Graciela Piñeiro

Unusual environmental conditions preserve a Permian mesosaur-bearing Konservat-Lagerstatte from Uruguay

The environmental characterization of the Lower Permian mesosaur-bearing strata of the Mangrullo Formation (Paraná Basin, northeastern Uruguay) has been controversial. Historically, marine conditions were suggested for this unit, despite the absence of any normal marine fossils. More recently, some authors have argued for freshwater to brackish settings, inferring fluctuating environmental conditions, which would have generated abrupt changes in the composition of the communities. Mesosaurs are the only tetrapods found in this unit, and they colonized the basin at the time of highest isolation, and apparently increased salinity, coincident with a gradual global rise in aridity. An assemblage of extremely low diversity (the “mesosaur community”) developed, with mesosaur reptiles, pygocephalomorph crustaceans, and the vermiform producers of the trace fossil Chondrites as the dominant components. This community may have existed under temporary hypersaline, lagoon-like conditions, as suggested by ecological, anatomical and physiological attributes of its member taxa. This interpretation is supported by sedimentological and mineralogical features of the enclosing rocks, also seen in the correlative Brazilian Iratí and South African Whitehill formations. In the Uruguayan deposits, as well as in their Brazilian correlatives, relatively close volcanic events affected the basin. This particular environment, where bottom waters were depleted of oxygen and hypersaline, retarded decay of the carcasses, and precluded the development of bioturbating organism, and together with bacterial sealing, favoured exquisite preservation of the fossils, including soft tissues. This leads us to consider the fossil-bearing strata of the Mangrullo Formation as a Konservat-Lagerstätte, the oldest known for South America.

A NEW PROCOLOPHONOID (REPTILIA, PARAREPTILIA) FROM THE UPPER PERMIAN OF URUGUAY

Abstract Pintosaurus magnidentis gen. et sp. nov., a latest Permian procolophonoid reptile from the Buena Vista Formation of Uruguay, is described and its relationships are considered. The new taxon is characterized by the absence of a lateral maxillary depression, a premaxilla with a dorsally expanded maxillary process, a maxilla-premaxillary junction almost straight and marked by a shallow, but well-defined step that displaces the premaxillary alveolar margin slightly medially with respect to the maxillary tooth row, and the presence of a large conical, fang-like palatine tooth, which is positioned next to the suture with the vomer. The transitional nature of Pintosaurus is reflected in the retention of several plesiomorphic features that contrast with the presence of synapomorphies that are shared with procolophonids. A phylogenetic analysis suggests a basal position for Pintosaurus within Procolophonoidea, either as the sister taxon of a clade that includes Coletta and procolophonids or as the sister taxon of Coletta itself. The results of our phylogenetic analysis, together with the Permian age of Pintosaurus, are consonant with the hypothesis of a Gondwanan origin of Procolophonoidea.

FIRST BASAL SYNAPSIDS (“PELYCOSAURS”) FROM THE UPPER PERMIAN-?LOWER TRIASSIC OF URUGUAY, SOUTH AMERICA

In their monograph Review of the Pelycosauria, Romer and Price (1940), proposed that the earliest synapsids (“pelycosaurs”) were cosmopolitan, despite the observation that amniotes appeared to be restricted to the paleotropics during the Late Carboniferous and Early Permian (290–282 Ma). Romer and Price (1940) accounted for the scarcity of terrestrial tetrapods, including “pelycosaurs,” in Lower Permian beds elsewhere to the absence of coeval continental deposits beyond North America and Europe. Indeed, most workers recognized a geographical and temporal gap between Permo-Carboniferous “pelycosaurs” and therapsid synapsids. Recent research has confirmed that varanopid and caseid “pelycosaurs” were components of therapsid-dominated Late Permian faunas preserved in Russia and South-Africa (Tatarinov and Eremina, 1975; Reisz, 1986; Reisz et al., 1998; Reisz and Berman, 2001). In this note, we report the first record of basal synapsids for South America (Fig. 1) and discuss its paleogeographic and temporal implications. These remains come from outcrops assigned to the Buena Vista Formation (Goso et al., 2001; Fig. 2), which filled part of the Parana Basin during Late Permian and probably Early Triassic (Bossi and Navarro, 1991). The Parana Basin is one of the major sedimentary basins of Gondwana, covering an area of nearly 1.5 million km2, in Brazil, Argentina, Paraguay and Uruguay (Franca et al., 1995; Zalan et al., 1990). At the beginning of basin development the Devonian marine and brackish conditions prevailed, but subsequently, during the Late Permian, there was a gradual change to continental sedimentation and the deposition of fossil-bearing red beds. Buena Vista Formation is characterized by reddish fine sandstone interbedded with lenticular clay layers and intraformational conglomerates (Fig. 2), (Bossi and Navarro, 1991; Goso et al., 2001). The paleoenvironment is related to the final withdrawal of the sea, and the upper part of this …

