# The Largest Mesosaurs Ever Known: Evidence from Scanty Records

Graciela Piñeiro, Pablo Núñez Demarco, Michel Laurin

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Graciela Piñeiro, Pablo Núñez Demarco, Michel Laurin. The Largest Mesosaurs Ever Known: Evidence from Scanty Records. Fossil Studies, 2024, 3 (1), pp.1. 10.3390/fossils3010001. hal-05344668

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Article

# The Largest Mesosaurs Ever Known: Evidence from Scanty Records

Graciela Piñeiro <fnref n="1" />,* [D], Pablo Núñez Demarco <fnref n="2" /> and Michel Laurin <fnref n="3" /> [D]

**Abstract:** Mesosaurs have long been considered to be small to mid-sized aquatic to semi-aquatic amniotes that lived in Gondwana during the Early Permian or Late Carboniferous, according to recent research that showed their ghost range extending back to the Pennsylvanian. Previous morphometric analyses based on several hundred mesosaur specimens, including materials from Uruguay, Brazil, South Africa, Namibia, and the Paris National History Museum, provided a comprehensive understanding of mesosaur ontogeny, documented from fetus to adults. As a result, it was possible to determine the approximate size of any individual, measuring just one isolated limb bone, vertebrae, or even cranial elements. Herein, we describe large, poorly preserved and incomplete skulls, as well as axial and appendicular bones, from the Mangrullo Formation Konservat-Lagerstätte of Uruguay that suggest the existence of gigantism in mature mesosaurs reaching more than twice the size of previously described adults and type specimens. The sporadic occurrence of these giant individuals contrasts sharply with the abundant remains of young mesosaurs and, in general, with what is commonly found in the fossil record of vertebrates. The poor preservation of the mature individuals and their presence in coastal areas of the basin is consistent with the hypothesis that older mesosaurs have spent more time near the coast. An alternative hypothesis suggesting pelagic lifestyles is less supported by the available data. Given the preservation of unborn and hatchlings, as well as early juvenile, mature and very mature individuals, the mesosaur record is considered exceptional among early amniotes.

**Keywords:** gigantism; ontogeny; lifestyle; Mesosauridae; Mangrullo Formation

## 1. Introduction

Body size has been extensively discussed in the context of theories about the origin of amniotes and of the amniotic egg [1,2] suggested that some stem-amniotes initially laid amamniotic eggs on land. Due to gas exchange constraints and the relatively high metabolic rates of amniotes, egg size was limited to a maximum diameter of about 1 cm. In turn, he postulated that the absence of a larval stage in amniote development would have constrained the adult body size. Based on these theoretical considerations (and the observed correlation between egg size and adult size in some amniotes), Carroll [1,2] suggested that the size of stem-amniotes would have decreased to no more than about 10 cm in snout-vent length, before increasing in size again, following the appearance of the

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Received: 18 September 2024

Revised: 17 December 2024

Accepted: 18 December 2024

Published: 25 December 2024

**Citation:** Piñeiro, G.; Núñez Demarco, P.; Laurin, M. The Largest Mesosaurs Ever Known: Evidence from Scanty Records. *Foss. Stud.* **2025**, 3, 1. https://doi.org/10.3390/fossils3010001

**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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conditions of the Creative Commons
Attribution (CC BY) license
(https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Departamento de Paleontología, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo CP. 11400, Uruguay</footnote>

<footnote n="2">² Instituto de Ciencias Geológicas, Facultad de Ciencias, Universidad de la República, Montevideo CP. 11400, Uruguay; pnunez@fcien.edu.uy</footnote>

<footnote n="3">³ Centre de Recherche en Paléontologie—Paris (CR2P), UMR 7207, CNRS/MNHN/SU, Muséum National d'Histoire Naturelle, CP 38, 57 rue Cuvier, 75231 Paris, cedex 05, France; michel.laurin@mnhn.fr</footnote>

<footnote>* Correspondence: fossil@fcien.edu.uy</footnote>
amniotic egg with its extra-embryonic membranes. Early amniote evolution would then be characterized by body size increase.

A general tendency for size increase has often been referred to as the “Cope’s rule” [3]. However, this expression is not justified by Cope’s writings, as convincingly demonstrated by [4]. Instead, it would be accurate to refer to it as the Depéret’s rule [5] since Depéret [6,7], unlike Cope [3], explicitly wrote that body size systematically increased through evolutionary time.

This scenario has been quantitatively tested at least twice. It was constructed timetrees of early stegocephalians [8], focusing on early amniotes and analyzing the evolution of body size on this tree using squared-change parsimony [9] to obtain nodal estimates, and phylogenetic independent contrasts [10] to estimate confidence intervals for these values. More recently, asymmetric linear parsimony was used [11] to refine nodal values in the presence of presumed trends and to detect potential changes in evolutionary trends that are predicted by [1,2] scenario on amniote origins (a decrease in size associated with oviposition on land, followed by a subsequent increase linked to the emergence of the amniotic egg).

These studies confirmed some of Carroll's predictions [1,2] but refuted others. While Laurin [8] confirmed a general trend of size increase in early amniotes (Depéret's rule), the size of these amniotes was not as small as Carroll [1,2] suggested. Even with a poorly constrained snout-vent length, estimates range from 10.5 cm to 1.64 m, whereas Carroll's [1,2] scenario specified a snout-vent length not exceeding 10 cm. More importantly, the smallest size estimated in hypothetical ancestors of amniotes was found deeper in the node, at the level of Tetrapoda or Cotylosauria [11]: (Table 5). Additionally, out of the 517 potential changes in trend shifts detected, only one was consistent with Carroll's [1,2] scenario.

Other studies on early amniotes (not specifically designed to test Carroll's scenario) similarly found trends of body size increase. For instance, Brocklehurst and Brink [12] found evidence of body size increase in edaphosaurids and sphenacodontids, though it did not infer the ancestral amniote size. The analyses of [13] suggest that the ancestral amniote was probably small, with body size subsequently increasing, at least in synapsids. However, that study modeled body size as a ternary discrete variable, where “small” represented a body mass of less than 1 kg, which is much higher than that implied by Carroll's [1,2] scenario on amniote origins. Nevertheless, the main conclusions of both of these studies are congruent with those of [8,11] and provide fairly limited support for that scenario.

