# Revision of the Cretaceous rudist genus Sellaea (Bivalvia, Hippuritida) with new insights on American forms: Phylogenetic and biogeographic implications

Valentin Rineau, Jean-Pierre Masse

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Valentin Rineau, Jean-Pierre Masse. Revision of the Cretaceous rudist genus Sellaea (Bivalvia, Hippuritida) with new insights on American forms: Phylogenetic and biogeographic implications. Cretaceous Research, 2022, 135, pp.105188. 10.1016/j.cretres.2022.105188. hal-03817332

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1 Revision of the Cretaceous rudist genus *Sellaea* (Bivalvia, Hippuritida) with new

2 insights on American forms: phylogenetic and biogeographic implications

3 *<sup>a</sup>Valentin Rineau and <sup>b</sup>Jean-Pierre Masse

4 <sup>a</sup>Center for Theoretical Study, Charles University & Czech Academy of Sciences, Jilská 1, 110

5 00 Praha 1, Czech Republic. ORCID: 0000-0001-8929-5201.

6 <sup>b</sup>CEREGE, Aix-Marseille Université, Centre Saint-Charles, Marseille Cedex 03, France

7 *Corresponding author. E-mail: valentin.rineau@gmail.com
8 Abstract

9 *Sellaea* is a key genus for understanding the expansion of rudists in the middle
10 Cretaceous. Its incursion into the Americas in the Albian is open to taxonomic debate, as
11 several researchers have hypothesised that the American *Sellaea* belongs to a new genus. A
12 taxonomic revision with analysis of new material from the Edwards Formation (Texas, USA)
13 coupled with a cladistic analysis using three-taxon analysis allows us to redefine the genus
14 *Sellaea*. The phylogeny reveals two distinct endemic groups, one belonging to the
15 Mediterranean province, and one to the Caribbean province. However, the involved
16 synapomorphies, related to the left valve posterior cavity, the shell margin, and the outline,
17 are not sufficient to create a new genus. The character related to pallial canals is the only one
18 to be homoplastic in the phylogeny of *Sellaea*. The presence of canals is therefore a variable
19 character state within the genus. The genus is redefined accordingly, allowing the relocation
20 of several species originally assigned to *Caprotina*, including species that lack pallial canals
21 and species with unknown right valves. *Sellaea minuta* is the only species retained over the
22 four *Sellaea* described from the Americas. The redefined genus *Sellaea* is Albian-
23 Cenomanian in extent, with the incursion of a single species into North America in the
24 Albian.

Keywords: Rudist bivalves, Albian, Caribbean province, Comanche shelf, Texas,
25 Three-taxon analysis.
26
27

## 1. Introduction<fnref n="1" />

The Early Cretaceous was an important period of diversification of rudist bivalves, which colonised carbonate platforms of warm, shallow seas of the Tethys, Pacific and Atlantic Ocean (Masse, 1992a, 1992b; Sha et al., 2020). Their diversification dynamics resulted in the evolution of an exceptional number of new morphoanatomical structures and taxa (Steuber et al., 2016). *Sellaea* is an Albian rudist genus recently hypothesised as a sister group of an important clade of rudists gathering Radiolitidae, Hippuritidae, and all canaliculated rudists (Rineau et al., 2020). *Sellaea* is thus a taxon of crucial importance for understanding the timing of the radiation of rudists in the Early Cretaceous.

Most of the *Sellaea* species were described in Italy (Di Stefano, 1889, 1898) at the end of the XIXth century, and have been subsequently discovered in Egypt (Steuber and Bachmann, 2002), Oman (Skelton et al., 2000) and Tibet (Rao et al., 2015). Davis (1980) made the first and only attempt to describe *Sellaea* forms in America. In America, *Sellaea* were mainly found in the Edwards Formation (Texas) of age middle to lower upper Albian, in the Comanche shelf. The Barremian-Cenomanian Comanche shelf was a mixed carbonate siliciclastic province. Rudists were widespread and abundant carbonate producers throughout the shelf from Florida to Texas (Scott et al., 2016). Comanchean rudists, relatively well known (Coogan, 1977; Davis-Strickland, 1988; Mitchell, 2013; Scott and Filkorn, 2007),

<footnote n="1">¹ Abbreviations of collections: PU, Museo Geologico di Torino (Turin, Italy); USNM, National Museum of Natural History, Smithsonian Institution (Washington DC, USA); UT, University of Texas, Jackson School Museum of Earth History, Non-vertebrate Paleontology Collections (Austin, USA). Abbreviations of morphoanatomical features: AAC-LV, anterior accessory (with variable or unknown function) cavity of the left valve; AM, anterior myophore; AT, anterior tooth; ATS, anterior tooth socket; BC, body cavity; CT, central tooth; CTS, central tooth socket; LV, left valve; PAC-LV, posterior accessory cavity of the left valve; PENC, posterior endomyophoral cavity; PMC, posterior myophoral (cavity in which always lies a myophore) cavity of the right valve; PM, posterior myophore; PT, posterior tooth; PTS, posterior tooth socket; RV, right valve. Abbreviations correspond to homologies formalised in Rineau et al. 2020.</footnote>
formed bioherms on the inner shelf and were abundant in reef communities (Scott, 1990).
46

They are recorded from the Hauterivian to the Albian (Coogan, 1977), and a stratigraphic
47

zonal pattern was developed by Scott and Filkorn (2007).
48

American *Sellaea* are only found in the middle to lower upper Albian, in the Edwards
49
50
Formation, where the Comanche shelf was a wide shallow carbonate area. They are easily
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recognised by their external morphology (a coiled left valve and a cylindro-conical uncoiled
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right valve) and their cavities (three large cavities on the left valve and two cavities plus
53
anterior pallial canals on the right valve) (Coogan, 1973, 1977; Dechaseaux et al., 1969). In
54
1980, Davis was the first to describe *Sellaea* species in America and erected four species from
55
Texas, *S. elongata* Davis, 1980, *S. globosa* Davis, 1980, *S. minuta* Davis, 1980, and *S. ziczac*.
56
However, the characters used to delineate the species are doubtful, e.g., borings taken as
57
pallial canals (Skelton and Masse, 1998), a decisive character for the placement of these shells
58
in *Sellaea* following di Stefano's definition of the genus. No author has subsequently taken up
59
the work of Davis, except a possible occurrence of '*Sellaea* sp. cf. *elongata*' (Alencáster,
60
1987) in Mexico. Moreover, subsequent authors have raised questions about these species,
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especially about whether or not they belong to the genus *Sellaea* (Skelton and Masse, 1998).
62
This would have particularly important consequences for rudist biogeography in general, to
63
understand in detail the relationships between new world and old world biogeographic areas
64
(Masse et al., 2007).
65

The purpose of this article is therefore to revise the American *Sellaea* based on
65
comparisons with the Italian type material of di Stefano (1889). The revision proposed herein
66
is mainly based on the description of new material from the Albian Edwards held in the
67
National Museum of Natural History, Smithsonian Institution and University of Texas
68
collections. Moreover, the taxonomic revision of American *Sellaea* is also an opportunity to
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analyse the relationship of all *Sellaea* with *'Caprotina'* sensu Di Stefano (op.cit.) (see
70
71 discussion in Skelton, 2013a). A phylogenetic analysis of *Sellaea* is produced for all *Sellaea*
72 plus affiliated *Caprotina* species. The phylogeny enables discussion of their relationships with
73 Italian forms and amending the diagnosis of the genus *Sellaea* Di Stefano (1889) accordingly.
74 This taxonomic revision and phylogenetic analysis finally provide a better understanding of
75 the biogeography of rudist bivalves in the middle Cretaceous.

