulus KONIG 1851 (see SCHULTKA 1994: 347), and Pringlia birtwelli (WOODWARD, 1872)."

Thus, the locality data for this specimen are vague, stratigraphy is worse, and there is no associated fauna. The other specimen, LL 11133, has fairly precise locality data (the Bickershawe Complex colliery tip near Leigh, Wigan), but the reported associated fauna (mostly terrestrial, with the exception of the bivalve *Naiadites*) is only moderately reliable
because “Unfortunately, the material is not preserved in situ, however all of the material listed above comes from a single, constrainable area of the spoil tip, and as such is likely to reflect original association.” (Anderson and Horrocks 1995: 649). Note that the bivalve *Naiadites* is usually called “non-marine”, but its distribution does not seem to fully support this assertion. At Joggins, it occurs, along with other bivalves, throughout the geological section of the coastal site of Joggins (Nova Scotia), even in the lowest levels of the section (though it is less abundant there than in higher levels), which show strong marine influence, as shown by the occurrence of punctate brachiopods and echinoderm fragments (Grey et al. 2011). Thus, the paleonvironment of *V. lievinensis* is poorly documented, and all that can be stated about it is that it was most likely not typically marine, but it may have been either freshwater or brackish.

What should we conclude from all this? What seems to be reasonably well-established is that chelicerates most likely originated in the marine environment (Aria 2022), like most ancient metazoan taxa. Extant xiphosurans are marine and at least some taxa are euryhaline, with abilities to tolerate very low salinities, but they never venture very far from the sea. The fossil record shows that Permo-Carboniferous xiphosurans inhabited marginal-marine, probably brackish environments. They may possibly have invaded freshwater environments, but there is currently no solid evidence of this; on the contrary, when associated with early stegocephalians, it is typically in localities that show some marine influence, as in Mazon Creek and Montceules-Mines (which is discussed below). Given their marine origin in the early Paleozoic, parsimony suggests that until convincing proof of truly freshwater Paleozoic xiphosurans is produced, this taxon should be considered to have inhabited marine and marine-influenced environments; the burden of the proof is authors who claim truly freshwater occurrences of xiphosurans in the Paleozoic. Goldring and Seilacher (1971: 440) reached this conclusion long ago and stressed that

"*The main habitats of limulids have always been in the shallow sea; but the fossilization potential for both, carcasses and tracks, was so much lower in the true biotope than in marginal and partly nonmarine environments that the fossil representation of the limulids is now stronger in these than in their main biotopes*"

Eyrypterids, marine, brackish or freshwater?

Eyrypterids are rare, but a good proportion of the known Permo-Carboniferous specimens have been found in localities that yielded stegocephalians, like Joggins (Carpenter et al. 2015), the Hamilton quarry (Cunningham et al. 1993), Mazon Creek (Clements et al. 2019), Kinney Quarry (Hunt et al. 1992; Braddy et al. 2021, 2023). A specimen was even found in the El Cobre Canyon (New Mexico), in locality 4564, low in the canyon (Hannibal et al. 2005).

The habitat of eyrypterids, as for several other extinct taxa, has proven difficult to assess. Gray (1988: 62) indicated that “Much the same kind of controversy evoked in discussion of limuloid habitats has been raised with regard to eyrypterids although these animals have no modern analogues.” Indeed, the fact that eyrypterids are extinct adds to the difficulty in inferring their paleoenvironment. Some authors (cited in Gray 1988: 62) suggested that the first ones, in the Ordovician (Fig. 3), were marine, and many of the first discovered eyrypterids were found in limestone, associated with a fairly typical marine fauna (Lebrun and Pylypenko 2023), but after that, as the group diversified taxonomically and presumably ecologically as well, the story becomes more complex. Indeed, Boucot and Janis (1983: 260) suggested that the Fish Bed Formation (Ludlow, Silurian) was “Non-marine as suggested by the eyrypterid fauna” (note that in this context, “non-marine” more likely means brackish coastal than freshwater).
940 Kjellesvig-Waering (1958: 1108) suggested that from the Pennsylvanian on, they were
941 restricted to freshwater. However, Kjellesvig-Waering later rejected the idea of non-marine
942 eurypterids (Gray 1988: 62).

<figure><img src="image_4.png" /><figcaption>Figure 3. *Pentecopterus*, one of the oldest (Darriwilian, mid-Ordovician) eurypteryds. Drawing by Patrick Lynch, published on Wikimedia commons (https://en.wikipedia.org/wiki/Eurypterid#/media/File:Eurypterids_Pentecopterus_Vertical.jpg) under the CC0 1.0 DEED licence.</figcaption></figure>

Nevertheless, Permo-Carboniferous eurypterids have often been interpreted as freshwater taxa (Lamsdell et al. 2019). This fact does not seem to be well-supported, as demonstrated by the citation chain that links papers that indicate that eurypterids, after the Devonian, were restricted to freshwater habitats: Lamsdell and Selden (2017: 104) cited Lamsdell and Braddy (2010) for this (in addition to an old paper from 1916 that I was unable to obtain), but the latter cited Braddy (2001) to justify the claim (and made no new contributions to this question). Braddy

943

944

945

946

947

948

949

950

951

952

953
954 (2001) reanalyzed data from Plotnick (1999), which did not tackle Permo-Carboniferous
eurypterids, as reflected by its title, "Habitat of Llandoverian-Lochkovian eurypterids". It seems
955 dangerous to conclude, from a study based on Silurian to Early Devonian eurypterids, that all
956 post-Devonian taxa were freshwater forms!
957

958 Furthermore, Plotnick's (1999) interpretations seem to minimize marine influence. To
demonstrate this, his ecological classification scheme needs to be explained. Plotnick (1999)
959 classified the biofacies assemblages from least (BA0) to most marine (BA6). This was a slight
960 modification of a scheme developed by Boucot and summarized in Boucot and Janis (1983:
961 252):
962

1 corresponds to the intertidal environment, as well as the brackish plus estuarine; 2 to the high subtidal; 3 to the remainder of the subtidal photic-phytal zone; 4 and 5 to the mid- and outer-shelf zone; and 6 to the shelf margin to upper bathyal region.
963
964
965
966

967 To this, Plotnick added BA0, for "probable nonmarine occurrences". The faunal composition
968 of some of the localities seems to match imperfectly the assigned assemblage. Out of the 94
969 localities, 8 are scored as BA 0, sometimes "or BA 1", and 46 are classified as possibly BA 1,
970 sometimes "or BA 2", which reflects substantial uncertainty. The localities classified as BA 0
971 (including those that might be BA1) exhibit some signs of marine influence; some localities
972 (like no. 2) have no associated fauna, or only plants, presumably terrestrial (like no. 62), some
973 are associated with vertebrates (ex. localities 1, 17, 51, 54, 64), and only one (71) is associated
974 with a more diversified biota, which includes xiphosurans. But note that while these vertebrates
975 were once considered freshwater because they occur in the Old Red Sandstone (then considered
976 to represent freshwater deposits), the argument no longer holds because many authors consider
977 the ORS as a marginal marine environment (Janvier 2007), as mentioned above (section on the
978 ORS). Thus, most of the localities classified as BA0 (non-marine) by Plotnick (1999) are
979 ecologically difficult to assess, and rather than truly freshwater, they could well represent
980 marginal-marine environments, such as proximal, low-salinity deltaic areas, bays where
981 freshwater discharge is important, and low-salinity lagoons. A more thorough ecological
982 assessment of these localities would be worthwhile.
983

983 The localities classified as BA 1 show stronger marine influence, like brachiopods, mostly
984 inarticulate, especially lingulids, or more rarely, articulate brachiopods like *Hindella*, *Atrypa*,
985 *Dalejina* (like localities 7, 18, 26, 27, etc.). Some, like localities 26–30, 32–35, 38, 48, even
986 yielded cephalopods, which suggests a fairly typical marine environment (see below). Some
987 have corals, in addition to cephalopods, like localities 3, 268, and 79, or crinoids, like localities
988 37, 48. A few BA 1 localities have acritarchs (no 42), stromatolites (66), trilobite fragments
989 (88), or possible cirripedes (92). Some BA 1 localities are hypersaline (91). Thus, most of the
990 localities considered BA1 by Plotnick (1999) seem to represent the marine end of the spectrum
991 encompassed by this category, namely, intertidal environments, as well as fairly salty brackish
992 and the distal estuarine, rather than the proximal, low-salinity estuarine environments.
993

