The armoured dissorophid Cacops from the Early Permian of Oklahoma and the exploitation of the terrestrial realm by amphibians

New specimens of the early Permian apex predator Varanops brevirostris at Richards Spur, Oklahoma, with histological information about its growth pattern

An articulated pelvic region and additional isolated material of Varanops brevirostris, which are indistinguishable from those of the generotype from the Cacops bonebed, demonstrate the presence of this large varanopid at the Richards Spur locality. The articulated specimen includes lumbar, sacral, and anterior caudal vertebrae, partial pelvis, femur, and proximal part of tibia, confirming the autapomorphies previously suggested for this species. These include the presence of distinct blade-like shapes of the neural spines in the sacral region, the presence of deeply excavated pubis, and the presence of a distinct transverse ridge on the ventral surface of the femur distal to the intertrochanteric fossa. It has also been found that the transverse ridges and grooves become larger during ontogeny since the juvenile specimen did not exhibit a well-developed ridge. Histological analysis of isolated limb bones and neutron computed tomography (nCT) of the articulated specimen indicate that the latter likely belonged to an adult individual. This is in contrast to the other varanopid at Richards Spur, the significantly smaller, more gracile predator Mesenosaurus efremovi, which also shows the presence of growth lines and the external fundamental system with an estimated minimum age of fourteen.

Revision of the Late Triassic metoposaurid “Metoposaurus” bakeri (Amphibia: Temnospondyli) from Texas, USA and a phylogenetic analysis of the Metoposauridae

Metoposaurids are a clade of large-bodied temnospondyls commonly found in non-marine Late Triassic deposits across northern Pangea. Three taxa are known from North America: Anaschisma browni, Apachesaurus gregorii, and “Metoposaurus” bakeri. While the osteology of most metoposaurids has been recently revised, that of a few taxa, including “Metoposaurus” bakeri remains poorly characterized. This taxon was formally described in 1931 as “Buettneria bakeri,” and its taxonomy has remained in flux ever since then. “Metoposaurus” bakeri is the earliest appearing metoposaurid in North America (Carnian of Texas), and Metoposaurus has frequently been utilized as an index taxon of the Otischalkian estimated holochron (‘land vertebrate faunachron’) and for biostratigraphic correlations with other geographic regions. The taxonomy of this species is therefore relevant for both taxonomic experts and biostratigraphers. Here we redescribe all material from the type locality of “M.” bakeri, the Elkins Place bone bed, and perform a phylogenetic analysis using a revised matrix assembled from several previous studies. Anatomical comparisons and phylogenetic analyses do not support placement in either Metoposaurus, a taxon otherwise only found in Europe, or Anaschisma, the only other large-bodied taxon from North America. Therefore, we erect a new genus, Buettnererpeton gen. nov., to accommodate this species. Metoposaurus is consequently absent from North America, and this genus cannot be used in global biostratigraphy. Phylogenetic analyses provide evidence that the phylogeny of the Metoposauridae remains extremely labile, with drastic differences in topological resolution and structure being linked to just a handful of characters and scores. Metoposaurids’ morphological conservatism and the increased recognition of intraspecific variation thus continue to be major confounds to elucidating the evolutionary history of this clade.

Modeling Skull Network Integrity at the Dawn of Amniote Diversification With Considerations on Functional Morphology and Fossil Jaw Muscle Reconstructions