76 **2. Historical background**

77 In 1889, Di Stefano described the fauna of Rupe del Castillo (Temiri Imerese, Sicily),
78 composed of numerous rudists. He erected numerous species, and defined two subgenera
79 within *Caprotina* to account for the presence or not of pallial canals in the RV: the subgenus
80 *Caprotina* 'sensu stricto', which includes species without pallial canals, and the subgenus
81 *Sellaea*, which includes species with anterior pallial canals near the anterior myophore of the
82 RV. All of these forms were considered as *Caprotina* by Di Stefano because of a prominent
83 posterior myophoral plate, cavities in the LV, and a myophoral cavity in the RV where is
84 inserted the posterior myophoral plate of the LV. However, none of the *Caprotina* described
85 is comparable with the type species of the genus *Caprotina striata* d'Orbigny, 1840 (Skelton,
86 2013a). Di Stefano assigned a Cenomanian age to these species. This age is now considered
87 doubtful because they have been likely described from slope deposits of rocks of different
88 ages (Rao et al., 2015). Finally, the fauna described by Di Stefano may have been found in-
89 situ in carbonate platforms in Cefalù (Camoin, 1983). In a second work, Di Stefano (1898)
90 redescribed several of these species and raised *Sellaea* at the generic rank. He postulated that
91 *Sellaea* is a transitional form between *Caprotina* and *Caprina*. The former subgenus
92 'Caprotina sensu stricto' contained originally *Caprotina stryx* Di Stefano, 1889. The
93 subgenus has never been amended, and *Caprotina* species described by Di Stefano (1889) and
94 are considered today by palaeontologists as species with doubtful generic attribution and are
cited using quotation marks (e.g., "'Caprotina' sensu di Stefano" in Skelton, 2013a). Nine species were originally allocated to *Sellaea: Sellaea cespitosa* (Di Stefano, 1889), *Sellaea ciofaloi* (Di Stefano, 1889), *Sellaea himerensis* (Di Stefano, 1889), *Sellaea laticoncha* (Di Stefano, 1889), *Sellaea orbignyi* (Di Stefano, 1889), *Sellaea pironae* (Di Stefano, 1889), *Sellaea plagioptychoides* (Di Stefano, 1889), *Sellaea sicula* (Di Stefano, 1889), and *Sellaea zitteli* (Di Stefano, 1889). Kutassy (1934) subsequently designated *S. zitteli* as the type species of the genus *Sellaea*. It is worth noting that species from Monte d'Ocre subsequently described by Parona (1909) and placed in the *Caprotina* (*C. distefanoi* Parona, 1909, *C. aprutina*, Parona, 1909 and also *C. roemeri* Gemmellaro 1865 redescribed in Parona's work) are highly similar to *Sellaea* but placed into *Caprotina* because of their unknown LV (i.e. unknown presence of pallial canals).

The first mention of *Sellaea* on the American continent (see Davis, 1980 for older mentions in abstracts or reports) was in the Treatise of Paleontology (Dechaseaux et al., 1969), where *Sellaea* was described as occurring in Sicily, Apennines, USA (Texas) and Mexico. The genus *Sellaea* is here characterised by two conical valves, a coiled LV and a straight RV, with three large and deep cavities in the LV, two on the posterior side and one on the anterior side, the AM-RV on an extension of the cardinal platform and separated from the shell wall by cavities or canals, and a PM-RV plate extending from tooth to shell wall.

The internal and morphological characters of the so-called 'Caprotina' sensu Coogan (1973) show that this form belongs to *Sellaea*. Two *Sellaea* sp. 'A' and 'B' are subsequently mentioned from the Albian Edwards by Coogan (1977). They are described solely based on RVs. He compared their morphoanatomy with the Di Stefano species. *Sellaea* is defined as identical to *Caprotina*, but with anterior canals on the RV. The first *Sellaea* sp. is close to *S. himerensis* in size, shape and internal arrangement of teeth and myophores, but differs in the pattern of pallial canals. Four subquandrangular canals and one elongated canal are present in
120 *Sellaea* sp. 'A', whereas *S. himerensis* possess seven subquadrangular canals and two smaller
121 ones in dorsal position. *Sellaea* sp. 'B' then is represented with multiple small canals in the
122 dorsal anterior margin, similarly to *S. plagioptychoides*. Coogan (1977) also proposed that a
123 species described from the Edwards Formation and assigned to *Plagioptychus* (Roemer, 1889)
124 might be potentially a *Sellaea*, but the information attached to this species, *Sellaea? cordata*
125 (Roemer, 1889), is far too fragmentary to assign it to a particular genus.

126 Davis (1980) proposed four new species in Texas following her M.Sc thesis (Davis,
127 1976): *Sellaea minuta*, *Sellaea ziczac*, *Sellaea globosa* and *Sellaea elongata*, based on
128 internal moulds or well-preserved specimens with transverse sections. *S. minuta* is small and
129 has thin calcitic and aragonitic layers, *S. ziczac* is distinguished by a fluted dorsal side, *S.
130 globosa* has rounded ventral canals and cavities, and *S. elongata* has a thin LV and a less
131 twisted RV than *Sellaea globosa* Di Stefano, 1889. They are typified by their relative valve
132 size, overall size, ornamentation, ligament position and cavities. The four species are typical
133 of the Albian Edwards (Fig. 1). Subsequently, only one mention of *Sellaea* sp. cf. *elongata* in
134 Mexico (Alencáster, 1987) attempted to follow the taxonomic framework established by
135 Davis for the American *Sellaea* with a putative identification at the specific level. Scott et al.
136 (2016) only refer to *Sellaea* sp. in the Albian Edwards of Texas.

137 Subsequently, Skelton and Masse (1998) questioned the assignment of the forms
138 described by Davis to the genus *Sellaea*. They hypothesised that no true pallial canals outside
139 the AM-RV are present on these shells. The state of preservation of Davis's specimens is
140 poor, and the pallial canals described are remarkably rounded and more likely to be borings.
141 In addition, the authors point out a feature that seems to further distance these species from
142 the genus *Sellaea*, namely that the posterior cavity of the LV appears systematically
143 undivided. Skelton and Masse, therefore, hypothesised that the American *Sellaea* specimens
144 might be assigned to a new genus, pending analysis of new material. However, they also
145

argued that in *Pachytraga*, the presence or absence of pallial canals outside the RV-AM is
related to intraspecific variation and that the same might also be true for *Sellaea*. These
thoughts highlight the importance of understanding the phylogenetic relationships among
*Sellaea* species to test if the diagnosis of the genus is correct or not.

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**3. Material and methods**

150

**3.1. Material studied**

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The present revision is based on both Italian and American specimens. The material
from the Museo Geologico di Torino (PU) refers to Italian material (Fig. 2A-F). For
American specimens, we focus on fossils from the National Museum of Natural History,
Smithsonian Institution (USNM) and University of Texas (UT) collections (Figs. 3-5) and on
a literature survey. Table 1 lists status, collection number and location of specimens studied
herein. Of a total of 20 specimens, only 13 have provenance information. All American
*Sellaea* with known provenance comes from the Albian Edwards in Texas near Waco (Fig. 1),
of middle to lower upper Albian age. The Edwards Formation was deposited during
transgressions on the expansive, flat and shallow Comanche platform where bioherms
developed (Scott, 1990). Rudists are numerous and diversified in this area, where they form
elongated bodies composed of caprinids, monopleurids and chondrodonts, and patch reefs
composed of radiolitids, toucasids and chondrodonts (Janson et al., 2015).

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Of the specimens studied from the UT, one is labelled as the holotype of *S. ziczac*
(Fig. 2G-H). It appears to be the only holotype that we were able to find from the four Davis'
species (1980), even if its status is doubtful – as it does not correspond to Davis’ original
description. The holotypes of *S. elongata*, *S. globosa*, and *S. minuta* are lost. We found four
other specimens from the UT, including a whole specimen sawed into several transverse
cross-sections and labelled *'Sellaea americana' Van de Flied* and designated as type on the
169 label. However, this specimen has never been described. Of the four specimens studied, one is
170 from Childress Creek, one from Benton Creek, and two from Coryell County. All are whole
171 specimens with transverse or longitudinal cross-sections. Among the USNM specimens 14
172 are presented herein (Fig. 3-5, Table 1). All specimens that have locality data are from Bluff
173 Creek or Lake Whitney. Of these specimens, some show polished cross-sections, some are
174 complete with their two valves connected, and some are isolated valves that allow us to
175 observe and compare in detail their internal anatomy, especially myophores and cardinal
176 apparatus (Fig. 3B, C, E, F, H). Of these specimens, one is labelled 'Caprotina pratheri
177 Stanton. Cretaceous - Edwards. Bluff Creek, 5-mi. Crawford. Cotypes', which is also an
178 unpublished name.