As previously argued [8,14,15], the small size of the earliest known amniotes may reflect a taphonomic bias favoring smaller sizes as for the most amniote fossils found at Joggins and Florence, where isolated bones fossilized in tree stumps that could only accommodate small animals (or their carcasses) [16,17]. Furthermore, the small size of the bones may have caused the misidentification of immature individuals as adults, given that they are completely disarticulated and it cannot be defined if the specimen corresponds to the associations of bones belonging to just one or more individuals [14,15]. This once happened to the embolomere *Calligenethlon* [18] from Joggins (Nova Scotia), which was initially interpreted as a taxon of small body size, whereas, in fact, this interpretation was based on juvenile specimens.

Below, we describe a few fragmentary specimens that show that *Mesosaurus tenuidens* [19] could reach a significantly larger size than previously thought. *Mesosaurus* is a mid-sized Permocarboniferous sauropsid found in South America and southern Africa that probably retains an aquatic habit from amphibious ancestors (according to hypothesis of [20]; and see also [15,21,22] or they may have been the first amniotes that were adapted to the aquatic environment from terrestrial relatives [23–28]).

Given the relatively basal position of *Mesosaurus tenuidens* (Amniota: Sauropsida) in amniote phylogenies, whether as the basalmost sauropsids [29–31]—which is our working
hypothesis—as slightly less basal sauropsids [32], or as basal parareptiles [33–39], its size could significantly impact our estimates of the ancestral amniote body size. [8]: (appendix 2) had estimated mesosaur cranial length at 10.1 cm and presacral (atlas-sacrum) length at 21.8 cm. However, new recently known evidence of mesosaur reproductive biology [40] and the specimens described herein suggest that the maximal mesosaur body size needs to be revised upward and, as a consequence, the hypotheses of the average size of early amniotes need to be updated.

In this paper, we describe remains of *Mesosaurus tenuidens* that reveal, for the first time, the large body size that the most mature individuals of this taxon could have reached. Our findings also allow us to re-evaluate previous hypotheses [14,15,28,41,42] that suggested semiaquatic habits of mesosaurs (rather than strictly aquatic, though as aquatic as that of pinnipeds or marine iguanas), which were more evident in the adult and mature ontogenetic stages This contrasts with earlier hypotheses suggesting a more fully aquatic lifestyle [25–27,43].

**2. Materials and Methods**

The materials described herein (Figures 1–6) were found in bituminous and non-bituminous shale of the Mangrullo Formation at the Picada de Cuello and El Baron localities in northern Uruguay (see [44] and Figure 1).

Our study relies on statistical modeling that helps us to determine an approximate average size of the animals to which the large isolated appendicular bones and, occasionally, a few articulated elements and the incomplete skulls described herein belong.

Fossils exhibit small irregularities due to the presence of fibrous gypsum crystals (and secondary iron oxides) (see Figures 7 and 8), resulting from their exposure to evaporitic mineral precipitation, present in the layers where the fossils are found. This mineralization is widely reported in the Mangrullo-Irati-Whitehill sea [24,45–49] related to the establishment of a drying environment, which facilitates disarticulation and a slow, gradual decay of both soft tissues and bones (see [50]).

**2.1. Description**

FC-DPV 3622, 3623. Incomplete mesosaur skulls preserved as part and counterpart, including recognizable postorbital and orbital regions (Figure 1). FC-DPV 3622 also preserves a highly deformed pre-orbital area, extending no further anteriorly than the level of the anterior margin of the naris. The morphology and anatomical arrangement of the preserved bones matches the configuration previously described for the mesosaur skull [19,21,25,34,51–57] (Figures 1 and 2).

FC-DPV 3620 and 1427 (Figure 3). Dorsal vertebrae. FC-DPV 3620 is a very large vertebra, possibly from the mid-dorsal series, given its tall vertebral spine and the horizontal position of the pre-zygapophyses (Figure 3A). The neural arch is swollen, but in the dorsal view, it does not display the square configuration (1-1 length-width) typical of mesosaur dorsal vertebrae (Figure 3B). Instead, this giant vertebra is slightly wider than it is long.

FC-DPV 2397 (Figure 4A). A large left scapulo-coracoid in lateral view; both elements that are disarticulated in early ontogenetic stages are clearly completely fused.

FC-DPV 3621, 2396 (Figure 5A,B). Large right humeri in dorso-medial view. The areas corresponding to the capitulum and the trochlea are more developed than in younger individuals (Figure 5C,D).

FC-DPV 1061. Fragment of the anterior region of the tail of a large mesosaur consisting of eight articulated vertebrae and partially preserved caudal ribs (Figure 6A).
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**Figure 1.** Largest known mesosaur skulls from the Mangrullo Formation Konservat-Lagerstätte of Uruguay. (A) Photograph of FC-DPV 3622, a badly preserved and incomplete mesosaur skull in dorsal view, including recognizable orbital and postorbital regions and a very deformed and fragmentary pre-orbital area. (B) Interpretive drawing of (A), showing the principal bones preserved that could be identified with acceptable confidence. (C) Photograph of the counterpart of (A). (D) Interpretive drawing of (C) showing the principal bones that could be identified, although the contacts between some of them is not well delimited. (E) Photograph of FC-DPV 3623, a very incomplete mesosaur skull preserving the postorbital region and the frontal interorbital bar; preorbital region is missing. (F) Interpretive drawing of (E). (G) Photograph of the counterpart of (E). (H) Interpretive drawing of (G), showing the difficulty in the delimitation of the present bones due to the fragmentary and poor preservation of the specimen. Abbreviations: f, frontal; j, jugal; la, lacrimal; ltf, Lower temporal fenestra; mx, maxilla; n, nasal; or, orbit; p, parietal; pp, postparietal; pf, parietal foramen; po, postorbital; pof, postfrontal; prf, prefrontal; pt, pterygoid; qj, quadrato-jugal; so, supraoccipital; sq, squamosal; st, supratemporal; t, tabular. Scale bars: 20 mm.
FC-DPV 1347. Isolated, almost complete caudal rib (probably the fourth) of a very large mesosaur (Figure 6B). Although it was not possible to calculate the exact size of the animal that bore such a rib, by comparison with the calculated humerus size and the large caudal fragment shown in Figure 6A we can assume that it features twice that size (but see below).