993 Last but not least, paleobiogeographic data show that at least some eurypterids, namely, the
994 pterygotoids "apparently could cross open oceans, and are found throughout the world in the
995 short time span of their existence (~40 Ma)", as Tetlie (2007: 559) concluded. However,
996 examination of the paleobiogeographical distribution patterns of some of the geologically most
997 recent eurypterids, namely the Adelophthalmoidea, shows that they were at least able to
disperse in coastal areas, given that by the Permian, they were present in the South China Block,
998 in addition to the core area of eurypterids in former Laurentia and Baltica (Tetlie 2007: fig. 5C).
999 Indeed, (Tetlie 2007: 572) admitted that adelophthalmoids and pterygotoids were "the most
1000
dispersed [eurypterids] throughout the world.” Adelophthalmoids are also among the eurypterids most frequently associated with Permo-Carboniferous stegocephalians. For instance, *Adelophthalmus brasdorensis* occurs at Joggins, *Adelophthalmus mazonensis* occurs at Mazon Creek, *Adelophthalmus sellardsi* is found in the Hamilton Quarry (in the marginal marine ostracode wackestone that also yielded rhynchonellid and productid brachiopods), and *Adelophthalmus* sp. is found in El Cobre Canyon (Cunningham et al. 1993; Hannibal et al. 2005; Tetlie 2007: table 5). More importantly, nearly 200 specimens of *Adelophthalmus luceroensis* have been found in the nearby Red Tanks Member of the Bursum Formation (Kues and Kietzke 1981; Braddy et al. 2021), which also yielded a diverse vertebrate fauna (Harris et al. 2004). These deposits are from a marine embayment, and the levels that produce *A. luceroensis* also yield microconchids, which reflect marine influence (Braddy et al. 2021:112).

Thus, the case for a freshwater interpretation of some eurypterids seems to be weak. Braddy (2001: 120) wrote “No marine fauna has ever been reported from this assemblage [Early Devonian Midland Valley of Scotland] and the presence of terrestrial plants and invertebrates indicates a freshwater lacustrine setting.” I did not try to reassess the paleoenvironment of these Early Devonian deposits because this paper focuses on the Frasnian to the Kungurian, but note that none of the three arguments given in this quote is fully satisfactory. The most convincing, the absence of marine faunas, is negative evidence, and given the controversial environmental tolerances of many Devonian taxa (such as microconchids, xiphosurans and eurypterids; see below), it is of limited value because some taxa that actually lived in marginal-marine, brackish environments may have been misinterpreted as freshwater forms. The presence of terrestrial plants (or any terrestrial organism) is irrelevant, as pointed out by Schultze (2009: 127); such plants do grow on coasts of seas and oceans, so they only indicate that land was nearby, but give no information about paleosalinity. Last but not least, “invertebrates” are a huge paraphyletic group that is found in all environments, and if the author meant “terrestrial invertebrates” (the sentence is ambiguous on this point), just like land plants, they only indicate that the shore was nearby. Some bias in favor of a freshwater interpretation was built into Braddy’s (2001) study. He classified benthic assemblages (B.A.) into the categories B.A. 0, “non-marine (e.g. lacustrine and fluvial settings), B.A. 1, for “marginal settings (e.g. lagoons, hypersaline environments, littoral settings and mud flats)”, B.A. 2, for “shallow marine settings”, and B.A. 3 +, which designates “deeper marine settings”. Braddy (2001: 121) indicated (p. 121) that “In the case of disputed environmental settings the lowest B.A [most continental] is used, for consistency.” Using this logic, with the data then available, would have led to consider the entire ORS entirely freshwater, and indeed, a locality with no paleoenvironmental data would be considered freshwater. A similar bias seems to occur in the eurypterid literature cited by Braddy (2001). Thus, in his discussion of the Upper Silurian Bertie Waterlimes, Braddy (2001: 124) indicated that it had been

"variously interpreted as representing an upper intertidal portion of a sabkha to subtidal sequence (Hamell, 1982), a near shore lagoon showing fluctuating salinity (Heckel, 1972; Copeland and Bolton, 1985), a wide lagoonal system behind a reef (Ruedemann, 1925; Monahan, 1931), a brackish to freshwater lagoon or estuarine deposit (Kindle, 1934), or a deltaic environment (O'Connell, 1916)."

Yet, Braddy (2001: 124) indicated that evaporites occur in the Bertie Waterlimes, which “contains a diverse eurypterid fauna (Clarke and Ruedemann, 1912) associated with a rare marine fauna, including **cephalopods**, bivalves, **lingulid brachiopods**, worms, gastropods, xiphosurans…”!
1048 The eurypterids that co-occur with microconchids probably inhabited marine-influenced habitats (see above, section on microconchids). One such co-occurrence with microconchids (reported erroneously as "Spirorbis") is in the Middle Devonian Gaspé Sandstone Series of New Brunswick and Québec provinces, in Canada (Gray 1988: 37). Similarly, one of the sites that yielded the most abundant and best-preserved eurypterid remains, in the Red Tanks member of the Madera Formation, contains microconchids, which were called "spirorbid worms") by Kues and Kietzke (1981: 709). This abundance and good preservation is not entirely consistent with Lamsdell et al.'s (2019: 1713) conclusion that "the eurypterid material [in coastal deposits; not specifically from the Red Tanks] was swept in from continental settings". The Red Tanks member also includes fusulinids and some marine and "non-marine" (possibly brackish or freshwater) strata.

1059 More importantly, at least four specimens (at least one of which belongs to the mycteropoid *Mycterops whitei*) have been found in three Late Pennsylvanian localities in two formations (Hushpuckney Shale, Swope Formation, Iowa, and Stark Shale Member, Dennis Formation, Nebraska) that have yielded "abundant conodonts" (Schram 1984). These specimens are fairly well-preserved because Schram (1984: 208) indicated that "the holotype alone is more complete than the known specimens of *M. ordinatus* Cope, 1886; *M. mathieui* (Pruvost), 1923; or *M. (?) blairi* Waterston, 1968." Thus, these mycteropoid remains must not have been carried far from their habitat, which suggests that they were marine.

1067 Gray (1988: 62) cautiously concluded that she could not dismiss the possibility "that there were marine, brackish and freshwater taxa throughout eurypterid history", but some more recent studies suggest a shift in opinion about eurypterid habitat. Thus, Tetlie and Poschmann (2008: 241) argued, after a "critical review" of this topic, that "There is much more of a marine influence in many of the sections yielding *Adelophthalmus* than has previously been acknowledged." More precisely, they suggested that "adelophthalmoids as a whole, throughout the history of the clade, inhabited environments situated near the coastal realm, predominantly those with reduced salinities, such as lagoons, estuaries or deltas", while recognizing that "some basal forms are known from fully marine deposits." Tetlie and Poschmann (2008: 241) suggested that *Adelophthalmus* was found in habitats with less salinity than other adelophthalmoids, especially in freshwater. However, this apparently referred to a brief episode of eurypterid history, "during Bashkirian and Moscovian times, when they are usually encountered within **coal-bearing strata** and associated with **terrestrial organisms** and 'freshwater bivalves'." Terrestrial organisms cannot indicate paleosalinity, and the coal-bearing strata have recently been reinterpreted as displaying marine influence (e.g., Carpenter et al. 2015; Ó Gogáin et al. 2016). Tetlie and Poschmann (2008: 242) admitted that "These horizons were usually interpreted as reflecting freshwater conditions, but nevertheless a marine influence is commonly, but not always, encountered within the respective sequences", and they concluded that "*Adelophthalmus* seems to be mainly confined to paralic or lowland basins, in depositional environments that had a close connection to the marginal marine realm." This interpretation differs only slightly from my interpretation of the habitat that I infer from the associated fauna, such as microconchids, xiphosurans, and xenacanthiforms, and from more recent literature about the relevant localities. The habitats of *Adelophthalmus*, which is arguably the most "freshwater" eurypterid, were obviously close to the sea, and Tetlie and Poschmann (2008) interpret them as freshwater, whereas I suspect, based on other associated faunal elements, that they were more likely to have been brackish environments, although perhaps with a fairly low salt content.

1094 More recent research suggests that Permo-Carboniferous eurypterids may have lived closer to the seas than previously thought. Thus, Braddy et al. (2023), in their study of *Hibbertopterus*
1096 (Mycteropoidea, Stylonurina) from the Carboniferous of Kinney Quarry (New Mexico),
1097 indicated that this site included sediments from "non-marine to shallow marine settings", but
1098 as in most other studies, "non-marine" only indicates an environment other than a typical
1099 marine setting in which coral reefs and echinoderms (among others) occur; this does not rule
1100 out brackish estuaries, deltas and lagoons. But more marine conditions were suggested by
1101 Feldman et al. (1993: 489-491) because brachiopods and crinoid debris occur there. Indeed,
1102 Braddy et al. (2023: 261) concluded that "The Kinney Quarry rocks represent a marine
1103 embayment" and that *Hibbertopterus* probably lived there (rather than the carcasses having
1104 been washed in). In light of their findings, and of a trackway of *Cyrtoctenus* reported from the
1105 Ecca Group in a paleoenvironment with possibly high salinity, Braddy et al. (2023: 261)
1106 admitted that previous ideas on Permo-Carboniferous eurypterid habitats "may need to be re-
1107 evaluated" and that they were probably euryhaline.