179 3.2. *Cladistic analysis*

180 The newly described American shells of *Sellaea* provide remarkable access to the
181 anatomy of the myocardial apparatus and the arrangement of the different cavities. These
182 specimens allow us to carry out a phylogeny of all the *Sellaea* species to test the insertion of
183 the American *Sellaea*. Two concurrent phylogenetic hypotheses are possible: the hypothesis
184 that two distinct groups correspond to two biogeographic provinces or that a single group is
185 composed of American and Italian *Sellaea*. Moreover, we tested the monophyly of the genus
186 *Sellaea* to discuss the pertinence of using RV pallial canals to define the genus. Finally, we
187 evaluated the phylogenetic position of the species (i) assigned to *'Caprotina'* sensu Di
188 Stefano (1889) because of a lack of canals and (ii) assigned to *Caprotina* by Parona (1909)
189 because of unknown RVs. The analysis allows us to clarify the evolution of canaliculated
190 rudists in the Early Cretaceous and the Caribbean and Mediterranean provincialism. The
191 formalisation of rudist homologies undertaken in Rineau et al. (2020) is a first guide to
192 proceed to the comparative anatomy of *Sellaea* shells to produce relevant characters for
193 cladistic analysis.
We undertook the phylogenetic analysis of a total of 14 species, including the 11
*Sellaea* species described by Di Stefano (1889), one species affiliated to *Sellaea* but assigned
to *Caprotina* by Di Stefano (1889) because of an absence of pallial canals (*Caprotina* stryx Di
Stefano, 1889), three *Caprotina* species affiliated to *Sellaea* with unknown RV (*Caprotina
roemeri* Gemmellaro, 1865, *Caprotina aprutina* Parona, 1909, *Caprotina distefanoi* Parona,
1909), and the American species redescribed herein. *S.? cordata* is not included because of its
doubtful supra-specific attribution and its high number of missing data. *Caprotina
quadripartita* (d'Orbigny, 1842) is probably a *Sellaea* species but is not included because of
its high number of missing data. We sought to code the maximum number of relevant
characters using all parts of the bivalve shell: cavities and pallial canals (four characters),
myophores (two characters), and shell shape and outline (two characters) (Table 2). We did
not state a-priori what should be the most informative characters. The phylogenetic analysis
was carried out using the cladistic method of three-taxon analysis (Nelson and Platnick, 1991)
which allows the construction of hierarchical characters (Cao et al., 2007) (SM 1). The
characters are decomposed into minimal trees (i.e., minimal hypotheses of degree of kinship
relationship) and the optimal cladogram retained is the one that maximises the number of
hypotheses by congruence maximisation (Hennig, 1966). The minimal assertions are
weighted following the method of Rineau et al. (2021) to improve the accuracy of the
analysis. We used Lisbeth 1.0 (Zaragüeta i Bagils et al., 2012) to build a matrix of minimal
statements, and PAUP 4.0b10 (Swofford, 2003) for the analysis of the matrix in branch and
bound. The testing procedure for the character states and their placement onto the phylogeny
are made following Rineau (2017).
216 **4. Comparative anatomy**

217 **4.1. Shape**

218 Although Davis (1980) used valve size to discriminate its four species, only the
219 holotypes had their dimensions recorded, precluding the use of statistical analyses. Moreover,
220 these external features used by Davis are generally described as dependent on the
221 environment and the age of the organism. This can be expected or supposed given the
222 morphological plasticity of the valves controlled by environmental parameters, arguing for
223 intraspecific variation in shell dimensions, shape, and coiling.

224 However, variations in the shell’s external features can be fruitfully compared to the
225 Italian material. Firstly, the Italian shells show a pronounced ventral flexure (‘sulcus’ in Rao
226 et al., 2015) on both valves (e.g., Fig. 2A, D, see also all *Sellaea* illustrated in Di Stefano
227 1889). In all Mediterranean and Asian specimens for which the ventral outline is visible
228 (Italy, Egypt, Tibet), the ventral flexure is present and slightly distorts the rounded outline,
229 which can give a subtriangular outline in some specimens (Parona, 1909; Rao et al., 2015; Di
230 Stefano, 1889; Steuber and Bachmann, 2002). However, in all the American specimens
231 studied here, the shell outline is always circular to oval, with no trace of ventral flexure (Figs.
232 3-5). Furthermore, Italian forms show a strongly developed and flared shell margin on the
233 LV (e.g., Fig. 2D) (Parona, 1909), giving a massive aspect of the shell. In American forms,
234 the margin is always narrow (e.g., Fig 3B). The absence of the ventral flexure and a strongly
235 developed calcitic layer in American *Sellaea* are two independent characters that favour the
236 hypothesis of two biogeographically distinct groups.

237 **4.2. Cavities, plates, pallial canals**

238 All *Sellaea* have a characteristic arrangement of cavities. They possess three main
239 cavities within their LV: the BC, the PAC-LV (divided or not), and the AAC-LV (e.g., Fig.
240

2A-E). The AM-LV lies in the AAC-LV in some species, and the PAC-LV is always empty of myophores. In the RV, the two main cavities are the BC and the PMC in which always lies the PM-RV (e.g., Fig. 3A-F, H). The presence of a deep AAC-LV is important as it allows the genus to be distinguished from *Caprotina* and *Chaperia*, and the absence of septa in it distinguishes the genus from all other canaliculated rudists.

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On the AAC-LV/BC wall, several Italian species possess an AM-LV in the form of a small longitudinal crest (*S. ciofaloi*, *S. laticoncha*, *S. plagioptychoides*) (see pl. 22, Fig. 11 in Parona, 1909). This feature is possibly also present in one American RV (Fig. 5I-L). Another qualitative variation concerns the CTS, which can develop dorsally and form a diverticulum stranded between the PT and AT in some species, as in *S. cespitosa*, *S. himerensis*, *S. zitteli*.

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*Caprotina quadripartita* and *Pachytraga davisarum* also possess all the *Sellaea* cavities (Skelton and Masse, 1998; Rao et al., 2015). *P. davisarum* was not assigned to *Sellaea* by Davis (1980) because of an undivided PAC-LV and contrary to the divided cavity of Italian specimens. However, this is also the case for all American forms reviewed here. Skelton and Masse (1998) hypothesised that the *Sellaea* specimens described by Davis (1980) belong to a new genus because of the absence of pallial canals on the RV and a systematically undivided PAC-LV. Our observations support the hypothesis that all American specimens have a deep PAC-LV without any division. However, only *S. himerensis* and *S. cespitosa* possess an undivided PAC-LV. In most of the Italian, Egyptian (Steuber and Bachmann, 2002), and Tibetan species (Rao et al., 2015), the PAC-LV is divided by a single septum (or even two, *e.g.*, *S. ciofaloi*). We note that the number of septa in the PAC-LV is also a variable feature inside the genus *Caprotina*.

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Skelton and Masse stated that ‘no true pallial canals are evident outside the anterior myophore of the RV in her figures’ (1998: p. 340). The pictures of *S. globosa* do indeed seem to refer to borings by their arrangement and their very rounded and spaced morphology.

263

264
Moreover, these ‘canals’ are present not only on the anterior side of the RV but also on the posterior side, which has never been described in *Sellaea*. Borings also appear to affect ‘*Sellaea* sp. B’ in Coogan (1977: pl. 3, fig. 6), while true canals appear to be present in ‘*Sellaea* sp. A’ (pl. 3, fig. 1b in Coogan, 1977). For *S. elongata* and *S. minuta*, pallial canals are visible on the RV. A close examination of the holotype of *S. ziczac* (Fig. 2G-H) does not reveal whether the specimen had pallial canals or not. However, the study of American *Sellaea* shells from UT and USNM reveals that the anterior pallial canals on the RV are almost always found in American specimens (Figs. 3-5). Finally, a group of shells from the USNM (Fig. 3A, C, E, F, H, I) has very shallow canals (Fig. 3E, F), or even no canals, with only a slight furrow (see AM in Fig. 3C). The high similarity of all these shells supports the hypothesis of a single species with variability in the presence and depth of anterior canals. Intraspecific variability of pallial canals is also present in *Pachytraga* species (Skelton and Masse, 1998). Variability of pallial canals does not seems to be related to the ontogeny, as the presence of canals or not is not related to the size of the shell.

### 4.3. *Myophores*

The rudists are known for the wide variety of their myophores and their mosaic evolution (Chartrousse, 1998; Rineau et al., 2020). Significant variability also exists in myophoral arrangement within *Sellaea*, and evolution between the anterior and posterior myophores is uncoupled. The AM varies in width, inclination, and orientation of the myophoral attachment surface (Fig. 6A-C). By contrast, the PM in *Sellaea* is quite constant (Fig. 6C) and is very similar to *Caprotina's* PM. The PM-LV is inserted on a plate protruding into the PMC; the PM-RV is vertical, located on the PMC/BC wall.

Variations of the AM-LV and AM-RV in *Sellaea* result in three types of myophoral arrangements. (i) The first (Fig. 6A) is classical and consists of a vertical insertion of the AM-
LV on the AAC-LV/BC wall, whereas the AM-RV is horizontal and does not bind into a cavity, but instead lies on a thickening. This type of arrangement is found only in American forms in which this condition is visible (Fig. 5A-H). (ii) The second arrangement (Fig. 6B) is present in Italian *Sellaea*, with an AM-LV inserted on a thickening of the AAC-LV/BC wall. This thickening partially covers the AAC-LV and the AM-LV which was rotated more horizontally. This variation is present in *S. zittelli*, *S. himerensis* and *S. cespitosa*. The morphology of this myophoral shelf suspended from the AAC-LV/BC wall is highly variable both in terms of the shape of the shelf, the width (it can be narrow or so wide that it almost completely covers the AAC-LV; Figs. 2A and 2B respectively) and the tilting (from highly tilted to almost horizontal; Figs. 2D and 2A respectively). Finally, the variations in AM-LV also have an ontogenetic component. They concern mainly the orientation of the myophoral plane during growth, with a flattening towards the ventral margin. These observations also apply to *Himeraelites*, which has a similar myophoral plate associated with the AM-LV that also varies strongly. (iii) The third (Fig. 6C), a rarer arrangement is found only in *S. plagioptychoides*, *S. sicula*, and one American specimen (USNM.PAL.770973; Fig. 5I-L). In these forms, the AM-LV is vertically inserted into the AAC-LV, but the AM-RV is located on a myophoral lamina that protrudes directly into the AAC-LV.