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Mesosaur ontogenetic skull series documented from specimens found in the Mangrullo Formation Konservat-Lagerstätte of Uruguay. (A–E) The photographs show a comparative view of the evolution of the skull size through mesosaur ontogeny. (A) FC-DPV 2242 early juvenile individual. (B) FC-DPV 2318, a juvenile to subadult specimen. (C) FC-DPV 2061, a subadult individual. (D) FC-DPV 2534, an adult individual. (E) FC-DPV 3622, the largest known mesosaur skull representing one of the mature individuals here described. See the significant gap in the skull size that can be observed between (D,E). Scale bar: 10 mm.</figcaption></figure>

## 2.2. Methods: The Morphometric Approach

In this study, measurements of the new large mesosaur specimens were conducted using the same methods as in [15,41]. We recorded the total length as well as the proximal, central, and distal widths of the large humeri and the dorsal vertebral centra and those of the humeri, along with the total length of the vertebral centra of the analyzed large disarticulated mesosaur specimens. Additionally, we recorded the antero-posterior length of the orbit and the maximum skull width at the middle orbital level of the new giant mesosaur skulls described herein. These measurements were compared with data from vertebrae of 69 mesosaur specimens (as reported in [41], and with 45 humeri and 39 skulls examined by [15]). Skull measurements were also compared to data of 109 skulls analyzed by [27] and from 22 skulls examined by [15].

Mesosaur total length estimates were obtained using the average vertebral size and the isometric growth relationships previously described for this taxon [15,41].

Skull data was reanalyzed using datasets from [15,27], calculating the slope (a) of the dataset and its error interval after applying the logarithmic equation:

$$ \log (y) = \log (x)a + \log (b) $$

where y and x represent the dimensions of specific parts or components, b is a constant, and a is the allometric coefficient. A coefficient a > 1 indicates positive allometry (y grows faster than x), while a < 1 indicates negative allometry (x grows faster than y). When a = 1 or close
to 1 with the 95% confidence interval including 1, the bivariate relationship is considered isometric [15,58–60].

To estimate the postcranial skeleton length, we assumed an average of 101 vertebrae in the mesosaur column [61]. By measuring the size of FC-DPV 3620, the largest known mesosaur dorsal vertebra described herein, and assuming it approximates to the mean vertebral length for this individual [41], we estimated postcranial length. Skull length can also be estimated using the isometric relationships established by [15], either from orbital length or stylopodium length. Additionally, there is a correlation between skull size and the mean vertebral length, allowing us to approximate postcranial skeleton length from a single measurement.

These measurements enable us to estimate the total size that these large specimens might have reached, albeit with a considerable margin of error, as the method is based on regressions involving substantially smaller animals. The measurement error was estimated by considering the error in measuring each sample (accounting for possible distortions due to compression or salt deposition on the bone surface) and the error in the linear adjustments used for reconstruction. Additionally, vertebral size variability was considered assuming that the studied isolated dorsal vertebra could represent both the longest and shortest vertebrae in the dorsal section.

## Data Presentation

As demonstrated by Feng et al. (2014), applying a log transformation to data that already follow a normal distribution can introduce a non-normal distribution and may even increase data variability. In our case, the mesosaur data exhibit a normal distribution, so a log transformation is not advisable. As we noted in [15], following the guidance of [62], all linear plots in this study use non-logarithmic data, while the allometric constants are calculated from logarithmic data. This approach also facilitates direct analysis and interpretation of the data and plots [62], particularly when our goal is to use these data to reconstruct the *Mesosaurus tenuidens* body size. Additionally, since our results confirm that the data are isometric, it is more appropriate to avoid logarithmic transformations altogether.

## 2.3. *Isometric Growth in Mesosaurs and Its Implications in Determining the Average Size of Mesosaurs*

Previous studies (e.g., [15]) demonstrated that isometry is the growth pattern (both inter- and intra-bone) identified for *Mesosaurus tenuidens*, the only valid mesosaur taxon [61]; but see also [27,63]. In addition, in a subsequent study, Verrière and Fröbisch [27] further analyzed the growth patterns in mesosaurs, supporting the proposal of [61] who suggested that *Mesosaurus tenuidens* is the only valid species while suggesting the presence of allometric relationships within mesosaur bones. However, the results provided by [27] should be approached with caution:

(i) Almost every measurement in [27] was exclusively compared to the average size of dorsal vertebrae (ASDV), with the authors justifying this choice by suggesting that ASDV is a good proxy for determining overall *Mesosaurus* size [27]. However, this proxy used by [27] is not optimal, given that it was demonstrated that ASDV has been shown to overestimate *Mesosaurus* length by including only the largest and more variable vertebrae in the *Mesosaurus* vertebral column [41]. Consequently, most of the allometric relationships found by [27] are inapplicable, as they are based on an inappropriate proxy and cannot reliably estimate *Mesosaurus* size. Moreover, the allometric relationships studied by these authors are largely tied to this proxy, complicating their interpretation; at most, these findings suggest the existence of
allometric relationships specifically with respect to the dorsal region. This does not refute or contradict previous findings.

(ii) In contrast, the isometric relationships used in the study of [15] were calculated using the average of all vertebrae, including cervicals and caudals, for a more accurate estimate, as recommended by [41].

(iii) Of the 23 relationships analyzed by [27], only 4 are not linked to the ASDV; of these, only one is relevant to the present study: the skull length vs. postorbital length ratio (a measurement in which both research groups found allometry). Given the substantial amount of data collected by these authors, we decided to include their data to recalculate the relationships with greater precision, and the results are presented in Figures 9 and 10 and particularly in Figure 11. Additionally, we analyzed the relationship between orbit size and postorbital length ratio, a parameter previously studied by [15] but not by [27].