One of the major questions in evolutionary vertebrate morphology is the origin and meaning of temporal skull openings in land vertebrates. Partly or fully surrounded by bones, one, two, or even three openings may evolve behind the orbit, within the ancestrally fully roofed anapsid (scutal) skull. At least ten different morphotypes can be distinguished in tetrapods with many modifications and transitions in more crownward representatives. A number of potential factors driving the emergence and differentiation of temporal openings have been proposed in the literature, but only today are proper analytical tools available to conduct traceable tests for the functional morphology underlying temporal skull constructions. In the present study, we examined the anatomical network in the skull of one representative of early amniotes, †Captorhinus aguti, which ancestrally exhibits an anapsid skull. The resulting skull modularity revealed a complex partitioning of the temporal region indicating, in its intersections, the candidate positions for potential infratemporal openings. The framework of †C. aguti was then taken as a template to model a series of potential temporal skull morphotypes in order to understand how skull openings might influence the modular composition of the amniote skull in general. We show that the original pattern of skull modularity (†C. aguti) experiences comprehensive changes by introducing one or two temporal openings in different combinations and in different places. The resulting modules in each skull model are interpreted in regard to the feeding behavior of amniotes that exhibit(ed) the respective skull morphotypes. An important finding is the alternative incorporation of the jugal and palate to different modules enforcing the importance of an integrated view on skull evolution: the temporal region cannot be understood without considering palatal anatomy. Finally, we discuss how to better reconstruct relative jaw muscle compositions in fossils by considering the modularity of the skull network. These considerations might be relevant for future biomechanical studies on skull evolution.

Returning to the roots: resolution, reproducibility, and robusticity in the phylogenetic inference of Dissorophidae (Amphibia: Temnospondyli)

The phylogenetic relationships of most Paleozoic tetrapod clades remain poorly resolved, which is variably attributed to a lack of study, the limitations of inference from phenotypic data, and constant revision of best practices. While refinement of phylogenetic methods continues to be important, any phylogenetic analysis is inherently constrained by the underlying dataset that it analyzes. Therefore, it becomes equally important to assess the accuracy of these datasets, especially when a select few are repeatedly propagated. While repeat analyses of these datasets may appear to constitute a working consensus, they are not in fact independent, and it becomes especially important to evaluate the accuracy of these datasets in order to assess whether a seeming consensus is robust. Here I address the phylogeny of the Dissorophidae, a speciose clade of Paleozoic temnospondyls. This group is an ideal case study among temnospondyls for exploring phylogenetic methods and datasets because it has been extensively studied (eight phylogenetic studies to date) but with most (six studies) using a single matrix that has been propagated with very little modification. In spite of the conserved nature of the matrix, dissorophid studies have produced anything but a conserved topology. Therefore, I analyzed an independently designed matrix, which recovered less resolution and some disparate nodes compared to previous studies. In order to reconcile these differences, I carefully examined previous matrices and analyses. While some differences are a matter of personal preference (e.g., analytical software), others relate to discrepancies with respect to what are currently considered as best practices. The most concerning discovery was the identification of pervasive dubious scorings that extend back to the origins of the widely propagated matrix. These include scores for skeletal features that are entirely unknown in a given taxon (e.g., postcrania in Cacops woehri) and characters for which there appear to be unstated working assumptions to scoring that are incompatible with the character definitions (e.g., scoring of taxa with incomplete skulls for characters based on skull length). Correction of these scores and other pervasive errors recovered a distinctly less resolved topology than previous studies, more in agreement with my own matrix. This suggests that previous analyses may have been compromised, and that the only real consensus of dissorophid phylogeny is the lack of one.

Early amphibians evolved distinct vertebrae for habitat invasions

Living tetrapods owe their existence to a critical moment 360-340 million years ago when their ancestors walked on land. Vertebrae are central to locomotion, yet systematic testing of correlations between vertebral form and terrestriality and subsequent reinvasions of aquatic habitats is lacking, obscuring our understanding of movement capabilities in early tetrapods. Here, we quantified vertebral shape across a diverse group of Paleozoic amphibians (Temnospondyli) encompassing different habitats and nearly the full range of early tetrapod vertebral shapes. We demonstrate that temnospondyls were likely ancestrally terrestrial and had several early reinvasions of aquatic habitats. We find a greater diversity in temnospondyl vertebrae than previously known. We also overturn long-held hypotheses centered on weight-bearing, showing that neural arch features, including muscle attachment, were plastic across the water-land divide and do not provide a clear signal of habitat preferences. In contrast, intercentra traits were critical, with temnospondyls repeatedly converging on distinct forms in terrestrial and aquatic taxa, with little overlap between. Through our geometric morphometric study, we have been able to document associations between vertebral shape and environmental preferences in Paleozoic tetrapods and to reveal morphological constraints imposed by vertebrae to locomotion, independent of ancestry.