## 4.4. Synthesis

We have seen that American *Sellaea* cannot be distinguished from Mediterranean *Sellaea* either by the number of septa inside the PAC-LV or by the presence of pallial canals. Morphological variations in the AM-LV are important for both American and Mediterranean *Sellaea*. The presence of an AM-LV shelf has never been shown in American *Sellaea* but is variable in Italian material. Moreover, the presence of a myophoral plate bearing the AM-RV is found in one American specimen and two Italian ones.
However, two features distinguish Italian and American species. The Tethyan *Sellaea* are distinguished by a ventral flexure, whereas all American specimens possess a rounded outline.

The American *Sellaea* possess a narrow margin, contrary to the flared margin of Italian specimens. All these elements lead us to conclude that sufficient morphological differences distinguish the American *Sellaea* from the Tethyan ones to classify them into separate species, but these differences are not sufficient to create a new genus.

## 5. Systematic palaeontology

The systematics here follow Carter et al. (2011) and Skelton (2013a, 2013b). Our revision focuses on the American *Sellaea*, and the Mediterranean forms are discussed in the remarks section of the genus to provide comparisons and to add justifications for the reallocated species.

Class: Bivalvia Linnaeus, 1758

Infraclass: Heteroconchia Gray, 1854

Order: Hippuritida Newell, 1965

Suborder: Radiolitoidea d'Orbigny, 1847

Superfamily: Caprinoidea d'Orbigny, 1850

Family: Caprinulidae Yanin, 1990

Genus *Sellaea* Di Stefano, 1889

Type species. *Sellaea zitteli* (Di Stefano, 1889) p. 28-30, pl. 8, figs. 1a, b, 2a, b, c, 3, by subsequent designation of Kutassy (1934).

Diagnosis. Rudist with coiled left valve and uncoiled conical right valve. On the left valve, presence of a posterior cavity and an undivided anterior cavity. Posterior cavity filled with
one or two septa in some species. The posterior myophore of the left valve is outwardly directed on a plate protruding into the posterior myophoral cavity on the right valve, facing the vertical posterior myophore of the right valve. On the right valve, canals can be present in a shell thickening located anteriorly from the anterior myophore.

Included species. *Sellaea aprutina* (Parona, 1909) nov. comb., *Sellaea cespitosa* (Di Stefano, 1889), *Sellaea ciofaloi* (Di Stefano, 1889), *Sellaea distefanoi* (Parona, 1909) nov. comb., *Sellaea himerensis* (Di Stefano, 1889), *Sellaea laticoncha* (Di Stefano, 1889), *Sellaea minuta* Davis, 1980, *Sellaea orbigny* (Di Stefano, 1889), *Sellaea pironae* (Di Stefano, 1889), *Sellaea plagioptychoides* (Di Stefano, 1889), *Sellaea quadripartita* (d'Orbigny, 1842) nov. comb, *Sellaea roemeri* (Gemmellaro, 1865) nov. comb., *Sellaea sicula* (Di Stefano, 1889), *Sellaea stryx* (Di Stefano, 1889) nov. comb., *Sellaea zitteli* (Di Stefano, 1889).

Remarks. *Sellaea* is Albian-Cenomanian (table 3). The genus was originally dated to Cenomanian in Sicily but the datation is doubtful. It was subsequently reported from the Albian in various localities (Chiocchini et al. 1994; Masse et al. 1997, 1998; Rao et al., 2015; Steuber and Bachmann, 2002). Occurences in Texas (Coogan, 1977; Dechaseaux, 1969; Scott et al., 2016), Mexico (?) (Alencáster, 1987), France (d'Orbigny, 1842), Italy (Chiocchini et al., 1994; Gemmellaro, 1865; Parona, 1898, 1909; Di Stefano, 1889, 1898), Tibet (Rao et al., 2015, 2017, 2020), Oman (Masse et al., 1997), Egypt (Steuber and Bachmann, 2002), and possibly from a Japanese guyot (Matthews et al., 1974). *S. quadripartita* described from the middle Cenomanian of Île Madame (Charentes, France) is a *Sellaea*, and not a *Caprotina*, as it possesses an AAC-LV (not present in *Caprotina*), extending the spatio-temporal range of *Sellaea* to Cenomanian of western France. *Sellaea* was assigned to the Caprinulidae by Skelton (2013a), without precise justification. According to the cladistic analysis of Rineau et al. (2020), *Sellaea* could be the sister group of a clade gathering Hippuritidae, Radiolitidae,
359

Caprinidae, Caprinuloideidae, Caprinulidae, Trechmanellidae, Antilocaprinidae, Ichthyosarcolitidae, pending cladistic analyses at higher taxonomic resolution.

360

361

The redefinition of the genus *Sellaea* puts an end to an uncomfortable taxonomic situation with species still assigned to the subgenus *Caprotina* but considered to be not related to *Caprotina* (Skelton, 2013a). As a result, recent authors cite the subgenus as *Caprotina* sensu stricto, with quotation marks (Rao et al., 2015; Skelton, 2013a; Skelton and Smith, 2000).

362

363

364

365

Following the present analyses, these forms, plus Parona's (1909) *Caprotina*, undoubtedly belong to the genus *Sellaea* despite absent or unobserved anterior pallial canals on the RV.

366

367

The cladistic analysis below (see section 6) therefore supports (i) the hypothesis of variations of pallial canals among *Sellaea* and (ii) the inclusion of *Caprotina* species of Di Stefano (1889) and Parona (1909) in *Sellaea*.

368

369

370

Mediterranean species. *Sellaea* species have a set of characters that distinguish them from the American *Sellaea*, but not to be separated into two genera. Mediterranean *Sellaea* have thick and flared margins. Their PAC-LV may be undivided or divided into two or more cavities.

371

372

373

Their AM-LVs have two typical morphologies: i) vertical on the AAC-LV/BC wall (Fig. 6A), or ii) on a myophoral thickening/plate at the top of the AAC-LV/BC wall (Fig. 6B). In the latter case, the myophoral thickening partially covers the anterior cavity, and the muscle insertion is oblique (fig. 2A). The shelf varies greatly, especially in its morphology and in the inclination of the myophore, which can range from sub-horizontal to sub-vertical. All have a biconvex ventral flexure on both valves which is mostly visible on the LV. These species include *S. aprutina*, *S. cespitosa*, *S. ciofaloi*, *S. distefanoi*, *S. himerensis*, *S. laticoncha*, *S. orbigny*, *S. pironae*, *S. plagioptychoides*, *S. quadripartita*, *S. roemeri*, *S. sicula*, *S. stryx*, *S. zitteli*. *S. sp.* from Sinaï (Steuber and Bachmann, 2002) and Tibet (Rao et al., 2015) also belong to this group due to the presence of a ventral flexure.

374

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376

377

378

379

380

381

382
383 American species. All *Sellaea* specimens found in America have features that distinguish them from Mediterranean *Sellaea*. American *Sellaea* have a narrow margin, the PAC-LV is never divided, and there is no ventral flexure. American specimens seem more gracile, with a thinner aragonitic layer, deeper cavities and no developed margin. Their AM-LV is always inserted vertically, without any thickening. All specimens of *Sellaea* reported from the USA belong to this group (Coogan, 1977; Davis, 1980; Dechaseaux, 1969). The specimen illustrated in Scott et al. (2016) is incomplete and cannot be determined more precisely.
384
385
386
387
388
389
390 American specimens are all known from the Albian Edwards, with a possible mention in Mexico (Alencáster, 1987).
391

392 **Sellaea minuta** Davis, 1980

393 Fig. 3B, C, E, F, H, 4E-L, 5A-L

394 1980 *Sellaea minuta* n. sp., p. 123, pl. 2, parts 2–4, text-figs. 5c, 6c.

395 1980 *Sellaea elongata* n. sp., p. 121, pl. 1, part 1, text-figs. 5a, 6a.

396 1980 *Sellaea globosa* n. sp., p. 123, pl. 1, part 3, pl. 2, part 1, text-figs. 5b, 6b.

397 1980 *Sellaea ziczac* n. sp., p. 126, pl. 3, part 2, text-figs. 4a, 5d, 6d.

398 1980 *Pachytraga davisarum* n. sp., p. 119, pl. 1, part 1.

399 1987 *Sellaea* sp. cf. *S. elongata* (Davis); Alencáster, p. 117

400 1998 *Pachytraga davisarum* Davis; Skelton and Masse, pp. 333, 339, fig.6c.

401 2020 *Sellaea minuta* ; Rineau et al., p. 6, 31, tab. 1, figs. 9c–d, 15k, 17m, 21d–h.