## 2.4. SEM Studies

To examine the microstructure and chemical composition of the thin encrusted layer covering the large mesosaur bones described herein, we performed an SEM analysis using energy-dispersive X-ray spectroscopy analysis (EDS) using a Thermo Scientific Ultra Dray EDS detector with NORAN System 7 X-Ray Microanalysis System (Figure 7). For this purpose, a sample was taken from one of the available humeri, and a transverse section of the bone was analyzed at its mid-shaft level, taking advantage of a natural fracture in the specimen. After this procedure, the bone was restored using a natural modeling clay as a soft adhesive.

## 3. Results

Our analysis shows that the size of FC-DPV 3620 (Figure 3), a large vertebra studied here, exceeds the size distribution typically observed in mesosaurs (Figure 9). In fact, the size of this vertebra surpasses the mean vertebral size of the mesosaur population (5.4 mm) by more than five times its standard deviation (1.8 mm), calculated without including this specimen. Similarly, the humerus shown in Figure 5A measures 72.3 ± 1 mm in length, far exceeding the average of 29.9 mm for previously described humeri (see [15,22,25,43,52,54] among others). This measurement places the size of this humerus over four standard deviations (8.8 mm) above the mesosaur population mean. The same is true for its proximal (12.4 ± 1 mm), middle (5.8 ± 1 mm), and distal (21.4 ± 1 mm) widths, which far exceed the averages typically observed in mesosaur humeri (3.8 mm, 3.0 mm, 9.7 mm) (Figure 10). Moreover, when examining the ratio between the length and width of the scapulocoracoids (Figure 4) and humerus (Figure 5), and projecting the previously found relationships for mesosaurs (e.g., [15]), the specimens align closely with the predicted size based on isometry, as shown in Figure 10.

The orbital length of the skulls shown in Figure 1 averages 32 ± 1 mm (Figure 1A,C) and 29 ± 1 mm (Figure 1E,G), respectively. These values greatly contrast with the previously observed average dimensions of 8.8 mm, thus nearly tripling the size of the largest specimen (12.8 mm) previously analyzed [15].

Using the data outlined in Section 2, it is possible to estimate that FC-DPV 3620 (the large dorsal vertebra, Figure 3) corresponds to a mesosaur that likely possesses a vertebral column of 1.55 ± 0.5 m and a skull between 0.13 and 0.16 m, resulting in an estimated total length of approximately 1.70 ± 0.20 m. Meanwhile, FC-DPV 3621 (one of the largest humeri (Figures 5A and 10) likely corresponds to a mesosaur with a skull of around 0.15 m, dorsal vertebrae measuring 14 mm of centrum length, and a total length of 1.56 ± 0.20 m, which is about twice the size of an average mesosaur (total length of 0.7 m).
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Mesosaur vertebrae from the Mangrullo Formation Konservat-Lagerstätte of Uruguay. Size comparisons between the largest known mesosaur dorsal vertebra (FC-DPV 3620, (A,C,E)) and other isolated dorsal vertebrae displaying the normal size for mesosaurs (FC-DPV 1427, batch of 12 dorsal vertebrae, (B,D,F)), in anterodorsal (A,B), lateral (C,D) and ventral (E,F) views. Scale bars: 10 mm (A,C,E,F) and 5 mm (B,D).</figcaption></figure>

Mesosaurs with large skulls such as those shown in Figure 1A–C,E–G likely had skull
lengths reaching 0.21 m, a total mean centrum length of approximately 20 mm and an
overall length of about 2.20 ± 0.30 m (Figure 11), which is three times the size of an average
mesosaurus (total length of 0.7 m).
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Mesosaur scapulo-coracoids from the Mangrullo Formation Konservat-Lagerstätte of Uruguay. (A) Comparison between the largest known mesosaur scapulo-coracoid (FC-DPV 2397, to the left) and progressively younger individuals ((B-E); FC-DPV 1249, 1250, 1487, and 1251, to the right) all in ventral view. Scale bar: 10 mm.</figcaption></figure>

The thickness of the big caudal rib described above (FC-DPV 1347, Figure 6B) does not seem to display a precise isometric pattern contrary to many other bones, likely due to the development of the pachyosteosclerosis condition, which, according to some studies, is associated with environmental changes, see [64]. However, the microanatomy of certain mesosaur bones suggests a relatively slow growth rate, which is similar to that observed in terrestrial ectotherms [65,66]. Finally, it was also suggested that the presence of osteosclerosis and pachyostosis is related to a return to an aquatic lifestyle as observed in extinct sloths from the Neogen of Perú [67].

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Mesosaur humeri from the Mangrullo Formation Konservat-Lagerstätte of Uruguay. (A,B) Comparison between the largest known mesosaur humeri (FC-DPV 3621, 2396) and those of progressively younger individuals ((C,D); FC-DPV 2357, 1257, 1508), in dorsal view. Scale bar: 10 mm.</figcaption></figure>

In the case of specimen FC-DPV 1347 (Figure 6B), it is possible to estimate the average size by considering the width ratio between ribs and humerus, see [61], though with a
much greater margin of error compared to what occurs in other bones. The measurements
showed that a rib of that size would correspond to a humerus between the size of specimens
FC-DPV 3621 and 2396 figured herein (see Figure 5A,B), and thus the rib would belong to a
*Mesosaurus* specimen with a size between 1.10 and 2 m in length.

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Mesosaurus caudal remains from the Mangrullo Formation Konservat-Lagerstätte of Uruguay. (A) FC-DPV 1061, fragment of the anterior region of the tail of a relatively large mesosaur, in ventral view. (B) FC-DPV 1347, isolated very large anterior caudal rib in ventral view (possibly the fourth one) displaying a size that is more than twice the already large dimension of the specimen shown in (A). Scale bar: 10 mm. Arrow points cranially.</figcaption></figure>

Reevaluation of the Isometric Relationships Found in Mesosaurs by Including Data from Verrière and Fröbisch (2022)

Our study reanalyzed isometry in mesosaurs by incorporating our results for isometry
in mesosaurs and complementing our data with those collected by [27] for mesosaur skulls,
including 95% confidence intervals (Figure 11). Notably, the results confirmed that isometric
relationships are present. This apparent contradiction with the previous findings reported
by [27] can be explained by the fact that their results were all close to isometry; however,
these authors did not consider the 95% confidence interval, which tends to include the
isometry threshold [58–60]. Therefore, these relationships should be statistically considered
isometric.
# 4. Discussion

## 4.1. *The Body Size of Mesosaurs*

Mesosaurs are often described as small to medium-sized aquatic amniotes, partly due to their elongated bodies and long tails that exceed the length of the rest of the body. Additionally, their long-snouted skull is composed of very thin cranial bones, and it bears numerous needle-like, marginal teeth. Despite the fragility of some of their cranial bones, mesosaur remains are abundant in the Gondwanan Permo-Carboniferous shale and silt deposits of Gondwana, including well-preserved, nearly complete skeletons with articulated skulls and partially preserved specimens that retain still recognizable soft tissues [28,40,44,47,68,69].