Morphology of the temporal skull region in tetrapods: research history, functional explanations, and a new comprehensive classification scheme

The morphology of the temporal region in the tetrapod skull traditionally has been a widely discussed feature of vertebrate anatomy. The evolution of different temporal openings in Amniota (mammals, birds, and reptiles), Lissamphibia (frogs, salamanders, and caecilians), and several extinct tetrapod groups has sparked debates on the phylogenetic, developmental, and functional background of this region in the tetrapod skull. This led most famously to the erection of different amniote taxa based on the number and position of temporal fenestrae in their skulls. However, most of these taxa are no longer recognised to represent natural groupings and the morphology of the temporal region is not necessarily an adequate trait for use in the reconstruction of amniote phylogenies. Yet, new fossil finds, most notably of parareptiles and stem-turtles, as well as modern embryological and biomechanical studies continue to provide new insights into the morphological diversity of the temporal region. Here, we introduce a novel comprehensive classification scheme for the various temporal morphotypes in all Tetrapoda that is independent of phylogeny and previous terminology and may facilitate morphological comparisons in future studies. We then review the history of research on the temporal region in the tetrapod skull. We document how, from the early 19th century with the first recognition of differences in the temporal region to the first proposals of phylogenetic relationships and their assessment over the centuries, the phylogenetic perspective on the temporal region has developed, and we highlight the controversies that still remain. We also compare the different functional and developmental drivers proposed for the observed morphological diversity and how the effects of internal and external factors on the structure of the tetrapod skull have been interpreted.

Size matters: the effects of ontogenetic disparity on the phylogeny of Trematopidae (Amphibia: Temnospondyli)

Trematopids are a clade of terrestrial Permo-Carboniferous temnospondyl amphibians. The intrarelationships of this clade are poorly known. This is largely attributable to a substantial disparity in size between type specimens, which range from the small-bodied lectotype of Mattauschia laticeps (< 4 cm skull length) to the large-bodied holotype of Acheloma cumminsi (> 15 cm skull length). Inferred correlation of size disparity with ontogenetic disparity has led previous workers either to omit taxa in phylogenetic analyses or to forgo an analysis altogether. Here, I take a specimen-level approach and multiple subsampling permutations to explore the phylogeny of the Trematopidae as a case study for assessing the effects of ontogenetic disparity on phylogenetic reconstruction in temnospondyls. The various analyses provide evidence that ontogenetic disparity confounds the phylogenetic inference of trematopids but without a directional bias. Tree topologies of most permutations are poorly resolved and weakly supported, reflecting character conflict that results from the inability of the analyses to differentiate retained plesiomorphies from juvenile features. These findings urge caution in the interpretation of phylogenetic analyses for which ontogenetic disparity exists, but is unaccounted for, and provide a strong impetus for more directed exploration of the interplay of ontogeny and phylogeny across Temnospondyli.

A redescription of the late Carboniferous trematopid Actiobates peabodyi from Garnett, Kansas