402 Neotype. The type specimen of Davis (1980) is lost. Here we designate as neotype a complete bivalve specimen in four transverse slices (UT WSA00003058.000, fig. 5A-H) from the Albian of Edwards formation in Texas (USA).
403
404
Diagnosis. *Sellaea* species with undivided PAC-LV. Shell margin narrow. AM-LV vertical on the inner shell. AM-RV horizontal on a slightly convex shelf (Fig. 6A) or vertical and inwardly directed (Fig. 6C), on a plate, protruding into the AAC-LV. One row of aragonitic pallial canals generally present between AM-RV and the outer shell layer. Shell outline round, without ventral flexure.

Description. Shell inequivalve; left valve regularly coiled; right valve uncoiled, conical to tubular. Both valves of same dimensions slightly rotated during growth. Transverse sections of both valves round to ovoid. A slightly angular outline of the right valve's anteroventral part may be present. Outer shell surface ornamented with irregular growth lines and by small costae; outer shell layer approximately of same thickness aragonitic inner layer on the ventral side. Ligament internal, vertical relative to commissure plane, in a ligamentary cavity connected to the shell surface by a ridge. Narrow shell margin.

Left valve. Left valve divided into four main cavities, one BC, one CTS, one elongated posterior cavity (PAC-LV) and one anterior cavity (AAC-LV). In transverse sections closer to the commissure, the AAC-LV is connected to a narrow CTS by a small gutter lying dorsally from the AT. The two anterior and posterior cavities are deep and elongated, each of them occupying one-quarter of the shell. PAC-LV longer and wider than AAC-LV, and both are generally kidney-shaped. Small ridge rarely present in the middle of the AAC-LV, attached to the AAC-LV/BC wall. CTS narrow, stranded between the PAC-LV and the BC, some with a ridge connected to the posteroventral side of the AT and a ridge located between the PT and the PM, leading to an 8-shaped CTS. A slightly inwardly curved plate protruding from the PAC-LV/CTS wall bears the PM on its inward side. The anterior side of the PAC-LV thickens in the location of the PM. AM vertical, outwardly directed, in the dorsal part of the AAC-LV/BC wall which thickens at this location. In some shells, ridges so developed that they merge and separate the deep dorsal part from the shallow ventral part of the CTS. CTS
connected to the AAC-LV by a very narrow connection dorsal from the AT. CTS/BC wall
connected to the PAC-LV/BC wall at half of the PAC-LV length. One proligamentary cavity
surrounds the ligamentary cavity. PT and AT straight and sub-conical to sub-quadrangular.

Right valve. Right valve divided into one BC and one kidney-shaped posterior cavity (PMC) in which lies the PM, vertical on the dorsal part of the PMC/BC wall, facing the LV myophoral plate. Small rounded PTS dorsal to the PMC. Horizontal AM shelf generally sub-horizontal and rounded, rarely vertical on a myophoral plate inwardly directed and protruding in the AAC-LV. Between the anterior shelf and the anterior shell wall lies a gutter or a row of canals. Canals generally rounded, always shallow, from 1 mm deep to approximately 1 cm for the largest individuals. CT surrounding the posterior part of the ATS, which is elongated anteroventrally.

Remarks. *Sellaea minuta* is the only species of *Sellaea* in the Caribbean province. It can be distinguished from the Mediterranean *Sellaea* by a PAC always undivided, the absence of ventral flexure, a non-flared margin and an AM-LV always vertical and not associated with shell thickening. The proligamentary cavity is a character defined in *Pachytraga tubiconcha* by Astre (1961). This species is described with a ligament cavity which is itself enclosed in a so-called proligamentary cavity. One specimen of *S. minuta* has a proligamentary cavity (Fig. 4H), thanks to a thin and well-preserved aragonitic layer. The proligamentary cavity is also present in *Sellaea* from Sinaï (Fig. 5C in Steuber and Bachmann, 2002). The presence or absence of pallial canals is considered here as intraspecific variation, as for *Pachytraga* (Skelton and Masse, 1998). One specimen of *Sellaea* aff. *S. minuta* (USNM.PAL.770973, fig. 5I-L) is essentially distinguished from other *S. minuta* by the presence of an AM-RV plate. This important character could imply a second species of *Sellaea* in the Caribbean province. However, as the specimen is alone and has no information on its provenance, the subsequent acquisition of new material from the Albian Edwards is necessary to clarify its status.
The species described by Davis are problematic for several reasons. From Davis' descriptions and illustrations, all we know is that the four species described are *Sellaea* of the American group. The identification of the species is impossible due to the lack of information on the myophores. For three of them, the types have been lost. The type of *S. ziczac* has been found but does not match the description. In addition, the interpretative drawings are problematic and some descriptions are dubious, notably by the identification of certain borings as pallial canals, or by the lack of data concerning the myophores. *S. minuta* is the only species described by Davis for which the presence of pallial canals on the anterior side of the RV is certain (Fig 3.2 in Davis, 1980). It was originally described as a new species because of a narrower bivalve shell than *S. globosa* and a thinner shell, characters that fall into the category of presumed intra-specific variation. It is also distinguished by different ornamentation, without specifying how this would be so. The *Sellaea* specimen used in Rineau et al. (2020) has been supposedly identified to this species because of the overall shape. *S. elongata* is described as a *Sellaea* with all its cavities, anterior canals on RV, an RV elongated and curving, a left valve slenderer than *S. globosa*, larger and less deformed than *S. minuta*. Without statistical analysis of this variability, defining species on these characters seems doubtful. *S. globosa* is a species described with canals ventrally and posteriorly on the RV, which are supposed to distinguish it from other *Sellaea*. However, these canals are in fact boring. Supposed to differ from *S. cespitosa* by the presence of 'few rounded divisions of the anterior AC, not divisions by vertical plates' (Davis, 1980: p. 123), the meaning of which is unclear, and which the illustrations do not allow to identify. The AM-LV of *S. globosa* is described as being on a 'wide extension of the cardinal platform and separated from shell wall by long subdivided AC [AAC-LV herein]' (Davis, 1980: p. 123). If homologous to the myophoral plate of Italian *Sellaea*, this shell could be included in the Mediterranean group (Fig. 6B). However, the type is lost and the illustrations do not show this feature. *S. ziczac* is
defined by an 'unusual ziczac of the dorsal area' (Davis, 1980: p. 127). Moreover, this feature
could not be identified on the holotype (Fig. 2G-H) which is an internal mould, and therefore
cannot be assigned to a particular species. Finally, *P. davisarum* was assigned to *Pachytraga*
by Davis (1980) because of its massive shell and undivided PAC-LV. It is unknown if the
species lack anterior pallial canals or if it is a taphonomic artefact. However, *Pachytraga
davisarum* possesses a PAC-LV, an AAC-LV, and a PMC, it might belong to *Sellaea*, even if
it does not bear anterior canals in the RV as shown in the phylogeny. However, the internal
features, and especially the myocardinal apparatus, are poorly known. For all these reasons,
we choose to synonymise all species from Davis (1980) and to give priority to *S. minuta*.

Type locality. Bluff Creek, Crawford, McLennan County, Texas, USA (31°32′54.1″ N
97°28′57.8″ W), Edwards Formation.

491 Occurrences. McLennan, Bosque/Hill and Bell counties (Fig. 1B, D), Edwards formation,
492 Texas (USA),.

493 Nomen dubium

494 *Sellaea? Cordata* (Roemer, 1889) **nom. dub.**

1889 *Plagioptychus* (?) *cordatus* n. sp., p. 291, pl.2, fig.2a-e.

496 1928 *Plagioptychus cordatus* Roemer; Adkins, p. 146.

497 1934 *Plagioptychus cordatus* Roemer 1889; Kutassy, pp. 173–174.

498 1937 *P. (?) cordatus* Roemer; MacGillavry, pp. 154–155.

499 1977 *Sellaea? Cordata* (Roemer); Coogan, p. 42, pl.3, figs.4–5.

500 There is not enough information to decide on the generic attribution of *Sellaea? Cordata*
501 (Mac Gillavry, 1937). For Mac Gillavry (1937), the species description is inconsistent. The
502 species was moved to *Sellaea* by Coogan (1977). In Coogan's work, *S.? cordata* is presumed
503

to be a specimen of *Sellaea* by inference because Roemer included it in *Plagioptychus* (1977: p. 42). However, there is no positive information to include this species in the genus *Sellaea*.

504

505

**6. Phylogeny of Sellaea**

506

The analysis includes a total of 14 species as operational taxonomic units for 8 characters (Table 2, SM 1). 7 characters are binary and one character is ordered with two informative states. The cladistic analysis by three-taxon analysis of the *Sellaea* species led to 90 optimal cladograms and a retention index (Kitching, 1998) of 79.23% (percentage of hypothesised phylogenetic relationships retained). In the next paragraphs, we discuss only the strict consensus, because it displays only the nodes present in all optimal trees (Fig. 7). The consensus tree can be seen in two parts: a part composed of clades 1 to 3 which are strongly supported by clear synapomorphies, and another part with the clades 4 to 10 which are more weakly supported with poor congruence and many nodes without clear synapomorphies (Fig. 7).