1108 I conclude from the above that the presence of eurypterids in the seas and marginal-marine
1109 environments is much better established than in truly freshwater deposits; I have not seen a
1110 single solid case for the latter, although localities where *Adelophthalmus* occurs without any
1111 other faunal element suggesting marine influence (if such localities exist) could plausibly
1112 represent a freshwater environment. However, I have not studied all eurypterid-bearing
1113 localities, so my conclusions on Permo-Carboniferous eurypterid paleoenvironments remain
1114 tentative; as Tetlie and Poschmann (2008: 242) admitted, our understanding of eurypterid
1115 paleoecology "is still in its infancy", and "only detailed examinations, i.e. bed-by-bed
1116 excavations of eurypterid-bearing sequences, may further improve our knowledge".
1117 Nevertheless, this interpretation of the habitat of eurypterids is not new. In his summary of the
1118 three main schools of thought on this topic, Plotnick (1999: 107) indicated that according to the
1119 "facies School" (one of the three main schools), first proposed by Kjellesvig-Waering (1961),
1120 eurypterids occurred in three facies: biofacies 1, fully marine; biofacies 2, near-shore, epeiric
1121 seas and lagoons, presumed to be somewhat hypersaline; biofacies 3, presumed "near-shore,
1122 brackish, deltaic environments. Note that even biofacies 3 is considered brackish, rather than
1123 truly freshwater. Kjellesvig-Waering (1961:794) explained that it included "the more brackish
1124 part of bays, and estuaries." This is further confirmed by Plotnick (1999: 107), who clarified
1125 that "Biofacies 3 of that school (not of his own scheme) was considered to be brackish water
1126 and landward of BA [Biofacies Assemblage] 1." This still seems to reflect the habitat of the
1127 most continental eurypterids; I have seen no evidence of eurypterids living far from the seas, in
1128 mid- to high-altitude, freshwater lakes and rivers.

1129 *Euthycarcinoids*

1130 These enigmatic arthropods of uncertain affinities (Ortega-Hernández et al. 2010; Giribet and
1131 Edgecombe 2019) occur in some Permo-Carboniferous localities that yielded stegocephalians,
1132 like Mazon Creek and Montceau-les-Mines (Rolfe 1985; Schram and Rofle 1994). The
1133 literature suggests that they inhabited a wide range of habitats from marine to freshwater, but
1134 this literature arguably displays the same problems in paleoenvironmental interpretation as that
1135 of eurypterids. Thus, Schram and Rolfe (1994: 142) indicated that while many euthycarcinoids
1136 seem to have been marine, those from Montceau-les-Mines "are thought to have inhabited fresh
1137 water (Rolfe, 1985)." However, Rofle (1985) may have suggested that the Montceau
1138 euthycarcinoids were freshwater inhabitants only because they were associated with
1139 "amphibians", which is not a good justification (Laurin and Soler-Gijón 2010). Simultaneously,
1140 he admitted that at Mazon Creek, they occur "only in the Essex fauna, i.e. with the most marine
1141 animals, in the distal region of the delta" (my translation throughout this paper, unless indicated
1142 otherwise). Neither paper explains why the Mazon Creek euthycarcinoids are marine, whereas
1143 the same taxa in Montceau should be considered freshwater. The euthycarcinoids of both
1144 localities are thought to be closely related; *Schramixerxes gerem* in Montceau is closely related
1145 to *Kottixerxes gloriosus* from Mazon Creek, whereas *Sottixerxes multiplex* from Montceau is
1146 related to *Pieckoxerxes pieckoeae* from Mazon Creek, according to Schram and Rolfe (1994:
1147 142). Indeed, Poplin and Heyler (1994) provides a different, arguably better-justified
1148 interpretation of the same findings:

1149 *Fossil remains of these animals were found in many regions of the world, generally in rocks derived from marine or coastal environments, such as Mazon Creek for instance. Thus their presence in the intermontane basin of Montceau-les-Mines poses a problem.*
1150
1151
1152

1153 Similarly, Racheboeuf et al. (2008: 12) interpreted the euthycarcinoids from Montceau as
1154 freshwater organisms, partly based on comparisons with euthycarcinoids from the “non-marine
1155 Braidwood facies of Mazon Creek” (p. 23). However, this quote raises two problems: first, it
1156 places these Mazon Creek euthycarcinoids in a different facies than Rofle (1985), and second,
1157 as discussed above (section on xiphosurans), Mazon Creek apparently lacks a freshwater fauna;
1158 it preserves aquatic, presumably euryhaline taxa, and terrestrial organisms (Clements et al.
1159 2019). Furthermore, Racheboeuf et al. (2008: 14) mentioned that Schram and Rolfe (1997: 211)
1160 considered euthycarcinoids to be marine.

1161 *Brachiopods*

1162 Brachiopods occasionally co-occur with stegocephalians in Permo-Carboniferous localities.  
1163 Extant brachiopods are exclusively marine animals, with a fairly low tolerance to salinity  
1164 variations. Some studies have suggested that in the Paleozoic, they may have lived in slightly  
1165 hypersaline or slightly brackish water (Fürsich and Hurst 1980). However, among extant  
1166 brqachiopods, even lingulids, which are probably the most tolerant brachiopods to salinity  
1167 variations, are “only moderately euryhaline, and optimally marine”, as shown by experimental  
1168 studies (Hammond 1983: 1311). They normally live in a salinity of at least 30‰ (Hammen and  
1169 Lum 1977), even though lower salinities may be tolerated for short periods. The  
1170 paleoenvironmental significance of lingulids has often been misinterpreted, according to  
1171 Cherns (1979). In his study of two nominal species of *Lingula* from the Lower Leintwardine  
1172 Beds (LLB), Ludlow, late Silurian, Cherns (1979: 42) stated that “Fossil lingulids, by analogy  
1173 with modem forms, are often inferred to have lived in shallow water”, but after analyzing his  
1174 data, he concluded (p. 45) that “Neither LLB species conforms to the commonly held view of  
1175 *Lingula* as an intertidal, very nearshore, or even brackish water, indicator.” Thus, the presence  
1176 of any brachiopod in a locality indicates a marine, fairly saline environment, contrary to what  
1177 has often been suggested.

1178 *Echinoderms*

1179 Echinoderms are infrequently associated with stegocephalians, but a few such associations are
1180 mentioned below, so it is worth reviewing briefly their osmotic tolerance. This taxon is
1181 overwhelmingly marine, a constraint probably linked to the fact that echinoderms have poorly
1182 developed circulatory, excretory, and gas exchange systems (Turner 2007: 464). Very few extant
1183 echinoderms, like *Ophiura albida*, *Amphiura chiajei* and *Asterias rubens*, have adapted to
1184 brackish waters (Cognetti and Maltagliati 2000: 9). Turner and Meyer (1980: 249) reported that
1185 “brackish-water populations of at least 22 species of echinoderms” were known, but more
1186 appeared to be discovered subsequently because Turner (2007: 464) reported that about 40
1187 species of echinoderms occur in estuaries. This is a tiny proportion of the 7584 currently
1188 recognized extant echinoderm species (as of 3 march, 2024) by the WORMS (World Register of Marine Species). Even the most euryhaline extant echinoderm (and the only one endemic to 1189 brackish water), the ophiuroid *Ophiophragmus filograneus*, which has been found occasionally 1190 in brackish water that had (momentarily) very low salinities, showed physiological stress when 1191 subjected to salinities below about 17‰ (Turner and Meyer 1980: 252), and experiments 1192 showed exposure to 8 ‰ salinity is deadly within 17 days (Turner 2007: 467). A record for 1193 echinoderms is a naturally-occurring population in Florida that was observed in a bay at 7.7 ‰ 1194 salinity in summer of 1958, but in that season, salinity fluctuated between 7.7 and 14 ‰ salinity, 1195 and after 1959, a subsequent study failed to recover any individuals (Turner 2007: 471), so it is 1196 possible that this bout of low salinity proved fatal for this population. Furthermore, some 1197 estuarine populations of echinoderms are sterile and continue to exist only through immigration 1198 from more marine environments. Thus, the presence of echinoderms is strong evidence that a 1199 truly marine environment either prevailed locally, or occurred in the vicinity, and that the 1200 salinity was probably above 15 ‰ on average, on a yearly basis, although it may have 1201 occasionally fluctuated down to a minimum of about 8 ‰, below which even the most 1202 osmotolerant echinoderms are quickly killed. 1203