Dissorophoids are a diverse clade of predominantly Permo-Carboniferous temnospondyls with a wide geographic distribution and broad ecological diversity. Each of the various dissorophoid clades first appears in the late Carboniferous, but their records are relatively sparse and fragmentary compared to those of the early Permian when dissorophoids reach their peak diversity and distribution, particularly in terrestrial environments where they are by far the most taxonomically diverse clade of non-amniote tetrapods. This provides an impetus for further study of the late Carboniferous terrestrial dissorophoids in order to contextualize the early stages in the clade's radiation into terrestrial ecosystems. Here we present a redescription of the late Carboniferous trematopid Actiobates peabodyi from Kansas, USA, which is represented by a nearly complete skeleton and which represents the earliest occurrence of trematopids in North America. Only the skull was previously described, and the taxon has been largely overlooked in the context of early terrestrial dissorophoid evolution. Here, we provide an updated cranial description, the first postcranial description, and a discussion of the position of A. peabodyi in the context of olsoniform evolution. Our most significant finding is the characterization of postcranial anatomy that is highly similar to that of later trematopid taxa. This high degree of conservatism indicates that the earliest trematopids were already well adapted for terrestrial environments, and post-Carboniferous radiations of olsoniforms may be attributed to an expansion of the dryland terrestrial environments in which these taxa already thrived, rather than to novel acquisition of adaptive features later in the clade's evolution.

Palaeophysiology of pH regulation in tetrapods

The involvement of mineralized tissues in acid-base homeostasis was likely important in the evolution of terrestrial vertebrates. Extant reptiles encounter hypercapnia when submerged in water, but early tetrapods may have experienced hypercapnia on land due to their inefficient mode of lung ventilation (likely buccal pumping, as in extant amphibians). Extant amphibians rely on cutaneous carbon dioxide elimination on land, but early tetrapods were considerably larger forms, with an unfavourable surface area to volume ratio for such activity, and evidence of a thick integument. Consequently, they would have been at risk of acidosis on land, while many of them retained internal gills and would not have had a problem eliminating carbon dioxide in water. In extant tetrapods, dermal bone can function to buffer the blood during acidosis by releasing calcium and magnesium carbonates. This review explores the possible mechanisms of acid-base regulation in tetrapod evolution, focusing on heavily armoured, basal tetrapods of the Permo-Carboniferous, especially the physiological challenges associated with the transition to air-breathing, body size and the adoption of active lifestyles. We also consider the possible functions of dermal armour in later tetrapods, such as Triassic archosaurs, inferring palaeophysiology from both fossil record evidence and phylogenetic patterns, and propose a new hypothesis relating the archosaurian origins of the four-chambered heart and high systemic blood pressures to the perfusion of the osteoderms. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Retention of fish-like odontode overgrowth in Permian tetrapod dentition supports outside-in theory of tooth origins

Teeth are often thought of as structures that line the margins of the mouth; however, tooth-like structures called odontodes are commonly found on the dermal bones of many Palaeozoic vertebrates including early jawless fishes. 'Odontode' is a generalized term for all tooth-like dentine structures that have homologous tissues and development. This definition includes true teeth and the odontodes of early 'fishes', which have been recently examined to gain new insights into the still unresolved origin of teeth. Two leading hypotheses are frequently referenced in this debate: the 'outside-in' hypothesis, which posits that dermal odontodes evolutionarily migrate into the oral cavity, and the 'inside-out' hypothesis, which posits that teeth originated in the oropharyngeal cavity and then moved outwards into the oral cavity. Here, we show that, unlike the well-known one-to-one replacement patterns of marginal dentition, the palatal dentition of the early Permian tetrapods, including the dissorophoid amphibian Cacops and the early reptile Captorhinus, is overgrown by a new layer of bone to which the newest teeth are then attached. This same overgrowth pattern has been well documented in dermal and oral odontodes (i.e. teeth) of early fishes. We propose that this pattern represents the primitive condition for vertebrates and may even predate the origin of jaws. Therefore, this pattern crosses the fish-tetrapod transition, and the retention of this ancestral pattern in the palatal dentition of early terrestrial tetrapods provides strong support for the 'outside-in' hypothesis of tooth origins.