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

**Figure 7.** Bone microstructure of the largest known mesosaur bones from the Mangrullo Formation Konservat-Lagerstätte of Uruguay; evidence of precipitation of gypsum crystals over the bone surface. (A) Transversal section of FC-DPV 3621, a large humerus shown in Figure 5A. (A) Cortical area showing the precipitation of the gypsum crystals forming encrusting masses over the external surface. (B,C) Close up of the gypsum crystals. (D) Microstructure of a medium-sized radius that we assumed to belong to a subadult or a young adult individual. Note that there are no encrustations over the cortical surface. (E) SEM-EDS analysis to determine the chemical elements present and forming the encrusting external layer, identifying S and Ca, which indicate the presence of calcium sulfate (gypsum).

This exceptional preservation has led to the designation of the Mangrullo Formation of Uruguay and the Iratí Formation of Brazil as Konservat-Lagerstätten for Gondwana [47]. Furthermore, mesosaur fossils from these units include different ontogenetic stages, from unborn individuals [47] to juveniles and adults.
Therefore, mesosaur ontogeny is well-documented, with body lengths ranging from 100 or 120 mm in hatchlings to 800 or 900 mm in young adults. The average mesosaur length is approximately 700 mm [15].

Previous studies suggested that mesosaurs are semiaquatic rather than fully aquatic amniotes [41]. In addition, in recent studies on mesosaur taxonomy and paleobiology (e.g., [14,30,31,41,61]), over 1000 specimens where examined, including both small and large individuals belonging to *Mesosaurus tenuidens*. The data also suggested that, at its mature stage, mesosaurs developed more terrestrial habits.

This hypothesis seems to be supported by the herein described fossils because their bad preservation (see Figure 4A, Figure 5A,B and Figure 6) could explain the scarcity of mature individuals in the mesosaur fossil record, given that the potential of fossilization in land is low [14,41,61].

Whereas [27] studied the relationship between sedimentary environment and mesosaur size, suggesting that complete articulated juveniles are more commonly found in shallow water deposits and complete articulated adults are more frequently associated with pelagic sediments, we describe here mostly incomplete and inarticulate bones of giant adults associated with evaporitic environments. These results, however, may not be contradictory, as young adults were likely more capable of reaching deeper waters, an environment where they were better preserved, while juveniles and older adults remained closer to the shore.

Such ecological configuration could also suggest a possible collecting bias that may explain the absence of large, mature specimens in mesosaur collections elsewhere, which mostly include articulated and nearly complete individuals averaging around 800 mm, as these are the best preserved.

Moreover, large mesosaur remains have not been found in deposits from the deepest parts of the Paraná Basin, located in the south-central South American region. Instead, they have been recovered from shallower deposits, such as those cropping in northern Uruguay [49] (Figure 8).

Given the vast number of complete and partial mesosaur skeletons that have been collected over more than a century, many of which are now part of North American and European institutional collections, it seems unlikely that giant mesosaurs inhabited deep waters, as none appear to have fossilized in offshore, deep-water facies.

These few very large specimens may represent the tail end of the size distribution (Figure 9), which implies that few individuals may have reached such a large size. Their scarcity can be clearly explained by these factors. However, considering that this population is as rare as the youngest mesosaurs, why are only the latter well-preserved, nearly complete, and articulated, while giant adults are incomplete, disarticulated and badly preserved? This seems to be not only due to their rarity but also due to the environment in which they lived. Furthermore, assuming that mesosaur population sizes follow a normal distribution, as is common, the presence of mesosaurs with vertebrae over 15 mm in length highlights an under-representation of individuals with vertebrae between 10 and 15 mm, which should be more prevalent than currently observed (Figure 9). This gap in representation could result from this population being more terrestrial and therefore poorly preserved (a situation exacerbated by a collection bias against poorly preserved fossils) or from high mortality rates among mesosaurs from juvenile to young adult stages, likely due to environmental pressures, possibly associated with the drying and disappearance of the sea in which they lived.

According to their fossil record, mesosaurs seem to have suffered high mortality rates at their early ontogenetic stages, meaning hatchling, early juvenile, subadult and young adult individuals. These stages include very well-preserved specimens and are the most
represented in the mesosaur record, whereas mature, very large mesosaurs are scarce and poorly preserved. A possible collecting bias may explain the absence of the large mature specimens in the mesosaur collections that only include individuals ranging 800 mm on average because the latter are the best preserved.

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

**Figure 8.** Generalized stratigraphic section of the Upper Carboniferous-Lower Permian Mangrullo Formation (Northeastern, Uruguay), showing the various sedimentary facies and fossils, including the position of the large mesosaur bones described in this contribution and the associated gypsum crystal layers. Colour codes from standard Munsell Chart. Abbreviations: C, conglomerate; S, silt; FS, fine sandstone. Adapted from [47].
At the moment, the record of mature giant mesosaurs reaching more than 2 m in length (see Figure 2) is exclusive to the Mangrullo Formation Konservat-Lagerstätte, consisting of sediments deposited in the shallowest area of the Paraná Basin [49].

## 4.2. Could the Mesosaur Gigantism Be Explained by the Influence of the Paleogeographic Bergmann Rule?

Our analyses of body size evolution in early amniotes should ideally consider the latitude (and associated temperature gradient) where the various taxa occurred. Bergmann's rule [70] suggests that taxa found at higher latitudes are larger than their tropical, low latitudes relatives. However, Bergmann's rule has so far been convincingly demonstrated to apply only to endotherms [71], which imply that it may not fully explain the relatively large size of mature mesosaurs.