507

508

509

510

511

512

513

514

515

516

The main result of the cladistic analysis is the confirmation of the hypothesis that the presence of anterior pallial canals (#4.1²) is unreliable as a defining character state of *Sellaea* because it is the only homoplastic state with #4.2 which have the lowest retention index of the analysis (#4, RI: 55.45%). The cladistic analysis shows that the pallial canals of the various *Sellaea* species cannot be homologous (#4.1 in Fig. 7). These canals are independently present in clades 7, 10, *S. cespitosa* and *S. himerensis*, not to mention the intra-specific polymorphism that concerns *S. minuta* (polymorphism handled in table 1 following Nelson and Ladiges, 1996 by retaining only the most derived state) in the same fashion as in *Pachytraga*. Thus it is impossible to distinguish one group of canaliculated *Sellaea* and one group without canals.

517

518

519

520

521

522

523

524

<footnote n="2">² In this section, #4 refers to character 4, and #4.1 refers to the state 1 of the character 4.</footnote>
525

Furthermore, the presence of several rows of canals (#4.2) is also homoplastic, supporting both nodes 7 and 10 (Fig. 7). The phylogenetic analysis of *Sellaea* thus demonstrates the need for the redefinition of the genus that has been undertaken here focusing not only on the presence of canals but also on cavities and myocardinal apparatus.

526

527

528

529

The first dichotomy of the strict consensus separates *Sellaea* species into two geographically distinct groups. The only Caribbean species *S. minuta* is the sister group of an exclusively Mediterranean clade supported by the presence of a divided PAC-LV (#1, RI: 67.27%), a flared margin (#7, RI: 100%), and a ventral flexure (#8, RI: 100%). Although *S. minuta* is not sufficiently distinct from other *Sellaea* to be considered a separate genus, it represents the only known incursion of the genus into the Caribbean province. The contribution of Mediterranean forms to Caribbean faunas due to marine currents transporting larvae has been hypothesised on many occasions (Masse et al., 2007a; Scott et al., 2016; Sha et al., 2020).

530

531

532

533

534

535

536

537

We also find that the Mediterranean clade is composed of *S. pironae* branched as a sister group of a clade gathering all other Mediterranean species (node 2) and supported by the presence of an AM-LV on a tilted plate above the AAC-LV (#5, RI: 100%). There is a wide range of myophore morphologies at the scale of the *Sellaea* genus, where they can be vertical, or on a more or less tilted horizontal shelf or a plate. In node 2, *S. roemeri* is the sister group of node 3 supported by the presence of a small shallow cupula at the base of the PM-LV (#2, RI: 64%). This cupula can also be found in *Caprotina* and *Chaperia*.

538

539

540

541

542

543

544

Clade 4 gathers all *Sellaea* with canals (#4.1) excepted *S. minuta*, and clade 5 gathers all of them which have multiple rows of canals (#4.2). The presence of multiple rows of canals is however homoplastic because of *S. sicula* which does not have this character state. The clade 7 gathering *S. sicula*, *S. laticoncha*, *S. plagioptychoides*, *S. ciofaloi*, *S. aprutina* is supported by one synapomorphy, an AM-RV on a plate protruding into the AAC-LV (#6, RI: 100%). It includes a clade 7 gathering species with a longitudinal crest (#3, RI = 100%). The

545

546

547

548

549
550 clades 4 to 8 are only supported by homoplastic states. Moreover, clades within the genus 551 *Sellaea* are not supported by any particular type of characters (e.g., canals, or myophores), 552 which rejects the idea of using specific characters (or overweighting them) to build 553 phylogenies of rudists (Domestoy, 1971; Skelton and Smith, 2000). On the contrary, rudist 554 evolution shows numerous examples of heterobathmy (i.e., mosaic evolution; Hennig, 1966), 555 for example on pallial canals or in myophores (Rineau et al., 2020).

556 The three-taxon analysis method is an efficient method to treat datasets with missing 557 data (Zaragüeta i Bagils and Bourdon, 2007). *S. aprutina*, *S. distefanoi*, *S. roemeri* were 558 previously placed into *Caprotina* because they look like *Sellaea* but have unknown RV 559 (Parona, 1909). Our analysis supports that the characters defined for the present analysis are 560 sufficient to test their phylogenetic position and that these three species are all included in a 561 clade (2) composed only of Italian *Sellaea*. Thus, our analysis supports their relocation to 562 *Sellaea* even with missing RV. *S. laticoncha*, originally placed in *Sellaea* by Di Stefano but 563 with unknown RV too, is also confirmed to be a *Sellaea*. The redefined genus *Sellaea* is then 564 monophyletic.

### 7. Biogeographic implications

566 Over the last decades, since the pioneer works of Coates (1973) and Kauffman (1973), 567 the existence in the Tethyan Early Cretaceous domain of two main biogeographic entities, the 568 Mediterranean and Caribbean provinces, characterised by distinct shallow water biological, 569 benthic assemblages, especially rudist bivalves, have been increasingly supported (Alencáster 570 and Pantoja-Alor, 1998; Chartrousse and Masse, 2004; Masse et al., 2007a; Scott and Hinote, 571 2007). Our study bears out the presence of the genus *Sellaea* in America, in agreement with 572 the hypothesis of Dechaseaux et al. (1969) and Davis (1980) and in contradiction with 573 Skelton and Masse (1998) views. The foregoing data, combined with the works of Harris and
Hodson (1922), Palmer (1928), Rojas and Iturralde-Vinent (1996), Coogan (1973, 1977), and Scott and Filkorn (2007), among others, suggest that acme of the Caribbean provincialism was during the Albian (Coates, 1973; Skelton, 1982). Endemic Caribbean forms include taxa ascribed to the family Caprinuloidea, whose genera possess a distinctive vertical stratigraphic distribution within this stage (Scott, 2002; Scott and Filkorn, 2007), the same been true for some representatives of the Polyconitidae (*Tepeyacia*) and Monopleuridae (*Pseudopetalodontia*) (Masse et al., 2007a; Pantoja-Alor and Skelton, 1999). Uncertainties still surround the taxonomic position of Caribbean representatives of *Toucasia/Pseudotoucasia*? and *Monopleura* which need further studies (see discussion in Masse and Fenerci-Masse, 2020; Pons et al., 2011). The generic position of some Radiolitidae (e.g. *Radiolites? abraensis* Coogan) (see discussion in Alencaster and Garcia Barrera, 2008), with a compact shell structure and rounded left valve, somewhat comparable to those of the Southwestern Asia genus *Auroradiolites* (Rao et al., 2015) is problematic too. Assuming that *Himeraelites*, a typical form of the Southern Mediterranean Tethyan margin (Masse et al., 1998, 2004: Skelton and Masse, 2000), is a potential sister group of a *Sellaea* clade, *Sellaea* might be a Mediterranean migrant into the Caribbean province. The hypothesis of *Caprotina* as a sister group (Rineau et al. 2020; Di Stefano, 1889) is also consistent. A similar and contemporaneous biogeographic expansion was also documented for the genus *Eoradiolites* (Radiolitidae) (Douvillé, 1900, 1909). In the Mediterranean province, it is worth noting that *Sellaea* was, during the Albian, restricted to the southern margin, then moved to the West European margin in the Cenomanian.

## 8. Conclusion

The genus *Sellaea*, first described by Di Stefano in 1889 in Italy, is now also known from Tibet, Oman, Egypt, France, and its presence in the USA is confirmed. Davis in 1980 erected four new species of *Sellaea* from the Albian Edwards of Texas. Our revision of the
American *Sellaea* and the description of new material allowed us to highlight the insufficiency of the current taxonomic framework of *Sellaea*. Following our revision of the genus, *Sellaea minuta* is the only American *Sellaea*. *S. minuta* is distinguished from Mediterranean *Sellaea* by the presence of a non-flared margin and by the absence of a ventral flexure. A cladistic analysis of *Sellaea* reveals that the above-mentioned characters are synapomorphic of an exclusively Mediterranean clade, with *S. minuta* as a sister group. However, these characters cannot be considered sufficient to raise a new genus for *S. minuta*. Moreover, the cladistic analysis reveals that pallial canals should not be used to define the genus due to the high plasticity of this character. Hence the genus *Sellaea* is redefined taking into account the fact that the presence of canals is variable among *Sellaea*, as for *Pachytraga*.

The redefinition of the genus puts an end to a taxonomic situation that led all modern authors to speak of *Caprotina* 'sensu stricto' about the subgenera described by Di Stefano (1889), notably by relocating all the *Caprotina* described by Di Stefano in the genus *Sellaea*. *Sellaea* possesses a high degree of plasticity in the myophores which, as *Himeraelites*, can vary greatly in their shape and orientation. *Sellaea* appeared with certainty in the Albian and spread throughout the world, where the endemic American form is distinguished by small specific morphological variations. The analysis undertaken in this work shows that highly important characters at the scale of genera or families are not important at the specific scale for disentangling the phylogenetic relationships between species of the same genus, due to mosaic evolution.