1204 *Cephalopods*

1205 Extant cephalopods are among the most strictly marine mollusks. Parsimony suggests that 1206 crown-cephalopods in general were also marine, and this crown-group is ancient, given that the 1207 divergence between nautilids and coleoids harks back deep into the Paleozoic. Indeed, some 1208 molecular studies placing it around the Late Devonian (Stöger et al. 2013), which is plausible 1209 because coleids are known from the Carboniferous at least (Klug et al. 2019) and nautiloids are 1210 much older still. What we know of the cephalopod fossil record (Leonova 2011) also suggests 1211 that they have always been exclusively marine forms. Unsurprisingly, cephalopods only 1212 infrequently co-occur with stegocephalians in Permo-Carboniferous localities, but several such 1213 examples are mentioned in this paper. Most extant cephalopod taxa tolerate only mild salinity 1214 fluctuations (Jereb and Roper 2016: 6). The most euryhaline taxa, like the squid *Loliguncula* 1215 brevis, can survive in water that has at least 17.9 ‰ salinity (Bartol et al. 2002), which is about 1216 half of the normal sea water salinity (about 35 ‰); they die in about 48 h in water that has 16.5 1217 ‰ salinity (Hendrix et al. 1981). Thus, the presence of cephalopods in a locality indicates strong 1218 marine influence, unless there is evidence of long-distance transportation. 1219

1219 *Freshwater jellyfish?*

1220 Many articles that describe supposedly fully continental (i.e., without any marine influence) 1221 deposits reported “freshwater jellyfishes” (e.g., Poujol et al. 2023: 5). Yet, an overwhelming 1222 majority of extant cnidarians are marine, with only fewer than 35 nominal species documented 1223 in freshwater habitats (Jankowski et al. 2008), as compared to over 12 000 nominal extant 1224 cnidarian species, according to the WORMS registry (https://www.marinespecies.org/ 1225 consulted on October 3, 2023). This amounts to 0.3% of the extant cnidarian specific 1226 biodiversity in freshwater ecosystems. In Permo-Carboniferous localities, putative “freshwater 1227 jellyfishes” and other cnidarians have been reported in sites that are now known to show strong 1228 marine influence. Most interesting is the case of Mazon Creek, which has yielded a variety of 1229 cnidarians (Clements et al. 2019: table 1), including medusae and the hydroid *Devotella* 1230 (Schultze 2009: 127). The most common cnidarian at Mazon Creek is *Essexella asherae* (Baird 1231 et al. 1985), which is a sea anemone, rather than a medusa (Plotnick et al. 2023). *Essexella* 1232 *asherae* appears to be autochtonous at Mazon Creek because of its abundance and its benthic 1233 lifestyle, and it was presumably a euryhaline, coastal marine taxon (Plotnick et al. 2023: 24)
1234 because Baird et al. (1986: fig. 2) showed that it is most abundant in the deepest, most marine
1235 (offshore) facies.

1236 **Devonian localities**

1237 *Established ideas and recent isotopic analyses*

1238 Most Devonian localities that have yielded stegocephalians were long interpreted as freshwater
1239 by most authors. Thus, Long and Gordon (2004: 703) stated that "A reasonable generalization
1240 is that they [Devonian stegocephalians] most likely inhabited large **freshwater river and lake
1241 systems**, environments similar to those inhabited by the East Greenland forms [*Ichthyostega
1242 and Acanthostega*]." However, Schultze (2009: 128) argued that "The most parsimonious
1243 interpretation of the environment of the earliest (Devonian) tetrapods is coastal marine".
1244 Subsequent isotopic study of *Ichthyostega* remains, and those of other Devonian
1245 tetrapodomorphs from East Greenland (Upper Devonian Celsius Bjerg group, *Remigolepis
1246 series*) and from the Zhongning Formation of Ningxia Hui (China), which has yielded the
1247 stegocephalian *Sinostega pani*, confirmed that these taxa "were euryhaline and inhabited
1248 transitional aquatic environments subject to high-magnitude, rapid changes in salinity, such as
1249 estuaries or deltas" (Goedert et al. 2018: 68). This result is particularly interesting because my
1250 earlier review (Laurin and Soler-Gijón 2010: fig. 4) accepted *Ichthyostega* and *Acanthostega
1251 as* the only Devonian stegocephalians for which the traditional freshwater habitat interpretation
1252 was not contradicted by indicators of marine influence; I now stand corrected! It would be
1253 interesting to perform similar analyses on other Devonian localities and formations, such as the
1254 Red Hill locality of the Catskill Formation, which yielded the stegocephalian *Hynerpeton*, and
1255 which was long interpreted as a freshwater taxon, but which might possibly have been
1256 euryhaline (Broussard et al. 2018).

1257 *Strud (Belgium)*

1258 The Famennian Strud locality (Belgium), which recently yielded a stegocephalian (Clément et
1259 al. 2004), has been interpreted as “one of the oldest continental – probably fresh-water –
1260 ecosystems” (Denayer et al. 2016). However, this hides considerable variability in the salinity
1261 of the environments represented in the strata of Strud, as the authors recognized:

1262 "*The depositional setting approximately corresponds to a ramp with both an
1263 increase in the marine influence and a deepening southwards (Thorez et al.
1264 1977). The proximal facies (northwards) frequently show a continental
1265 influence and are thus dominantly sandy and silty, whereas the distal facies
1266 (southwards) are more mixed with frequent carbonate intercalations (Ciney
1267 area and southwards) indicating deposition in a 'deeper' part of the basin (c.
1268 50 m deep after Thorez & Dreesen, 1986)."

1269 Indeed, faunal elements that indicate a fairly typical marine environment occur at various levels
1270 of the Strud succession and more generally, in the Dinant Synclinorium (Thorez et al. 1977).
1271 These will be reviewed from the base to the top. In the late Fransian/early Famennian Fasiole
1272 Formation, Denayer et al. (2016: 113, 116, 121) report spiriferid brachiopods and a sandy
1273 crinoidal limestone. The overlying Bois-des-Mouches/Citadelle de Huy Formation includes,
1274 near its top, a "coarse crinoidal limestone" (Denayer et al. 2016: 115), as well as "rhynchonellid
1275 brachiopods, locally abundant but not broken", which "suggest a more marine environment"
1276 (Denayer et al. 2016: 123). The late Famennian Poulseur Member of the Comblain-la-Tour
1277 Formation contains "some brachiopod shells, often dissolved" (Denayer et al. 2016: 115), and
1278 a "bioturbated marine sandstone" (Denayer et al. 2016: 123). Finally, near the top of the
1279 succession, the Complain-au-Pont Formation contains a crinoidal limestone (Denayer et al. 2016: 115). Lamsdell et al. (2019:1709) indicated that the 1.4 m thick channel-filling succession in which they found eurypterid material displayed “no evidence for marine influence”. This may have been true of the 1.4 m section, but is this a large enough area to expect to find traces of marine influence in what was apparently a deltaic or estuarine environment? And eurypterids only indicate that the sea was near (see above).

<figure><img src="image_5.png" /><figcaption>Figure 4. Optimization of habitat in stegocephalians (with a few other sarcopterygians provided to better optimize near the base of Stegocephali) on a phylogeny. This figure is slightly updated from Laurin and Soler-Gijón (2010: fig. 4), which represented the interpretations of the authors at the time. For the data shown, only the habitat of *Ichthyostega* and *Acanthostega* has been updated to reflect the findings of Goedert et al. (2018), along with the geological age of *Sinostega*. To facilitate comparisons, the phylogeny was kept the same as in Laurin and Soler-Gijón (2010: fig. 4); in any case, minor changes in the phylogeny of various taxa would not change the overall pattern, and the relationships between the main clades have been confirmed by two recent studies based on extensive datasets (Marjanović and Laurin 2019; Laurin et al. 2022). The geological timescale, shown only from the Devonian through the Cisuralian, has been colored using the RGB Color Code provided by the Commission for the Geological Map of the World</figcaption></figure>

1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
The stegocephalian from Strud was found in beds B and D of the Royseux Member of the Evieux Formation (Clément et al. 2004; Denayer et al. 2016: 114), which did not yield typical marine fossils. Because of this, these beds of the Royseux Member have been interpreted as a freshwater to brackish, fluviatile to estuarine environment. However, this member is between the Poulseur Member and the Complain-au-Pont Formation, both of which contain marine fossils. Thus, the open sea was probably not very far from the habitat of the Stud stegocephalian. Indeed, Denayer et al. (2016: 123) interpreted the Royseux Member of the Evieux Formation) as “a lagoonal deposit in a back-barrier position. It consists of fining-upwards sequences associated with red beds, evaporitic dolomite (sabkha sequence) and anhydrite pseudomorphs, locally with dark shale”. The presence of evaporitic dolomite and of anhydrite pseudomorphs
1307 implies saline (plausibly brackish) water, which became hypersaline at least occasionally. Were
1308 the scant remains of the stegocephalians washed in, which would imply a freshwater habitat, or
1309 did this stegocephalian inhabit the estuary and other marginal-marine habitats, as apparently
1310 did the other Devonian stegocephalians? In this case, the fragmentary nature of the specimen
1311 does not preclude the possibility that it represents an allochtonous taxon.