Optimal swimming speed estimates in the Early Permian mesosaurid Mesosaurus tenuidens (Gervais 1865) from Uruguay

Abstract Mesosaurid biology has been subject of continuous debate since the first description of Mesosaurus tenuidens by Paul Gervais in 1867. Controversy surrounds their environmental and feeding preferences. Most studies suggested that mesosaurids were marine reptiles and perhaps piscivorous predators. Nonetheless, recent work suggests that they inhabited a salty, eventually hypersaline shallow epicontinental sea and that pygocephalomorph crustaceans were their preferred food item. Here, we present results of the first biomechanical study about optimal swimming capabilities in Mesosaurus tenuidens, which along with the comparative analysis of the limb morphology support the hypothesis that these animals were slow swimmers living in shallow waters. The study is based on the revision of several almost complete mesosaurid specimens and isolated, well-preserved bones housed in palaeontological collections in Uruguay, Brazil, France and Germany. We studied the relative size and proportions of the bones, as well as their morphology and anatomical position to produce a three-dimensional reconstruction of the original appearance of an undamaged, complete skeleton. Our results suggest a fairly low optimal swimming speed for mesosaurids, which is consistent with capture of fairly slow prey like pygocephalomorphs, possibly by filter-feeding, rather than by active pursuit of fast prey.

A Reassessment of the Taxonomic Position of Mesosaurs, and a Surprising Phylogeny of Early Amniotes

We reassess the phylogenetic position of mesosaurs by using a data matrix that is updated and slightly expanded from a matrix that the first author published in 1995 with his former thesis advisor. The revised matrix, which incorporates anatomical information published in the last twenty years and observations on several mesosaur specimens (mostly from Uruguay) includes seventeen terminal taxa and 129 characters (four more taxa and five more characters than the original matrix from 1995). The new matrix also differs by incorporating more ordered characters (all morphoclines were ordered). Parsimony analyses in PAUP 4 using the branch and bound algorithm show that the new matrix supports a position of mesosaurs at the very base of Sauropsida, as suggested by the first author in 1995. The exclusion of mesosaurs from a less inclusive clade of sauropsids is supported by a Bremer (Decay) index of 4 and a bootstrap frequency of 66%, both of which suggest that this result is moderately robust. The most parsimonious trees include some unexpected results, such as placing the anapsid reptile Paleothyris near the base of diapsids, and all of parareptiles as the sister-group of younginiforms (the most crownward diapsids included in the analyses). Turtles are placed among parareptiles, as the sister-group of pareiasaurs (and in diapsids, given that parareptiles are nested within diapsids). This unexpected result offers a potential solution to the long-lasting controversy about the position of turtles because previous studies viewed a position among diapsids and among parareptiles as mutually exclusive alternatives.

The Feeding Habits of Mesosauridae

Mesosauridae comprises the oldest known aquatic amniotes which lived in Gondwana during the Early Permian. Previous work in the Uruguayan mesosaur-bearing Mangrullo Formation suggested that mesosaurids lived in an inland water body, inferred as moderately hypersaline, with exceptional preservational conditions that justified describing these strata as a Fossil-Lagerstatte. Exquisitely preserved articulated mesosaur skeletons, including gastric content and associated coprolites, from the Brazilian Irati Formation in the State of Goias (central-western Brazil) indicate excellent conditions of preservation, extending the Konservat-Lagerstatte designation to both units in the Parana Basin. The near-absence of more resistant fossil remains, like actinopterygian and temnospondyl bones, demonstrates the faunistic poverty of the mesosaur-bearing “salty sea”. Our studies of the alimentary habits of mesosaurids through the use of stereoscopic microscopy, light and electronic microscopy, and X-ray diffractometry suggest that the diet of mesosaurids was predominantly composed of pygocephalomorph crustaceans (possibly not exceeding 20 mm in length). However, the presence of bones and bone fragments of small mesosaurs in the gastric content, cololites, coprolites, and possible regurgitalites may also indicate cannibalistic and/or scavenging habits. Cannibalism is relatively common among vertebrates, particularly during conditions of environmental stress, like food shortage. Likewise, the apparent abundance of pygocephalomorph crustacean fossils in the Irati and Mangrullo Formations, outside and within the studied gastric, cololite, and coprolite contents, might have to do with environmental stress possibly caused by volcanic activity, in particular ash spread into the basin during the Early Permian. In this context, casual necrophagy on the dead bodies of small mesosaurs and large pygocephalomorphs might have been an alternative alimentary behavior adopted for survival in mesosaurs.