A small caseid synapsid, Arisierpeton simplex gen. et sp. nov., from the early Permian of Oklahoma, with a discussion of synapsid diversity at the classic Richards Spur locality

The fossil record of caseids, a clade of faunivorous to large herbivorous Permian synapsids, is unusual in having a poorly documented history. Although Kungurian caseids are common in the well-known continental deposits of North America, and the fossil record of the group extends into the middle Permian (Guadalupian), with the presence of the large caseid Ennatosaurus in the Mezen Basin faunal assemblage, only two other occurrences are known in older Permian age sediments. One is an undescribed caseid from the Bromacker Quarry in Germany, and the second is Oromycter from the lower Permian of Richards Spur, Oklahoma. The former is known from several articulated skeletons, but the latter is known only from a handful of skeletal elements, including elements of the snout and lower jaw, some phalanges, and a few vertebrae. Here the fragmentary tooth bearing elements and dorsal vertebrae of another small caseid from Richards Spur are described, with a discussion of its significance in the context of caseid evolution, and the continuously expanding faunal list and taxic diversity at this locality.

Caudal autotomy as anti-predatory behaviour in Palaeozoic reptiles

Many lizards can drop a portion of their tail in response to an attack by a predator, a behaviour known as caudal autotomy. The capacity for intravertebral autotomy among modern reptiles suggests that it evolved in the lepidosaur branch of reptilian evolution, because no such vertebral features are known in turtles or crocodilians. Here we present the first detailed evidence of the oldest known case of caudal autotomy, found only among members of the Early Permian captorhinids, a group of ancient reptiles that diversified extensively and gained a near global distribution before the end-Permian  mass extinction event of the Palaeozoic. Histological and SEM evidence show that these early reptiles were the first amniotes that could autotomize their tails, likely as an anti-predatory behaviour. As in modern iguanid lizards, smaller captorhinids were able to drop their tails as juveniles, presumably as a mechanism to evade a predator, whereas larger individuals may have gradually lost this ability. Caudal autotomy in captorhinid reptiles highlights the antiquity of this anti-predator behaviour in a small member of a terrestrial community composed predominantly of larger amphibian and synapsid predators.

Histological characterization of denticulate palatal plates in an Early Permian dissorophoid

Denticles are small, tooth-like protrusions that are commonly found on the palate of early tetrapods. Despite their widespread taxonomic occurrence and similar external morphology to marginal teeth, it has not been rigorously tested whether denticles are structurally homologous to true teeth with features such as a pulp cavity, dentine, and enamel, or if they are bony, tooth-like protrusions. Additionally, the denticles are known to occur not only on the palatal bones but also on a mosaic of small palatal plates that is thought to have covered the interpterygoid vacuities of temnospondyls through implantation in a soft tissue covering; however, these plates have never been examined beyond a simple description of their position and external morphology. Accordingly, we performed a histological analysis of these denticulate palatal plates in a dissorophoid temnospondyl in order to characterize their microanatomy and histology. The dentition on these palatal plates has been found to be homologous with true teeth on the basis of both external morphology and histological data through the identification of features such as enamel and a pulp cavity surrounded by dentine. In addition, patterns of tooth replacement and ankylosis support the hypothesis of structural homology between these tiny teeth on the palatal plates and the much larger marginal dentition. We also provide the first histological characterization of the palatal plates, including documentation of abundant Sharpey’s fibres that provide a direct line of evidence to support the hypothesis of soft tissue implantation. Finally, we conducted a survey of the literature to determine the taxonomic distribution of these plates within Temnospondyli, providing a broader context for the presence of palatal plates and illustrating the importance of maintaining consistency in nomenclature.