1312 **Carboniferous localities and formations**

1313 *Minto Formation (New Brunswick)*

1314 The early Moscovian (Pennsylvanian) Minto Formation of New Brunswick has yielded an
1315 interesting metazoan fauna that includes a few stegocephalian remains, including a jaw that
1316 plausibly belongs to a colosteid, small limb bones, and a vertebral centrum that has been
1317 plausibly attributed to an embolomere, in addition to finned tetrapodomorph material, mostly
1318 assigned to *Megalichthys* (Ó Gogáin et al. 2016). While the stegocephalian material is scant (15
1319 specimens, out of the total of 722 vertebrate specimens), Ó Gogáin et al. (2016: 713) suggested
1320 that the vertebrate remains had undergone “minimal transportation”. Nevertheless, the
1321 possibility that the fragmentary stegocephalian remains of this locality represent para-
1322 autochtonous occurrences cannot be dismissed. This site is interesting because the
1323 paleoenvironment appears to represent marginal-marine environments with a salinity gradient
1324 that ranges from open marine shallow environments to tidal estuaries. This interpretation is
1325 supported by framboidal pyrite, sponge spicules, spirorbiform microconchids, echinoid spines,
1326 bioclasts of punctate brachiopods in the most marine facies, and spirorbiform microconchids,
1327 along with the bivalve *Naiadites*, in the brackish embayment facies. The xenacanthiform
1328 remains occur in all facies (salinities), which is consistent with their inferred broad osmotic
1329 tolerance, which appear to have ranged from fully marine to freshwater (see above). Other taxa
1330 appear to have displayed osmotic preferences: dipnoans, some rhizodonts (*Archichthys*,
1331 *Strepsodus*) and *Megalichthys* occur mostly in the open marine facies, whereas *Rhizodus* and
1332 *Rhizodopsis* are most common in brackish tidal estuaries (Ó Gogáin et al. 2016: 714). The
1333 stegocephalians are most abundant in the brackish embayment. Ó Gogáin et al. (2016: 717)
1334 suggested that euryhaline habit could explain why many Permo-Carboniferous gnathostome
1335 remains “show enriched strontium isotope values indicative of continental or freshwater
1336 influences”.

1337 *Joggins*

1338 Joggins has yielded many stegocephalian remains, including the oldest known amniotes
1339 (Carroll 1964; Archer et al. 2015: 662). The diversity, abundance, and quality of preservation
1340 of stegocephalian remains (e.g., Godfrey et al. 1991), many of which are articulated (Holmes
1341 et al. 1998) or at least composed of many clearly associated skeletal elements (Mann et al.
1342 2020), suggests that little transport had occurred (Falcon-Lang et al. 2006: 561). This locality
1343 has long been interpreted as an intra-montane, freshwater basin (Carroll 1970: 17), and some
1344 isotopic analyses support this interpretation (Brand 1994). As we saw above (isotopic section),
1345 Brand (1994) suggested that the Joggins Formation sediments were non-marine (continental),
1346 but the few Sr isotopic analyses performed by Carpenter et al. (2015) led them to conclude
1347 that these results were equivocal, and only indicated that the environment was probably
1348 not fully marine. Indeed, Carpenter et al. (2015) concluded, based on faunal data, that the
1349 marine influence at Joggins was fairly strong, at least at several stratigraphic levels.

1350 Other recent studies support marine influence in the Joggins Formation (Davies et al. 2005),
1351 and similar conclusions had been reached much earlier by Duff and Walton (1973). The Joggins
1352 cliffs preserve a variety of habitats that reflect 14 or 15 cyclothems (probably linked to 1353 regressions and transgressions), with some levels representing marginal marine levels, and 1354 other levels that exhibit less marine influence (Grey et al. 2011; Carpenter et al. 2015). The 1355 marine levels are represented by the open water (OW) facies that contains limestone and coal; 1356 other facies include poorly drained floodplain units (PDF) and well-drained alluvial plain units 1357 (WDF). The presence of framboidal pyrite in the most marine levels also indicate normal 1358 marine salinity levels (Grey et al. 2011: 262), and agglutinating foraminifers, which are 1359 characteristic of shallow marine environments, occur both in Joggins and the nearby Sydney 1360 basin, where the Florence locality is located (Schultze 2009: 127-128).

1361 The OW facies has been interpreted as reflecting a "brackish sea" in recent studies (e.g., 1362 Carpenter et al. 2015), but it yields fossils that show that a stenohaline, marine fauna was present 1363 nearby; these include echinoderm fragments (from blastoids, crinoids, or both), fragments of 1364 punctate brachiopods (Grey et al. 2011: 260), and actinistian scales (Schultze 2009: 127). In 1365 addition, the OW facies yields other taxa that may have been euryhaline rather than frankly 1366 marine, including microconchid tubes, chondrichthyan scales and teeth, and a lungfish tooth 1367 plate. Grey et al. (2011: 262) concluded that « The Joggins Formation records a waning marine 1368 influence over time » and that at some point (at least in the OW facies showing the strongest 1369 marine influence, especially near the base of the formation), it was close to the Paleo-Tethys 1370 Ocean. Grey et al. (2011: 262) noted "antithetic abundances of brackish ostracodes and 1371 freshwater bivalves", but at Joggins, bivalves are present even in layers that produced fragments 1372 of punctate brachiopods (Grey et al. 2011: 260), so without a finer analysis that discriminates 1373 between the bivalve taxa present at various levels, it is unclear that these are indeed freshwater 1374 (rather than brackish water, or even marine) bivalves. Grey et al. (2011: 262) supported earlier 1375 conclusions that the Joggins Formation represents "fluvial deposition into shallow brackish 1376 waters" and suggested that this brackish environment was inhabited only by euryhaline 1377 organisms. In extant ecosystems, typical brackish faunae composed primarily of taxa with 1378 marine relatives inhabit waters where the salinity is between 5 ‰ and 30‰, and especially in 1379 the upper half of this salinity range (Cognetti and Maltagliati 2000). Carpenter et al. (2015: 662) 1380 suggested that a western extension of the Tethys Ocean had spread through a hypothetical mid- 1381 Euramerican seaway and reached the area of Joggins at highstand of cyclical transgressions 1382 possibly driven by Milankovitch-scale orbital cycles. They also suggested that the brackish sea 1383 present at Joggins "probably represent the distal extension of the marine bands that characterize 1384 coal measure successions in north-west Europe", which suggests marine influence in many 1385 other Permo-Carboniferous localities that have yielded stegecephalians. These interpretations 1386 are supported by the reinterpretation of supposedly endemic gnathostome taxa (Ctenoptychius 1387 cristatus, actually a synonym of the chondrichthyan *Ageleodus pectinatus*; *Gyracanthus* 1388 *duplicatus*, actually an undetermined gyracanthid acanthodian; *Conchodus plicatus*, actually an 1389 indeterminate *Sagenodus* dipnoan) as fairly cosmopolitan taxa that also occur in fully marine 1390 environments. Thus, Carpenter et al. (2015: 682) interpreted the Joggins aquatic gnathostome 1391 fauna "as having a distinctly euryhaline or diadromous mode of life", and they even suggested 1392 (p. 683) that in the Pennsylvanian, a much greater proportion of gnathostomes were euryhaline 1393 than today (only 3–5% now), and that this represented an important intermediate step toward 1394 the subsequent establishment of vertebrates in freshwater habitats. Carpenter et al. (2015: 683) 1395 interpreted this as resulting from important glacio-eustatic fluctuations that resulted in the 1396 "widespread development of brackish estuaries and epeiric seaways". Ironically, these recent 1397 works suggest that Dawson's (1865) much earlier interpretation of Joggins as a coastal deposit 1398 was correct, after all.