The ontogenetic transformation of the mesosaurid tarsus: a contribution to the origin of the primitive amniotic astragalus

The hypotheses about the origin of the primitive amniotic tarsus are very speculative. Early studies argued that the origin of the astragalus, one of the largest proximal bones in the tarsus of basal amniotes, was produced by either the fusion of two, three, or even four of the original tarsal bones, the intermedium, the tibiale and the proximal centralia (c4 and c3), or that the intermedium alone transforms into the primitive astragalus. More recent studies have shown that the structure of the tarsus in Captorhinus supports the former hypothesis about a fusion of the intermedium, the tibiale, the proximal centrale (c4) and eventually c3, producing a purportedly multipartite structure of the amniotic astragalus, but the issue remained contentious. Very well preserved tarsi of the Early Permian aquatic amniote Mesosaurus tenuidens Gervais, 1864–1865, which represent the most complete ontogenetic succession known for a basal amniote (the other exceptional one is provided by the Late Permian diapsid Hovasaurus boulei Piveteau, 1926), suggest that there is more than one ossification center for the astragalus and that these fuse during late embryonic stages or maybe early after birth. A non-hatched Mesosaurus in an advanced stage of development shows that the tarsus is represented by a single bone, most probably the astragalus, which seems to be formed by the suturing of three bones, here interpreted as being the intermedium, the tibiale, probably already integrated to the c4 in an earlier stage of the development, and the c3. An amniote-like tarsal structure is observed in very basal Carboniferous and Permian tetrapods such as Proterogyrinus, Gephyrostegus, the diadectids Diadectes and Orobates, some microsaurs like Tuditanus and Pantylus and possibly Westlothiana, taxa that were all considered as true amniotes in their original descriptions. Therefore, the structure of the amniotic tarsus, including the configuration of the proximal series formed by the astragalus and the calcaneum, typically a pair of enlarged bones, could have been established well before the first recognized amniote walked on Earth. Accordingly, the tarsus of these taxa does not constitute specialized convergences that appeared in unrelated groups, they might be instead, part of a transformation series that involves taxa closely related to the early amniotes as some hypotheses have suggested.

A Reassessment of the Taxonomic Position of Mesosaurs Based on Two Data Matrices

The Early Permian mesosaurs are the oldest known primarily aquatic amniotes. Despite the interest that they have generated over time, their affinities remain controversial. Recently, two hypotheses have been supported, in which mesosaurs are either the sister-group of all other sauropsids, or the sister-group of other parareptiles. We recently upheld the former hypothesis, but in the latest study on mesosaur affinities, MacDougall et al. published a study highly critical of our work, while upholding the hypothesis that mesosaurs are basal parareptiles. We expect that the debate about mesosaur affinities will continue in the foreseeable future, but we wish to respond to the two central comments published by MacDougall et al. in 2018, who argue that variability in the temporal fenestration of early sauropsids, combined with the omission of several recently-described parareptile taxa, explain the differences in topologies between their study and ours. Reanalyzing our data matrix and theirs without characters linked with temporal fenestration, and removing from their matrix the parareptile taxa that they added (and that we omitted) does not alter the resulting topologies. Thus, their main conclusions are false; the differences in taxonomic position of mesosaurs must result from character choice and scoring differences.

Was Mesosaurus a fully aquatic reptile

Mesosaurs have been considered strictly aquatic animals. Their adaptations to the aquatic environment are well known and include putative viviparity as the reproductive strategy along to the presence of several skeletal characters such as a long, laterally compressed tail, long limbs, particularly the foot larger than the manus, pachyostotic bones, mainly in the ribs, and osteosclerosis in the humerus. They also possessed non-coosified girdle bones and incompletely ossified epiphyses, although there could be an early fusion of the front girdle bones to form the scapulocoracoid in some specimens. Some of these features, however, are shared by most of the basal tetrapods which are considered as semiaquatic and even terrestrial. The study of vertebral column and limbs provides essential clues about the locomotor system and the lifestyle of early amniotes. In this study, we have found that the variation of the vertebral centrum length along the axial skeleton of Mesosaurus tenuidens fits better with a semi-aquatic morphometric pattern, as shown by comparisons with other extinct and extant taxa. Whereas well-preserved mesosaur skeletons are mostly represented by juveniles and young adults that inhabited aquatic environments, more mature individuals might hypothetically have spent time on land. This is tentatively pointed out by taphonomic factors such as the scarce representation and poor preservation of remains of mature individuals in the fossiliferous levels, and also by anatomy of the appendicular bones, and particularly the strongly ossified epiphyses and tarsus.