Indeed, mesosaurs were likely ectotherms, and their paleogeographic range was limited to a small part of Gondwana, within a relatively narrow paleolatitude range, suggesting that Bergmann's rule cannot account for the size differences between mesosaur populations. Nevertheless, that rule may be suggesting that mesosaur ancestors were smaller and lived in tropical low latitudes at the end of the Carboniferous period, a suggestion that may be a little speculative, but more work will be required to prove it.

## 4.3. Body Size of Early Amniotes

The suggestion of [1] that the ancestors of the early amniotes were small but experienced a rapid increase in body size after the appearance of the amniotic egg has sparked numerous studies (e.g., [8,13,72,73]).

These works highlight the significance of the origin of amniotes and the evolution of body size in this context. The implications of our findings on mesosaur size in this context (see Figures 9–11) depend partly on mesosaur taxonomic affinities, which is controversial. While “traditional” hypotheses supported by data matrices positioned them as basal amniotes or basal parareptiles [21,29,30,39], a recent study has linked them to the recumbirostran lepospodyls, particularly based on the isometric growth pattern shared by both groups, as well as by the elongated body and their semiaquatic habits [15].

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Size distribution of 2769 vertebrae from 69 specimens of *Mesosaurus tenuidens*, showing the outlying position of vertebra FC-DPV 3620 (Figure 3), marked with a diamond symbol. Based on data provided by [41].</figcaption></figure>
As the present knowledge about the growth patterns in basalmost amniotes is ex-
tremely poor, the isometric growth pattern shared by mesosaurs and recumbirostran mi-
crosaurs could be a convergent condition that does not imply relatedness, but this should
be studied.

Other not-in-depth-enough studied proposals (e.g., [52,55,57,74]) have explored the
possibility of a relationship of mesosaurs to the basal synapsids, mainly due to the presence
of a small lower temporal fenestra in their skull and anatomical similarities in the postcra-
nium. However, this hypothesis was largely dismissed by most researchers; as such, a
relationship has never been recovered in a matrix-based phylogenetic tree, and the presence
of a lower temporal fenestra is not as informative to infer relationships as was previously
believed [39,75]. A possible relationship among mesosaurs and parareptiles was already
discussed in previous papers, where both phylogenetic and comparative anatomical studies
did not support such a relationship, see [15,30,31].

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

**Figure 10.** Length and width ratios of the humerus of *Mesosaurus tenuidens*, indicating the position of the humeri (A,B,C,D) described in Figure 5. Note how far A is from the other specimens. Based on the data presented by [15].

Therefore, understanding body size evolution in early amniotes and their predecessors
is complicated and in addition to the problem evoked above, several other factors, including
potential taphonomic filters and uncertainties regarding the ontogenetic stage of various
specimens may have influenced perspectives on this debated topic [8, 11, 13–15, 73].

In two of the previously proposed taxonomic positions for mesosaurs (basal Synapsida
or basal Sauropsida), it seems that their ancestors may have been small, but recent studies,
however, suggest that while there was an evolutionary trend toward increasing body size in
reptiliomorphs [8] and synapsids, the trends are less clear in reptiles (i.e., amniotes) [8,11],
which appear to show a weaker tendency toward size increase and explored less mor-
phospace [13].
<figure><img src="image_14.png" /></figure>

**Figure 11.** Different size ratios observed in the skull of *Mesosaurus tenuidens*. (A) Length between the posterior border of the orbit and the posterior border of the skull (PBO-PBS) vs. orbital length. (B) Maximum skull width vs. orbital size. (C) Maximum skull length vs. orbital size. The measurements taken are indicated in the top left corner of each figure. Red dots: data from [15]; blue dots: data from [27]. Solid lines indicate the linear adjustments for the data; dashed lines extend the relationship to the observed size to giant mesosaurs; dotted lines indicate the confidence intervals. Red lines indicate original results of [15], and black lines resulted using data from [15] plus data of [27]. The black regression line uses the red and blue data points and is obviously dominated by the blue points. The 1A and 1C points were not considered for the linear fit. Right plots show the value of the slope (a in the isometric equation), as well as its 95% confidence interval, calculated for log-transformed data. Relationships are identified as isometric if the 95% confidence interval includes 1 (e.g., [58–60]). The position of the skulls depicted in Figure 1A,C and Figure 1E,G (FC-DPV 3622 and 3623, respectively) is marked on the graphs.

The recent discovery of reproductive strategies in mesosaurs, a finding for which specialists had waited for more than a century, represents the oldest known evidence for amniotic reproduction and to the origin of embryonic membranes.
One isolated, almost complete fetus curled inside an unmineralized shell and another smaller embryo in an earlier stage of development preserved inside the belly of a mesosaurus female, indicate embryo retention in mesosaurs and possible viviparity. However, the possibility that they laid eggs on land in an advanced stage of development cannot be yet dismissed [40].

This finding if the reproductive strategy of mesosaurs is shared by all the earliest amniotes or if mesosaurs occupy a fairly basal position within Amniota, supports the hypothesis of [76] suggesting that the amniotic egg appeared in aquatic to semiaquatic taxa that laid eggs on land (although close to the water); this is also consistent with the previously proposed theory that the amniotic embryonic membranes originated in a context of embryo retention [77–79].

It also suggests that the size of the earliest amniotes was not as small as [1,2] predicted, since the eggs and the hatchlings for the basal taxon *Mesosaurus tenuidens* do not support such hypothesis since the hatchling mesosaurs could have reached 100 to 120 mm in lenght.

In the light of the aforementioned evidence, we evaluated the possibility of performing a complementary study including an optimization of the character "average adult size" in early amniotes, based on the two "traditional" main tree topologies (i.e., that from [30,31] and that from [21,33,38]).

However, we were not convinced that such a study is timely for our paper because while we have confident data to estimate the average size of mesosaurs, there is insufficient information for the state of this character in several of the other taxa to be examined.

Instead, taking into account that we are describing here remains of a taxon for which we can know the average size of both the earliest and the mature-most ontogenetic stages, a circumstance that is meet in very few of most extinct and even also some extant taxa, such a study would require revision of the data used for the optimization of the character "average size" in the other basal amniote taxa.