1399 *Hamilton fossil-Lagerstätte*
This fossil Konservat-Lagerstätte from Kansas preserves a Stephanian coastal community (Cunningham et al. 1993). The stegocephalian fauna includes temnospondyls (Trimerorhachoidea, Dissorophoidea, the second-most frequently represented vertebrate taxon, and Eryopoidea), some of which are represented by larvae, as well as amniotes (Captorhinomorpha, Diapsida, Edaphosauridae, and Ophiacodontidae). Other vertebrates include the acanthodian *Acanthodes*, which is the most common vertebrate, Xenacanthida (*Expleuracanthus*?, *Orthacanthus*, and *Xenacanthus*), Hybodontoidea (*Hamiltonichthys*), Actinopterygii ('Elonichthys' and other), Dipnoi (*Gnathorhiza*, *Sagenodus*), Actinistia, and Tetrapodomorpha (Megalichthyinae). Eurypterids and numerous remains of "spirorbids" (actually microconchids) from Hamilton may represent euryhaline forms (see above), but brachiopods and echinoderms provide the best evidence of normal marine conditions in at least some levels (in the marine limestone and mudstone), which also yielded temnospondyls. Many of the vertebrates are articulated and fairly well-preserved, which suggests that they have not been transported far from where they lived, although others show signs of flotation. Furthermore, vertebrate coprolites include remains of marine metazoans (Cunningham et al. 1993: 230), which shows that they fed in a marine environment. Much of the local fauna appears to have been preserved in its environment; Cunningham et al. (1993: 227) stated that "Some productid brachiopods, for example, are preserved in apparent growth position, nested between clasts, with their delicate spines intact. Additionally, some colonies of an encrusting cystoporate bryozoan (*Fistulipora?*) apparently are in growth position". Other fossils from Hamilton, such as bryozoans, echinoderms, and fusulinids, are fragmented, abraded, and micritized and may be reworked from older sediments (Cunningham et al. 1993: 227). These could have been transported (not necessarily far), but they are easily distinguishable from the much better-preserved autochtonous elements. Schultze et al. (1994: 443) interpreted the temnospondyls as autochtonous and concluded that they "retained their tolerance to salinity from their marine ancestors and were able to spawn in near-shore environments."

The Hamilton locality displays three main lithologies: a conglomerate, an ostracode wackestone, and an assemblage of laminated limestones and mudstones. Most marine and brackish fossils mentioned above occur in all three lithologies, except for the echinoderms, which may occur only as lithoclasts of wackestone, in the laminated limestones and mudstones, which also includes the vertebrate fossils. Patterns in the lamination suggest that the limestones and mudstones were deposited in a tidal environment, and Cunningham et al. (1993: 235) suggested that high sedimentation rate and variable salinity may have enhanced fossilization. The deposits probably formed in a lagoon or estuary and some tidal creeks. Cunningham et al. (1993: 234) concluded that despite the fact that previous studies had "emphasized the freshwater nature of the laminated limestones from the Main Quarry based on some faunal elements, systematic sieving of the fossil-bearing beds has revealed the presence of marine invertebrate fossils throughout the sequence" and suggested that "The mixed composition of the fossil assemblage is interpreted as indicative of brackish or, most probably, variable paleosalinity." Thus, the vertebrates that inhabited this locality were plausibly euryhaline, coastal forms. Hamilton is among the most marine-influenced Permo-Carboniferous localities (all of which yielded stegocephalians) compared by Schultze and Maples (1992).

## Mazon Creek

Much has been written about the biota and paleoenvironment of this locality, including in various sections (above) of the present paper. Here, only a few additional points need to be added. Above, I indicated that many taxa found in Mazon Creek may have been euryhaline, but exceptions exist. Beyond the obvious case of allochtonous (mostly terrestrial) taxa, a fairly diverse cephalopod assemblage is documented (Saunders and Richardson 1979), and this even
1448 includes coleoids (Doguzhaeva et al. 2007), which have a poor fossil record. As mentioned
1449 above, the vast majority of cephalopods are stenohaline, marine forms, and only a few taxa, like
1450 the extant squid *Lolliguncula brevis* are moderately euryhaline (Bartol et al. 2002). Shark egg
1451 capsules are also present, and this also suggests strong marine influence because extant
1452 chondrichthyans lay eggs only in the marine environment, including along the coasts, notably
1453 in estuaries, but never in freshwater (Schultze and Soler-Gijón 2004: 326; Schultze 2009: 127).
1454 Holothurians, which also occur (Baird et al., 1986), are another typically marine taxon, like
1455 most other echinoderms.

1456 Thus, Mazon Creek undoubtedly exhibits a stronger degree of marine influence than most other
classical Permo-Carboniferous stegocephalian-bearing localities. This is paradoxical because a
1457 detailed study of tidal rhythmites of various Carboniferous and Holocene localities led Archer
1458 et al. (1995: 411) to conclude that “the Francis Creek rhythmites [in the area of Mazon Creek]
1459 may have formed in a **significantly inland** setting with a **strong fluvial influence** and
1460 overprinting on the tidal cycles.” Thus, the sedimentological data point at a brackish-water
1461 estuarine environment where stenohaline marine forms could not have lived. The cephalopods
1462 documented from Mazon Creek may have ventured there at high tide when salinity was the
1463 highest and may have made only brief incursions there.
1464

1465 Other than the cephalopods, the obviously allochtonous elements include the remains of
1466 terrestrial embryophytes and of terrestrial arthropods, such as myriapods, arachnids (including
1467 scorpions), and insects (Clements et al. 2019: 5). The possibility that some bivalves and
1468 “stegocephalians” were washed-in freshwater faunal elements was considered briefly by
1469 Clements et al. (2019: 5) based on the previously assumed habitats of these taxa, but these
1470 authors admitted that the occurrence of these bivalves with “polychaetes” raised serious doubts
1471 about this interpretation, and that the habitat of the stegocephalians was equally poorly
1472 constrained. The exceptionally good preservation of these stegocephalian remains, with dermal
1473 scales (e.g., Mann et al. 2021), skin and toepads (Mann and Gee 2020) suggests that they have
1474 not been transported far, without guaranteeing that they were truly autochtonous.

1475 *Red Tanks Member of the Bursum Formation*

1476 Although not among the classical stegocephalian-bearing Permo-Carboniferous localities, the
1477 Red Tanks Member of the Bursum Formation recently yielded a fairly diversified assemblage
1478 of late Pennsylvanian stegocephalians (the temnospondyls *Eryops*, *Trimerorhachis* and an
1479 undetermined taxon, the embolomere *Archeria*, a diadectid, a caseid, *Edaphosaurus* and
1480 *Dimetrodon* cf. *D. milleri*) and other vertebrates (the chondrichtyans *Petalodus* and *Deltodus*,
1481 undetermined actinopterygians, as well as the dipnoan *Gnathorhiza bothrotreta*) from “mixed
1482 marine-nonmarine sequences” (Harris et al. 2004: 267). The presence of typically marine levels
1483 in limestone layers of this member is demonstrated by the presence of conodonts and fusulinids,
1484 among others. In addition to this limestone, the Red Tanks Member includes “nonmarine
1485 mudstones and siltstones of a coastal plain environment”, “mudstones of a brackish to
1486 freshwater environment”, “nearshore and fluvial sandstones, limestone conglomerates of a
1487 high-energy nearshore environment, limestone horizons composed of different types of
1488 bioclastic wackestones and mudstones, and rare packstones indicating deposition in an open to
1489 restricted shallow marine shelf environment” (Harris et al. 2004: 267–268). Thus, this member
1490 seems to preserve several paleoenvironments, most of which show some marine influence, as
1491 well as some freshwater environments. The stegocephalians and other vertebrates were found
1492 in several lithologies, mostly in mudstones, siltstones, sandstones and conglomerates, but also,
1493 more rarely in shales and, in the limestone levels, as in localities 4640 and 5349 (Harris et al.
1494 2004: fig. 2, appendix). Thus, the vertebrate community preserved in the Red Tanks Member
1495 plausibly inhabited a variety of habitats, ranging from freshwater to brackish, possibly even
1496 marine. However, given their rather fragmentary nature, some or all of these might represent
1497 allochtonous elements.