## 9. Acknowledgements

We thank Simon Mitchell, Robert W. Scott, and the editor-in-chief of Cretaceous Research Eduardo Koutsoukos for their helpful comments. For access to Italian specimens from Monte d'Ocre, we thank Annalaura Pistarino (Museo Regionale di Scienze Naturali,
Torino). For access to American specimens, we thank Ann Molineux and Liath Appleton (University of Texas, Austin) and Mark Fleurence (Smithsonian Institution, National Museum of Natural History, Washington). We thank Philippe Loubry for the photographs of specimens.

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Alencáster, G., Garcia Barrera, P. 2008. Albian radiolitid rudists (Molluscs, Bivalvia) from Eastern and Central Mexico. Geobios 41, 571–587.

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651 Chartrousse, A., 1998. The myocardinal organization of coalcomaninid rudists
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844

## 11. Captions

845

## Figures.

846

Figure 1. Middle-late Albian paleogeographic map of the Comanche shelf in Texas (redrawn

847

from Scott 2016), and detail of locations of the material listed in Table 1 and described herein

848

from the Edwards Formation. FSB: Fort Stockton Basin; CTP: Central Texas Platform; MB:

849

Maverick Basin. Shoreline in dotted lines, platforms in beige. A, Lake Whitney dam,

850

Bosque/Hill County; B, Childress Creek, Bosque County, former type locality of *Sellaea*
851 *minuta* Davis, 1980 (the type is now lost); C, type locality of *Pachytraga davisarum* Davis,
852 1980 and *Sellaea globosa* Davis, 1980; D, Bluff Creek, Crawford, McLennan County, new
type locality of *S. minuta*; E, type locality of *Sellaea elongata* Davis, 1980; F, Coryell
853 County, type locality of *Sellaea ziczac* Davis, 1980.
854

855 Figure 2. Photographs of Italian and American LV of *Sellaea*. **A**, *Sellaea cespitosa* Stefano
856 1889, figured in Parona 1909, inner surface of left valve; Albian, Abruzzi, L'Aquila, Monte
d'Ocre, Fossa Mezza Spada, Italy (PU18487). **B**, *Sellaea zitteli* Stefano 1889, inner surface of
857 left valve; Albian, Abruzzi, L'Aquila, Monte d'Ocre, Fossa Mezza Spada, Italy (PU18492). **C**,
858 *Sellaea himerensis* Stefano 1889, figured in Parona 1909, external view (up) and inner surface
859 of left valve (bottom); Albian, Abruzzi, L'Aquila, Monte d'Ocre, Fossa Mezza Spada, Italy
860 (PU18488). **D**, holotype of *Sellaea distefanoi* nov. comb. Parona 1909, inner surface of left
861 valve; Albian, Abruzzi, L'Aquila, Monte d'Ocre, Fossa Mezza Spada, Italy (PU18364). **E**,
862 holotype of *Sellaea aprutina* nov. comb. Parona 1909, inner surface of left valve; Albian,
863 Abruzzi, L'Aquila, Monte d'Ocre, Fossa Mezza Spada, Italy (PU18363). **F**, *Sellaea stryx* nov.
864 comb. Stefano 1889, figured in Parona 1909, inner surface of broken left valve; Albian,
865 Abruzzi, L'Aquila, Monte d'Ocre, Fossa Mezza Spada, Italy (PU18366). **G** and **H**, holotype of
866 *Sellaea ziczac* Davis 1980, internal mould of left valve; Albian Edwards, Coryell County,
867 Texas, USA (UT TX000001246.004; 31°6.5'N 97°54'W). Pictures A-F provided by
868 Annalaura Pistarino and G-H by Liath Appleton. Scale bar, 10 mm.
869

870 Figure 3. Photographs of *Sellaea minuta* specimens. **A**, internal mould with partial
871 preservation of the outer and inner shell layers (USNM.PAL.770981). **B**, inner surface of
872 broken left valve; Albian Edwards, Bluff Creek, Crawford, McLennan County, Texas, USA
873 (USNM.PAL.770980). **C**, inner surface of broken right valve; Albian Edwards, Bluff Creek,
874 Crawford, McLennan County, Texas, USA (USNM.PAL.770979). **D**, transverse section of
875 the right valve; Lake Whitney dam, Bosque/Hill County, Texas, USA (USNM.PAL.770975).
876 E, inner surface of right valve; Albian Edwards, Bluff Creek, Crawford, McLennan County,
877 Texas, USA (USNM.PAL.770984). F, inner surface of right valve; Albian Edwards, Bluff
878 Creek, Crawford, McLennan County, Texas, USA (USNM.PAL.770986). G, posterior view
879 of internal mould with preservation of the shell layer of the right valve; Lake Whitney dam,
880 Bosque/Hill County, Texas, USA (USNM.PAL.770977). H, inner surface of broken right
881 valve; Albian Edwards, Bluff Creek, Crawford, McLennan County, Texas, USA
882 (USNM.PAL.770978). I, anterior view of internal mould; Lake Whitney dam, Bosque/Hill
883 County, Texas, USA (USNM.PAL.770976). Pictures provided by Philippe Loubry. Scale bar,
884 10 mm.

885 Figure 4. Photographs of *Sellaea minuta*. A-D, transverse sections of a right valve
886 (USNM.PAL.770972). E-F, transverse sections of a right valve (USNM.PAL.770970). G,
887 transverse section of a left valve (USNM.PAL.770974). H, transverse section of a left valve;
888 Childress Creek, Bosque County, Texas, USA (UT NPL00094416.000). I, transverse section
889 of a right valve. J-K, longitudinal polished sections of a bivalve specimen; Bell County,
890 Texas, USA (UT NPL00094417.000). L, transverse section of a right valve
891 (USNM.PAL.770969). Pictures provided by Philippe Loubry. Scale bar, 10 mm.

892 Figure 5. Photographs of American *Sellaea*. A-H, neotype of *Sellaea minuta*, seven transverse
893 serial sections of articulated specimen proceeding from LV-upper left (A) to RV-lower left
894 (H); Albian Edwards, Bluff Creek, Crawford, McLennan County, Texas, USA (UT
895 WSA00003058.000; 31°32'54.067"N 97°28'57.838"W). I-L, *Sellaea* aff. *S. minuta*, four
896 transverse serial sections; (USNM.PAL.770973). Pictures provided by Philippe Loubry. Scale
897 bar, 10 mm.

898 Figure 6. Sketch of three myophoral organisations in *Sellaea*. A, the AM-LV is vertical in the
899 AAC-LV, outwardly directed. B, the AM-LV is horizontal, on a thickening covering the
900 AAC-LV. C, the AM-LV is vertical in the AAC-LV, outwardly directed and facing the AM-
RV carried on a plate. For all species, the posterior myophoral organization which is only displayed in C is constant. Legend: AAC-LV: anterior cavity of the left valve; AC-RV: anterior cavity of the right valve; CTS: central tooth socket; PMC: posterior myophoral cavity.

Figure 7. Strict consensus of 90 optimal trees (RI = 79.23%) found in three-taxon analysis displaying the phylogenetic relationships among the *Sellaea* species with the corresponding character states (synapomorphies, homoplasies, unassigned instances). The cladogram shows clearly an initial divergence between Mediterranean and American species of *Sellaea*. Losses are not shown because contrary to parsimony analyses, loss cannot be used to support relationships in three-taxon analysis. Character descriptions (the first number corresponds to the character number – in bold in the phylogeny – as in table 2, the number in parentheses to the character state). 1 – posterior cavity (PAC-LV): undivided (0); divided in one or more (1) (RI = 67.27%). 2 – Central tooth socket: root (0); diverticulum between the cavity and the PM-LV (1) (RI = 64%). 3 – Anterior cavity (LV): root (0); longitudinal crest on the BC/AAC-LV wall (1) (RI = 100%). 4 – Anterior right valve canals: AM-RV large and thick (0); presence of pallial canals (1); multiple rows of canals (2) (RI = 55.45%). 5 – Anterior myophore (LV): myophore on the inner shell (0); myophore on a tilted plate above the AAC-LV (1) (RI = 100%). 6 – Anterior myophore (RV): myophore on a horizontal shelf (0); myophore on a plate protruding into the AAC-LV (1) (RI = 100%). 7 – Shell margin: narrow (0); flared (1) (RI = 100%). 8 – Shell outline: rounded (0); ventral flexure (1) (RI = 100%).

## Tables.

Table 1. List of specimens of *Sellaea* studied and figured in the present study, with associated data on their localisation and status. All specimens with a known localisation are middle to lower-upper Albian.
Table 2. Taxon-character matrix for 11 species and 8 characters, and their associated hierarchical coding (Cao et al., 2007; Zaragüeta i Bagils et al., 2012). ? : missing data.