Thus, to optimize the character "average size", reliable estimates of the average size of all included OTUs would be required. Our new data on mesosaurs show that the size reached by a taxon during its life cannot be assessed without a large sample size of individuals.

Therefore, we think that the body size data that we obtained for *Mesosaurus* are not comparable with what is known for most other early amniotes, and we prefer not to include an optimization. However, this amazing case of mesosaurus preservation shows that our hypotheses on the size of the earliest amniotes will require a revision.

As we have suggested a Late Carboniferous–Early Permian age for the deposits of the Mangrullo Formation containing mesosaurs, see [44,68], *Mesosaurus* will be a good support for hypotheses that argue against the claim that the earliest amniotes were very small (e.g., [8]).

### 4.4. Attritional, Mass Mortality or Both: How the Presence of Very Mature Individuals Can Modify the Models Suggested for Mesosaur Taphonomy and Environments

Although mesosaurs are famous for the preservation of a high number of almost complete and well preserved specimens, and the proof of that is the high interest demonstrated by many researchers interested in studying them [80], disarticulation of the carcasses also occurred and such bones and skeletal parts may have been removed and possibly buried more than once [81], even though weathering or fractures are very infrequent and postmortem transport may have been insignificant.

In many skeletons, the bones have suffered diagenetic dissolution and were preserved as molds that provide very detailed part and counterpart impressions of their external anatomy and in some cases of internal soft tissues [40,47]. Some taphonomic categories
were described for the mesosaurs of Uruguay [22,47,80], which were also described to be present in Brazil and Africa, as reported by [24] and [81], respectively.

*Mesosaurus tenuidens* appears to be the only tetrapod living in the Mangrullo lagoon (see Figure 8), as well as in the Iratí (Brazil), Whitehill (South Africa) and Huab (Namibia) areas [61,63]. Just like mesosaurs, Uruguayan pygocephalomorph crustaceans are also dominant and represented until now by just a single species, *Hoplita ginsburi* [68]. As in mesosaurs, several ontogenetic stages of this crustacean taxon can be recognized in the fossiliferous settings that preserve the “Mesosaur Community” [47].

Taphonomic groups found in the Mangrullo Formation include (i) articulated, well-preserved fragmentary or almost complete skeletons, including the skull, (ii) articulated well-preserved fragmentary skeletons lacking skulls, (iii) isolated well-preserved bones and molds, and (iv) bone beds formed by tri-dimensional bones along to partially articulated axial skeletons, representing the disarticulation of one, two and a maximum of five individual skeletons [22,47].

Similar categories can be observed for pygocephalomorphs, which include: (i) complete and very well-preserved specimens, (ii) complete and partially preserved dorsal carapaces, (iii) complete abdomen and tail fan, (iv) partial abdomen or isolated abdominal segments, and (v) exuvia [68].

It was suggested that both mesosaurs and pygocephalomorphs developed gregarious behaviors [40], and this is supported here based on the presence of monodominant assemblages (bone or carapace aggregations belonging to just one species), which also preserve a representation of individuals in different ontogenetic stages [15,27,61,63,68].

Early juvenile, subadults and adult mesosaur individuals have been previously identified [22,55,80,82,83], and more recently reported [27], but juvenile, subadult and adult individuals along to fetuses and hatchlings were only described from the Uruguayan Mangrullo Formation [14,15,40,41].

A high mortality rate among individuals preserved in these previously mentioned stages of development is also denoted from the assemblages, specifically showing an overrepresentation of young individuals (the most vulnerable) mixed with the adults (possible parental care), suggesting that the phenomenon affected all of the population [84]. Aggregations composed of early juvenile and young adults could perhaps be related to mortality events during the reproductive seasons.

This type of representation often co-occurs with catastrophic events that may have produced immediate mass mortality, or with short-term sudden mass death events [81]; but see also [84] that may have lead to the definitive extinction of the group.

Although very convincing, this taphonomic model does not consider very large, old individuals of the population because their existence was unknown. The material that we describe herein documents a previously missing ontogenetic stage and may support the hypothesis that catastrophic events over the basin affected the entire population.

Hypothetically, the mature individuals may have rested on the shore, only returning to water for food or they may have lived segregated from the younger populations in other aquatic environments and the fossilized skeletons may have returned to shore shortly before death, or their carcasses could have been washed ashore, possibly far from the habitat occupied by the younger members. Although some studies have considered that adult mesosaurs may have been pelagic, the compact, osteosclerotic bones seen in mesosaurs [26,63,85] typically characterize taxa that inhabit shallow waters, where bone ballast is useful [65,66,86]. Alternatively, osteosclerosis in mesosaurs may also have developed in response to hypersalinity, as apparently occurred for some Neogene marine mammals from the Paratethys [87], and which is thought to have prevailed in the mesosaur
sea [47]. These hypotheses are consistent with the scarcity of the very large individuals in the mesosaur assemblages and match with the poor preservation of these specimens.

Catastrophic events such as volcanic activity and evaporitic deposits are constrained to the uppermost section of the Mangrullo Formation that bears mesosaur remains. Attritional assemblages are also observed, and time averaging could have existed, particularly from the time of the first colonization of the basin by mesosaurs, but data on the sedimentary accumulation rates for the Mangrullo Formation are not available.

Although strong storms and tsunamis have been proposed as catastrophic events that produced sudden mass mortality of mesosaurs in Brazil [81], volcanic activity in the Paraná Basin cannot be dismissed.

The presence of 20 to 30 mm thick bentonite layers, interbedded with shale deposits, documents the incidence of such volcanic activity in the Mangrullo Formation, as in the Irati Formation in Brazil and the Whitehill deposits in South Africa [88]. These events could have produced high mortality rates among juvenile pygocephalomorphs, the main prey item of mesosaurs [69], thus altering the food chains of the aquatic ecosystem.

This can be evidenced by the high abundance and good preservation of the mesosaur and pygocephalomorph remains associated with these bentonite layers [40,55,56].