1498 *Montceau-les-Mines*

1499 Montceau-les-Mines has been considered by many authors to represent an intramontane,
1500 freshwater basin (e.g., Perrier and Charbonnier 2014). Thus, Racheboeuf et al. (2008: 12)
1501 indicated that the Montceau biota "is a mixture of both strictly aquatic, **freshwater** faunal
1502 components and terrestrial ones". Also, they argued that "the closest Upper Carboniferous
1503 marine deposits were located at least several hundred kilometres SW of Montceau". However,
1504 this argument rests on previous paleoenvironmental interpretations that allowed reconstructing
1505 ancient coastlines, which is weak given that numerous changes in interpretation over the years,
1506 such as those concerning the Old Red Sandstone. This paleogeographic argument was criticized
1507 long ago by Schultze (1995: 258) who pointed out that "This method stacks one interpretation
1508 on another (the palaeogeographic position on an assumed distinction of marine from freshwater,
1509 the questionned palaeoenvironment)". Furthermore, Schultze (2009: 130) pointed out that many
1510 taxa (conchostracans, ostracodes, the bivalve *Anthraconaia*, eurypterids, *Acanthodes*,
1511 chondrichthyans, and actinopterygians) interpreted as freshwater indicators in Montceau also
1512 occur in the clearly strongly marine-influenced environment of Mazon Creek. Schultze (2009:
1513 133) went further and stated that "The lack of undoubtedly freshwater forms in the Paleozoic
1514 is a specific problem." This may be slightly overstated because aquatic insect larvae, though
1515 uncommon, do suggest freshwater or low salinity, if autochtonous, despite the few exceptions
1516 consisting in extant insect larvae that can develop in hypersaline water (Pallares et al. 2015).

1517 The isotopic analyses by Fischer et al. (2013) and Luccisano et al. (2023) suggested that some
1518 localities in the area (Buxières-les-Mines, and the Muse) represent continental environments
1519 with little or no marine influence (see section "Freshwater chondrichthyans"?), but these studies
1520 did not sample Montceau, and I am unaware of relevant isotopic analyses on that locality.
1521 Montceau is about 40 km South of the Muse, and about 110 km East of Buxières-les-Mines, so
1522 Montceau would have been a little closer to the Paleotethys than Buxières and the Muse
1523 (Schultze 2009; Mercuzot et al. 2022), and this is consistent with the presence of taxa that
1524 suggest marine influence and that have not been described from Buxières and the Muse. These
1525 include acritarchs, the annelid *Palaeocampa anthrax* (Pleijel et al. 2004), which belongs to the
1526 marine taxon Amphinomida, and abundant remains of the xiphosuran *Liomesaspis* (Anderson,
1527 1997; Racheboeuf et al. 2002). As mentioned above (section on xiphosurans), even the most
1528 euryhalie extant xiphosuran does not venture more than about 100 km from the seas. All of
1529 these were dismissed by Racheboeuf et al. (2008: 13) as indicators of marine influence, but
1530 accepted as such by Pleijel et al. (2004).

1531 Another possible indicator of marine influence is *Myxineidus gononorum*, based on a fossil that
1532 was initially described as a hagfish, even though a more recent study raised doubts about its
1533 identity and suggested that it might be a lamprey (Germain et al. 2014). Hagfishes are purely
1534 marine, but lampreys inhabit both the seas and freshwater. The fossil displays two rows of
denticles forming a chevron pattern, which is characteristics of hagfishes and what appears to
1535 be impression of a wrinkled skin, which is reminiscent of a dead hagfish, but a faint halo around
1536 the head looks like the peculiar mouth of lampreys (though it appears to be devoid of the
1537 keratinous denticles present on the mouth of extant lampreys). However, Germain et al. (2014:
1538 133) wondered if "this pattern [reminiscent of a lamprey's mouth] somewhat reflects the
1539 original body shape of the animal, or is merely and incidental artefact due to the distribution of
1540 microbially induced films during the decay of the animal", and they also admitted that "Quite
1541
1542 a similar halo is sometimes observed in other fossiliferous nodules from Montceau-les-Mines, not necessarily associated with soft tissue preservations." In the end, their suggestion that the fossil represented a lamprey seemed to depend as much on the presumed freshwater habitat of Montceau as on its morphology, because they stated that their new interpretation "would perhaps resolve the controversial problem of the presence of a hagfish in the reputedly freshwater, intramontane basin of Montceau-les-Mines". Indeed, Germain et al. (2014: 134) stated that the Montceau basin "provides no clear evidence for any marine influence, be it temporarily." Given the evidence mentioned above that suggest marine influence in that basin, especially the annelid *Palaeocampa* and the xiphosuran *Liomesaspis*, it seems better to base a taxonomic assignment of *Myxineidus* on its morphology, which unfortunately leaves serious doubts about its affinities.

1553 Stegocphalians are represented by temnospondyls (*Branchiosaurus petrolei*, *Micromelerpeton boyi*, and fragments of *Actinodon*), nectrideans (*Sauravus costei*, *S. spinosus*, and *Montcellia longicaudata*), the aistopod *Phlegethontia longissima*, and fragments of a synapsid (*Stereorachis? blanziacensis*); none of these are very well-preserved, even though several specimens preserve traces of soft tissues (Perrier and Charbonnier 2014). It is thus unclear if this fauna was autochtonous; significant transport cannot be excluded.

1559 *German basins*

1560 In my previous review (Laurin and Soler-Gijón 2010), I presented various arguments supporting marine influence in the Saar-Nahe basin. These obviate the need for a detailed treatment of this basin, but a few additional points can be made. The presence of glauconite in that basin also supports marine influence (Schultze 2009: 131) because it normally forms in the marine environment (Baldermann et al. 2013). In addition, microconchids are present in various horizons in the basin (Schultze 2009: 131 and references cited therein), and as indicated above (section "Freshwater microconchids"?), these indicate marine influence.

1567 In the central German basins, which extend from the Thüringer Wald Permo-Carboniferous basins to the Döhlen Basin farther east, Schultze (2009: 132) reported the presence of chondrichthyan egg capsules. As indicated above (section on Mazon Creek), these suggest fairly strong marine influence. In addition, xiphosurans (the limulid *Euproops*) occur there, in several levels, and they can be locally abundant given that Schultka (2000) described 45 individuals from at least four levels of a single locality. As explained in the section "Xiphosuran habitat through time", this suggests at the very least proximity to the sea. In north-east Germany, the Rotliegend has yielded "hydromedusas" (Legler et al. 2004), which also suggests marine influence (Schultze 2009: 132).

1576 It is impossible to assess the allochtonous or autochtonous nature of all stegocephalian remains found in all localities of the German basins, but the *Glanochthon latirostre* and *Archegosaurus decheni* found in a shark (*Triodus sessilis*) in the Lower Permian Lake Humberg, in the Saar-Nahe Basin, probably all lived in the same environment and there is no reason to infer significant transport (Kriwet et al. 2008).

1581 *Bohemia*

1582 The Bohemian basins in the Czech Republic were already discussed by Laurin and Soler-Gijón (2010), so only a few comments are needed here. These basins are best known for the famous Late Carboniferous (Westphalian D, Kasimovian) Nýřany locality (Schoch 2022: 14) in the Plzeň-Manětín Basin (Klembara et al. 2014). Nýřany yielded a great diversity of
1586 stegocephalians: temnospondyls, such as *Cochleosaurus bohemicus* (Sequeira 2003),
1587 *Cheliderpeton vranyi* (Werneburg and Steyer 2002), *Anthracobamus fritschi*, *Branchiosaurus*
1588 salamandroides, ‘Platyrhinops’ *fritschi*, *Mordex calliprepes*, *Mattauschia laticeps* (Schoch
1589 2022), the baphetid *Loxomma bohemicum* (Fritsch 1883), the enigmatic *Gephyrostegus*
1590 *bohemicus* (Godfrey and Reisz 1991), which has long been considered an anthracosaur,
1591 although some analyses suggest a more crownward position (Marjanović and Laurin 2019),
1592 amphibians that are still often referred to as “microsaurs” (a paraphyletic group), such as
1593 *Microbrachis pelikani* (Vallin and Laurin 2004), *Crinodon limnophyes*, *Ricnodon copei*,
1594 *Sparodus validus*, *Hylopleusion longicostatum* (Carroll and Gaskill 1978), as well as other
1595 amphibians, such as the nectrideans *Sauropleura scalaris* and *Urocordylus angustatus* (Fritsch
1596 1883), some amniotes, among others. Nýřany and other nearby Carboniferous strata have
1597 typically been interpreted as freshwater environments (Sequeira 2003: 21; Opluštil et al. 2005),
1598 but Laurin and Soler-Gijón (2010) presented an alternative interpretation. In addition to the
1599 presence of chondrichthyan egg capsules and microconchids already evoked by Laurin and
1600 Soler-Gijón (2010), the presence of limulid xiphosurans and of the eurypterid *Adelophthalmus*
1601 (Schultze 2009: 132) suggest marine influence in at least some parts of the basin and some
1602 horizons. Medusae have been described there; they have been interpreted as freshwater taxa
1603 (Kozur 1984), but see above (section “Freshwater jellyfish?”) for reasons to question this
1604 interpretation. The taxonomic diversity suggests that most stegocephalians found in that locality
1605 were probably amphibious or aquatic. The Nýřany assemblage has been interpreted as
1606 representing a brief, local accumulation, which seems plausible given the good preservation of
1607 most specimens (Sequeira 2003: 21). Ichnofossils also show that temnospondyls occurred
1608 locally (Turek 1989). All of this supports the hypothesis that most of these specimens were not
1609 carried far.