Palate anatomy and morphofunctional aspects of interpterygoid vacuities in temnospondyl cranial evolution

Temnospondyls were the morphologically and taxonomically most diverse group of early tetrapods with a near-global distribution during the Palaeozoic and Mesozoic. Members of this group occupied a range of different habitats (aquatic, amphibious, terrestrial), reflected by large morphological disparity of the cranium throughout their evolutionary history. A diagnostic feature of temnospondyls is the presence of an open palate with large interpterygoid vacuities, in contrast to the closed palate of most other early tetrapods and their fish-like relatives. Although the function of the interpterygoid vacuities has been discussed in the past, no quantitative studies have been performed to assess their biomechanical significance. Here, we applied finite element analysis, to test the possibility that the interpterygoid vacuities served for stress distribution during contraction of the jaw closing musculature. Different original and theoretical skull models, in which the vacuities differed in size or were completely absent, were compared for their mechanical performance. Our results demonstrate that palatal morphology played a considerable role in cranial biomechanics of temnospondyls. The presence of large cranial vacuities were found to offer the dual benefit of providing additional muscle attachment areas and allowing for more effective force transmission and thus an increase in bite force without compromising cranial stability.

Vertebral Development in Paleozoic and Mesozoic Tetrapods Revealed by Paleohistological Data

Basal tetrapods display a wide spectrum of vertebral centrum morphologies that can be used to distinguish different tetrapod groups. The vertebral types range from multipartite centra in stem-tetrapods, temnospondyls, and seymouriamorphs up to monospondylous centra in lepospondyls and have been drawn upon for reconstructing major evolutionary trends in tetrapods that are now considered textbook knowledge. Two modes of vertebral formation have been postulated: the multipartite vertebrae formed first as cartilaginous elements with subsequent ossification. The monospondylous centrum, in contrast, was formed by direct ossification without a cartilaginous precursor. This study describes centrum morphogenesis in basal tetrapods for the first time, based on bone histology. Our results show that the intercentra of the investigated stem-tetrapods consist of a small band of periosteal bone and a dense network of endochondral bone. In stereospondyl temnospondyls, high amounts of calcified cartilage are preserved in the endochondral trabeculae. Notably, the periosteal region is thickened and highly vascularized in the plagiosaurid stereospondyls. Among “microsaur” lepospondyls, the thickened periosteal region is composed of compact bone and the notochordal canal is surrounded by large cell lacunae. In nectridean lepospondyls, the periosteal region has a spongy structure with large intertrabecular spaces, whereas the endochondral region has a highly cancellous structure. Our observations indicate that regardless of whether multipartite or monospondylous, the centra of basal tetrapods display first endochondral and subsequently periosteal ossification. A high interspecific variability is observed in growth rate, organization, and initiation of periosteal ossification. Moreover, vertebral development and structure reflect different lifestyles. The bottom-dwelling Plagiosauridae increase their skeletal mass by hyperplasy of the periosteal region. In nectrideans, the skeletal mass decreases, as the microstructure is spongy and lightly built. Additionally, we observed that vertebral structure is influenced by miniaturization in some groups. The phylogenetic information that can be drawn from vertebral development, however, is limited.

A new Early Permian reptile and its significance in early diapsid evolution

The initial stages of evolution of Diapsida (the large clade that includes not only snakes, lizards, crocodiles and birds, but also dinosaurs and numerous other extinct taxa) is clouded by an exceedingly poor Palaeozoic fossil record. Previous studies had indicated a 38 Myr gap between the first appearance of the oldest diapsid clade (Araeoscelidia), ca 304 million years ago (Ma), and that of its sister group in the Middle Permian (ca 266 Ma). Two new reptile skulls from the Richards Spur locality, Lower Permian of Oklahoma, represent a new diapsid reptile: Orovenator mayorum n. gen. et sp. A phylogenetic analysis identifies O. mayorum as the oldest and most basal member of the araeoscelidian sister group. As Richards Spur has recently been dated to 289 Ma, the new diapsid neatly spans the above gap by appearing 15 Myr after the origin of Diapsida. The presence of O. mayorum at Richards Spur, which records a diverse upland fauna, suggests that initial stages in the evolution of non-araeoscelidian diapsids may have been tied to upland environments. This hypothesis is consonant with the overall scant record for non-araeoscelidian diapsids during the Permian Period, when the well-known terrestrial vertebrate communities are preserved almost exclusively in lowland deltaic, flood plain and lacustrine sedimentary rocks.