Another environmental catastrophe that has affected the Mangrullo lagoon was drought, evidenced by the abundance of fibrous gypsum crystals distributed in several layers thorough the Mangrullo Formation profile, along with bentonite levels and mesosaur skeletons and isolated bones (see Figure 8). A performed SEM study of the surface of the large mesosaur bones described herein confirms the presence of a thin layer of gypsum. Small quantifications of Fe in the EDS analysis (Figure 7) probably indicates the presence of pyrite, which is very common in some levels.

Evaporitic intrasedimentary growth of gypsum crystals and dolomitization were described in the Irati Formation of Brazil by [48], confirming geochemical analyses and sedimentological profiles from [89]. Moreover, Oelofsen [24] also described gypsum and halite crystals in the dolomitic and shale deposits of the Whitehill formations. The presence of layers of gypsum crystals in these units indicates subaerial exposure, hypersaline shallow water bodies and high evaporaton rates [45,46,49].

Xavier et al. [48] indicated that evaporites can be diagenetic and form within sediments long after their deposition. This raises the possibility that the evaporitic minerals are not strictly contemporaneous with the fossils. However, the evaporite deposition we discuss herein corresponds to specific desiccation events particularly affecting the last sediments deposited. The thinness of the evaporitic layers and their rarity are also indicators that these events were sporadic and particularly affected the last sedimentary layers where the fossils were found. If diagenetic, the phenomenon would have been widespread and persistent over time and it would have affected multiple layers or left a significant deposit, which is not the case for what we see in the Uruguayan Mangrullo Formation. This implies two things: (1) the deposition and the evaporitic events are close in time, and thus mesosaurs lived under such evaporitic conditions and were affected by them judging by the high number of skeletal remains associated with the gypsum crystal levels (Figure 8), and (2) the depositional environment already involved a shallow water body near the coast.

This connects these mesosaurs to a coastal environment rather than a deep-sea envi-
ronment, and, under this hypothesis, we can argue that giant mesosaurs found in Uruguay
are associated with a coastal marine, rather than an open marine, environment.

Droughts and volcanic eruptions may have affected the aquatic biota, thus producing mortality events where the animals died under atmospheric deterioration and strong desiccation conditions that led to the disappearance of the Irati-Whitehill sea in no more than four million years, after which it was replaced by a desert [88].
The occurrence of such an event is supported by the presence of the gypsum crystals and other evaporitic minerals and by the study of palynomorph assemblages that denote a dominance of species adapted to arid environments [90].

All this evidence is consistent with paleoclimatic hypotheses that suggest progressive aridification starting at the Upper Carboniferous and continuing into the Permian [88,91], favored by tectonic activity related to the Pangea formation.

**5. Conclusions**

In this contribution, we describe the remains of the largest known mesosaurs documented so far. The bones, consisting of two fragmentary skulls, a dorsal vertebra, one scapulocoracoid and at least two humeri, plus a proximal fragment of the tail and an isolated caudal rib, were analyzed using morphometric techniques and SEM analyses. Their sizes were compared to a large sample of the same bones of *Mesosaurus tenuidens*, representative of the body size that was previously known for this taxon.

Our analyses revealed that the size of these mesosaurs exceeds the typical size distribution previously recorded for *Mesosaurus tenuidens*, the only valid mesosaur species.

These individuals were larger than the mean by four to five times its standard deviations, with skulls measuring between 150 and 200 mm in length and a total body size between 1.5 and 2.5 m.

Additionally, comparisons between the dimensions of the humerus and the relationship between cranial features such as the antero-posterior orbital length and the length between the posterior border of the orbit and the posterior border of the skull show that the humerus and cranial proportions maintain ratios consistent with the previously observed isometric growth in smaller mesosaurs.

This indicates that the isometry in the skull and long bones of mesosaurs is largely preserved, even in the giant, mature individuals.

To explore the significance of such large mesosaurs in the population of the Mangrullo Formation, we considered the potential impact of Bergmann's rule. Although this is an important paleogeographic principle in the study of isolated populations of the same species or distinct taxa whose body size may have been affected by changes in temperature and food availability (including competition for nutrients), it does not explain the presence of such large specimens of mesosaurs in the Mangrullo Formation.

Therefore, we conclude that the great variability in body size found in mesosaurs corresponds to their ontogenetic stage and growth pattern. The previously recognized smaller body sizes likely represent assemblages dominated by early juveniles, subadults and young adults, potentially reflecting catastrophic mass mortality events that interfere with the attritional normal addition of carcasses of animals that died by varied causes.

Bentonite layers and gypsum crystals found in the Mangrullo Formation result from the occasional input of ashfalls over the lagoon derived from the increased tectonic activity related to the Pangea formation. These volcanic events combined with the observed gradual drought in the Iratí-Whitehill sea and the concomitant desertification are here suggested as the main trigger that might explain the mesosaur extinction, at least in the Uruguayan region of the Paraná Basin.

**Author Contributions:** Conceptualization, G.P., P.N.D. and M.L.; methodology, G.P. and P.N.D.; software, P.N.D.; validation, G.P., P.N.D. and M.L.; formal analysis, G.P., P.N.D. and M.L.; investigation, G.P. and P.N.D.; resources, G.P., P.N.D. and M.L.; data curation, G.P. and P.N.D.; writing—original draft preparation, G.P.; writing—review and editing, G.P., P.N.D. and M.L.; visualization, G.P., P.N.D. and M.L.; supervision, G.P.; project administration, G.P.; funding acquisition, not funding available. All authors have read and agreed to the published version of the manuscript.
**Funding:** This research received no external funding

**Data Availability Statement:** Data from vertebrae are available in: Núñez Demarco et al. (2018) [41]; Piñeiro et al., 2021 [61]; Núñez Demarco, 2022 [15]; Verrière and Fröbisch, 2022 [27]).

**Acknowledgments:** We wish to thank Magela Rodao for her highly professional assistance in the SEM studies. We are grateful to the Academic Editor Ismar de Souza Carvalho and to the two anonymous reviewers that provided critical revisions that highlighted the relevance of this work. We finally want to thank the Hastings family, owners of the El Baron Ranch, for always supporting our field work activities and for kindly being involved with the study and the protection of mesosaurs and other fossils found in their land, a precious heritage for the Uruguayan Paleontology and for elsewhere.

**Conflicts of Interest:** The authors declare no conflict of interest.

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