Correspondence between numbers and character state definitions given in figure 7 caption.

Table 3. List of all *Sellaea* species and their corresponding ages.
<figure><img src="image_2.png" /></figure>

Dallas

A

B

Waco

C

D

E

F

Austin

100'

200 km
A.

<figure><img src="image_3.png" /></figure>

B.

<figure><img src="image_4.png" /></figure>

C.

aac-rv

<figure><img src="image_5.png" /></figure>

D.

<figure><img src="image_6.png" /></figure>

am

at in pts

ct

pt in pts
pmc

H.

<figure><img src="image_7.png" /></figure>

am

at

pts
pmc

G.

<figure><img src="image_8.png" /></figure>

E.

<figure><img src="image_9.png" /></figure>

F.

<figure><img src="image_10.png" /></figure>

arvc

ats

pts

pmc

I.

<figure><img src="image_11.png" /></figure>

arvc

aac-lv

bc
pm-lv.

<figure><img src="image_12.png" /></figure>

am-rv

<figure><img src="image_13.png" /></figure>

pt

cts

at

am

rv

pdc

pts

am

rv

pdc

pts

am

rv

G.

E.

<figure><img src="image_14.png" /></figure>

am-rv

F.

ligamentary
cavity

<figure><img src="image_15.png" /></figure>

proligamentary
cavity

<figure><img src="image_16.png" /></figure>

am-rv

I.

H.

<figure><img src="image_17.png" /></figure>

D.

<figure><img src="image_18.png" /></figure>

K.

aac-lv

L.

at

aac-lv

am-rv

ats

pdc

A.

<figure><img src="image_19.png" /></figure>

at

pdc

pts

am

rv

pdc

pts

am

rv

pdc

pts

am

rv

B.

<figure><img src="image_20.png" /></figure>

am-rv

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

C.

<figure><img src="image_23.png" /></figure>

J.

I.
<figure><img src="image_24.png" /><figcaption>A.</figcaption></figure>

<caption>B.</caption>

<figure><img src="image_25.png" /><figcaption>E.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>J.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>H.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>I.</figcaption></figure>

<caption>L.</caption>
<figure><img src="image_29.png" /><figcaption>B.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>C.</figcaption></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

Anterior muscle

<figure><img src="image_33.png" /></figure>

Posterior muscle

<figure><img src="image_34.png" /></figure>

Left valve (LV)

<figure><img src="image_35.png" /></figure>

Right valve (RV)

LV umbo

<figure><img src="image_36.png" /></figure>

Anterior

Posterior

RV umbo
<figure><img src="image_37.png" /></figure>
<table><thead><tr><th>Species</th><th>Status</th><th>Collection number</th><th>Localisation</th></tr></thead><tbody><tr><td>Sellaea aprutina nov. comb.</td><td>Holotype (Parona 1909)</td><td>PU18363</td><td>Abruzzi, L'Aquila, Monti d'Ocre, Fossa Mezza Spada, Italy</td></tr><tr><td>Sellaea caespitosa</td><td>figured (Parona 1909)</td><td>PU18487</td><td>Abruzzi, L'Aquila, Monti d'Ocre, Fossa Mezza Spada, Italy</td></tr><tr><td>Sellaea distefanoi</td><td>Holotype (Parona 1909)</td><td>PU18364</td><td>Abruzzi, L'Aquila, Monti d'Ocre, Fossa Mezza Spada, Italy</td></tr><tr><td>Sellaea himerensis</td><td>figured (Parona 1909)</td><td>PU18488</td><td>Abruzzi, L'Aquila, Monti d'Ocre, Fossa Mezza Spada, Italy</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770981</td><td></td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770975</td><td>Lake Whitney dam, Bosque/Hill County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770977</td><td>Lake Whitney dam, Bosque/Hill County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770976</td><td>Lake Whitney dam, Bosque/Hill County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770972</td><td></td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770969</td><td></td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770980</td><td>Bluff Creek, Crawford, McLennan County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770979</td><td>Bluff Creek, Crawford, McLennan County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770984</td><td>Bluff Creek, Crawford, McLennan County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770986</td><td>Bluff Creek, Crawford, McLennan County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770978</td><td>Bluff Creek, Crawford, McLennan County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td>Neotype (herein)</td><td>UT</td><td></td></tr><tr><td>Sellaea minuta</td><td></td><td>WSA00003058.000</td><td>Bluff Creek, Crawford, McLennan County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770970</td><td></td></tr><tr><td>Sellaea minuta</td><td></td><td>USNM.PAL.770974</td><td></td></tr><tr><td>Sellaea minuta</td><td></td><td>UT NPL00094416.000</td><td>Childress Creek, Bosque County, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td></td><td>UT NPL00094417.000</td><td>Bell county, Texas, USA</td></tr><tr><td>Sellaea minuta</td><td>Holotype of S. ziczac (Davis 1980)</td><td>UT TX000001246.004</td><td>Coryell County, Texas, USA</td></tr><tr><td>Sellaea zitteli</td><td></td><td>PU18492</td><td></td></tr></tbody></table>
<table><thead><tr><th>Character number</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th></tr><tr><th>Hierarchical coding</th><th>(0(1))</th><th>(0(1))</th><th>(0(1))</th><th>(0(1(2)))</th><th>(0(1))</th><th>(0(1))</th><th>(0(1))</th><th>(0(1))</th></tr></thead><tbody><tr><td>Sellaea aprutina nov. comb.</td><td>1</td><td>?</td><td>1</td><td>?</td><td>1</td><td>?</td><td>?</td><td>1</td></tr><tr><td>Sellaea cespitosa</td><td>0</td><td>1</td><td>0</td><td>1</td><td>1</td><td>0</td><td>1</td><td>1</td></tr><tr><td>Sellaea ciofaloi</td><td>1</td><td>?</td><td>1</td><td>2</td><td>?</td><td>?</td><td>?</td><td>1</td></tr><tr><td>Sellaea distefanoi nov. comb.</td><td>1</td><td>1</td><td>0</td><td>?</td><td>1</td><td>?</td><td>1</td><td>1</td></tr><tr><td>Sellaea himerensis</td><td>0</td><td>1</td><td>0</td><td>2</td><td>1</td><td>0</td><td>1</td><td>1</td></tr><tr><td>Sellaea laticoncha</td><td>1</td><td>0</td><td>1</td><td>?</td><td>?</td><td>?</td><td>?</td><td>1</td></tr><tr><td>Sellaea minuta</td><td>0</td><td>0</td><td>0</td><td>1</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Sellaea orbignyi</td><td>1</td><td>0</td><td>0</td><td>2</td><td>?</td><td>0</td><td>?</td><td>1</td></tr><tr><td>Sellaea pironae</td><td>1</td><td>0</td><td>0</td><td>?</td><td>0</td><td>?</td><td>?</td><td>1</td></tr><tr><td>Sellaea plagioptychoides</td><td>1</td><td>?</td><td>1</td><td>2</td><td>?</td><td>1</td><td>?</td><td>1</td></tr><tr><td>Sellaea roemeri nov. comb.</td><td>1</td><td>0</td><td>0</td><td>?</td><td>1</td><td>?</td><td>?</td><td>1</td></tr><tr><td>Sellaea sicula</td><td>1</td><td>?</td><td>0</td><td>0</td><td>?</td><td>1</td><td>?</td><td>1</td></tr><tr><td>Sellaea stryx nov. comb.</td><td>1</td><td>1</td><td>0</td><td>0</td><td>1</td><td>0</td><td>1</td><td>1</td></tr><tr><td>Sellaea zitteli</td><td>1</td><td>1</td><td>0</td><td>2</td><td>1</td><td>0</td><td>1</td><td>1</td></tr></tbody></table>
<table><thead><tr><th>Species</th><th>Age</th></tr></thead><tbody><tr><td>Sellaea aprutina (Parona, 1909) nov. comb.</td><td>Albian</td></tr><tr><td>Sellaea cespitosa (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea ciofaloi (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea distefanoi (Parona, 1909) nov. comb.</td><td>Albian</td></tr><tr><td>Sellaea himerensis (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea laticoncha (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea minuta (Davis, 1980)</td><td>Middle/lower upper</td></tr><tr><td>Sellaea orbigny (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea pironae (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea plagioptychoides (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea quadripartita (d'Orbigny, 1842) nov. comb</td><td>Cenomanian</td></tr><tr><td>Sellaea roemeri (Gemmellaro, 1865) nov. comb.</td><td>Albian</td></tr><tr><td>Sellaea sicula (Di Stefano, 1889)</td><td>Albian</td></tr><tr><td>Sellaea stryx (Di Stefano, 1889) nov. comb.</td><td>Albian</td></tr><tr><td>Sellaea zitteli (Di Stefano, 1889)</td><td>Albian</td></tr></tbody></table>