1610 **Conclusion**

1611 This literature survey illustrates a recurring theme that pervades the history of paleontological
1612 research on the Paleozoic paleoenvironments. The absence of typically marine indicators, such
1613 as coral reefs, echinoderms, and a diversified brachiopod fauna has been interpreted as
1614 indicating a "non-marine environment", which was often implicitly or explicitly assumed to be
1615 freshwater. However, "non-marine environments" thus defined (very broadly) include estuaries,
1616 deltas, coastal mangroves, lagoons and salt marshes, which occur between truly marine and
1617 freshwater environments on, or near the coast, as well as brackish or salt lakes, which occur
1618 even far from coasts. This seems to have been too often forgotten. Thus, the paleoenvironment
1619 of many Permo-Carboniferous localities that have yielded stegocephalians need to be
1620 reassessed, even in the comparatively well-studied Permian redbeds of Texas (Fig. 1).
1621 Freshwater ecosystems may well be very ancient; Gray (1988: 1) even boldly suggested that
1622 "The freshwater ecosystem may be as old as most life." That may well be, but various factors
1623 may have conspired to yield a very fragmentary picture of the history of freshwater ecosystems.
1624 The ichnofossil record, which is more abundant than body fossils for many taxa, suggests a
1625 marine origin of early life; Buatois et al. (2005: 322) stated that "Virtually all Precambrian
1626 ichnofaunas represent the activity of open-marine biotas that presumably inhabited nearshore
1627 to deep-marine areas under normal salinity conditions". Oceans and seas cover more of the
1628 Earth's surface than continents, and more importantly, most sediments are deposited by the
1629 largest, most powerful rivers, in deltas that are predominantly located at the edge of seas
1630 (including epicontinental seas) and oceans. In fact, Peters and Husson (2017: 323) concluded
1631 that "Most of the surviving volume of sedimentary rock (~75%) was deposited in and adjacent
1632 to shallow seas on continental crust". By contrast, non-marine sediments decrease in quantity
1633 with increasing age. This implies that in the Permo-Carboniferous, we expect far more
1634 fossiliferous localities (though not necessarily those that yielded stegocephalians) to represent
1635 marine and coastal (plausibly brackish) environments than freshwater ones. These factors, plus
1636 the higher erosion rates of high-altitude deposits compared to low-altitude sediments that we
1637 can expect from basic physical principles, may result in a low fossilization potential of
1638 freshwater organisms, especially ancient ones, except for those living close to the seas, where
1639 their remains may be carried after death.

1640 On the contrary, marginal-marine environments, where much sedimentation occurs, should be
1641 fairly well-represented in the fossil record, but they may be difficult to interpret because
1642 coastlines can vary quickly, especially in deltas, and even on a daily basis, tides result in short-
1643 term salinity variations in some coastal habitats. Thus, the exact environmental preferences and
1644 tolerances of long-extinct organisms that inhabited these coastal environments are difficult to
1645 assess. These organisms appear to have included many Paleozoic stegocephalians. Of course,
1646 this does not mean that all Paleozoic stegocephalians lived in brackish or normal-marine salt
1647 water. Like extant teleosts that occupy a great range of aquatic environments, many
1648 stegocephalians may have been adapted to freshwater habitats, and in some cases, independent
1649 evidence exists for this (e.g., Witzmann and Brainerd 2017).

1650 Above, I raised the question of a bias in favor of freshwater interpretation of localities devoid
1651 of typically marine fossils, and discussed some cases for which marginal-marine, brackish
1652 environments seem more plausible. While I focused on the body fossil record which I know
1653 best, I note that Buatois et al.'s (2005) review of the ichnological record suggests a similar
1654 pattern. Localities or formations that were initially interpreted as freshwater and now seem to
1655 more plausibly represent brackish environments include the Sequatchie Formation (Upper
1656 Ordovician) from Georgia and Tennessee (p. 325), the Kanawa Formation (Middle
1657 Pennsylvanian) from West Virginia (p. 327), the Permian Rio Bonito Formation from southern
1658 Brazil (p. 328), "many Cretaceous units" (p. 331), and the Ekalaka Member of the Paleocene
1659 Fort Union Formation in Montana (p. 334), just to mention the cases reported by Buatois et al.
1660 (2005). Furthermore, "in virtually every case, ichnological interpretations were supported fully
1661 by the companion microfossil studies" (Buatois et al. (2005: 331), as well as by neoichnologic
1662 studies (p. 337).

1663 This review mostly supports the preliminary conclusions that I presented more than a decade
1664 ago (Laurin and Soler-Gijón 2010). For most of the localities that I had discussed earlier, like
1665 Joggins, my suspicions of some marine influence have been strengthened by this new literature
1666 review. One notable exception is the habitat of some Famennian (Late Devonian)
1667 stegocephalians from Greenland. At that time, nothing suggested marine influence, so I had
1668 accepted the freshwater habitat that had been inferred in the literature, even though it appeared
1669 anomalous in an optimization of habitat use in early stegocephalians (Laurin and Soler-Gijón
1670 2010: fig. 4), in which *Ichthyostega* and *Acanthostega* displayed an acquisition of a freshwater
1671 lifestyle from an ancestor that was inferred to have lived in brackish water. The isotopic
1672 analyses performed by Goedert et al. (2018) show that there is no need to invoke such transitions
1673 to a freshwater habitat in these Devonian taxa, which appear to have inhabited brackish, coastal
1674 marine environments. A new optimization of lifestyle on a stegocephalian phylogeny (Fig. 4)
1675 shows the updated scores for *Ichthyostega* and *Acanthostega*. Laurin and Soler-Gijón (2010:
1676 table 2) had also considered Strud to be a freshwater locality, and while the recent literature still
1677 interprets it this way (Denayer et al., 2016), most levels in the Dinant Synclinorium are clearly
1678 marine (Thorez and Dreesen 1986), and even in the more continental strata (on the northern
1679 edge of the basin) that have yielded stegocephalian remains, the sea could not have been far.
1680 More research on the habitat of Devonian stegocephalians would be useful.

1681 The picture that emerges from all this is that early stegocephalian diversification seems to have
1682 occurred to a large extent close to coasts, including those of epicontinental seas, and to a lesser
1683 extent farther inland, and on land and possibly in freshwater. Is this pattern genuine, or does it
1684 reflect a taphonomic artefact that reflects the extent of sedimentation in deltas of the largests
1685 rivers on the coasts, along with erosion of sediments deposited farther from the coasts? If the
1686 latter is correct, a large evolutionary radiation of stegocephalians may have occurred in
1687 freshwater habitats but be poorly known because of taphonomic bias. Some localities, like
1688 Buxière-les-Mines, the Muse and Nýřany, may represent these freshwater localities, as
1689 suggested by the traditional interpretations. What was the salinity of the coastal environments
1690 in which stegocephalians diversified? As we saw above, the mere fact that tides occurred, as
1691 shown by tidal rhythmites, does not necessarily indicate brackish water because tidal effectes
1692 can propagate inland along rivers (Feldman et al. 1993: 495), but most localities where tidal
1693 rhythmites have been recognized yielded fossils of taxa of marine origin, which suggests
1694 brackish water.

1695 This survey may have raised more problems than it has solved, and unfortunately, time
1696 constraints prevented me from reassessing the paleoenvironment of the many Carboniferous
1697 stegocephalian-bearing localities and formations, such as the Garnett quarry, and of the more
1698 numerous Cisuralian localities. Hopefully, this review has shown that it is time to have a fresh
1699 look at the development of continental ecosystems from the Late Devonian through the
1700 Cisuralian.

1701 Acknowledgments

1702 I thank Amin Khaleghparast (Tehran, Iran) for sending me two images that he had contributed
1703 to create to use as figures, Jean Goedert (CR2P, Paris) for information about isotopic methods
1704 to assess paleosalinity, Gloria Arratia (U. of Kansas, Lawrence) for commenting on the short
1705 section of the draft on the habitat of early actinopterygians, and Simon Braddy (Manorbier,
1706 U.K.) for sending one of his papers on eurypterids and providing the e-mail address of a retired
1707 colleague. Two reviewers and the editor, Florian Witzmann, provided feedback that improved
1708 the paper. However, the opinions expressed in this paper and the errors that the text may contain
1709 are my sole responsibility.

1710

1711
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