Osteoderm microstructure indicates the presence of a crocodylian‐like trunk bracing system in a group of armoured basal tetrapods

Buchwitz, M., Witzmann, F., Voigt, S. and Golubev, V. 2012. Osteoderm microstructure indicates the presence of a crocodylian‐like trunk bracing system in a group of armoured basal tetrapods. —Acta Zoologica (Stockholm) 93 : 260–280. The microstructure of dorsal osteoderms referred to the chroniosuchid taxa Chroniosuchus, Chroniosaurus, Madygenerpeton and cf. Uralerpeton is compared to existing data on the bystrowianid chroniosuchian Bystrowiella and further tetrapods. Chroniosuchid osteoderms are marked by thin internal and relatively thick external cortices that consist of lowly vascularised parallel‐fibred bone. They are structured by growth marks and, in case of Madygenerpeton, by lines of arrested growth. The cancellous middle region is marked by a high degree of remodelling and a primary bone matrix of parallel‐fibred bone that may include domains of interwoven structural fibres. Whereas the convergence of Bystrowiella and chroniosuchid osteoderms is not confirmed by our observations, the internal cortex of the latter displays a significant peculiarity: It contains distinct bundles of shallowly dipping Sharpey’s fibres with a cranio‐ or caudoventral orientation. We interpret this feature as indicative for the attachment of epaxial muscles which spanned several vertebral segments between the medioventral surface of the osteoderms and the transversal processes of the thoracic vertebrae. This finding endorses the hypothesis that the chroniosuchid osteoderm series was part of a crocodylian‐like trunk bracing system that supported terrestrial locomotion. According to the measured range of osteoderm bone compactness, some chroniosuchian species may have had a more aquatic lifestyle than others.     

Palaeohistology and external microanatomy of rauisuchian osteoderms (Archosauria: Pseudosuchia)

Abstract: The presence of postcranial dermal armour is plesiomorphic for Archosauria. Here, we survey the external microanatomy and histology of postcranial osteoderms (i.e. dorsal paramedian and caudal osteoderms) of rauisuchians, a widely distributed assemblage of extinct predatory pseudosuchians from the Triassic. The osteoderms of eight rauisuchian taxa were found to be rather compact bones, which usually lack significant bone remodelling or large areas of cancellous bone. The presence of highly vascularized woven or fibrolamellar bone tissue deposited in the core areas indicates higher growth rates during earlier life stages, whereas a more compact parallel‐fibred bone matrix indicates reduced growth rates in later development. This pattern of change corroborates earlier studies on long bone histology. With the exception of a bone tissue found in the sample of Batrachotomus kupferzellensis, which might be the result of metaplastic ossification, the general mode of skeletogenesis is comparable with intramembraneous ossification. The lack of cancellous bone tissue and remodelling processes associated with bone ornamentation, as well as the predominantly intramembraneous mode of ossification, indicates that rauisuchian osteoderm formation differs profoundly from that of the osteoderms of the only extant pseudosuchian lineage, the crocodylians.     

Osteoderm histology of Proterochampsia and Doswelliidae (Reptilia: Archosauriformes) and their evolutionary and paleobiological implications

Postcranial osteoderms are commonly developed in the major lineages of Archosauriformes, including forms such as proterochampsids and doswelliids. Here, we survey the histology of osteoderms of the doswelliids Archeopelta arborensis and Tarjadia ruthae, and the proterochampsids Chanaresuchus bonapartei and Pseudochampsa ischigualastensis to understand better the morphogenesis of these skeletal elements. Whereas, the Doswelliid osteoderms possess a trilaminar organization, in which two cortices (external and basal) can be differentiated from an internal core of cancellous bone, these elements are compact structures in proterochampsids. The osteoderms of P. ischigualastensis are avascular and they consist entirely of parallel‐fibered bone. Conversely, the osteoderms of C. bonapartei are well vascularized structures composed of zones of woven‐fibered bone and annuli of parallel‐fibered bone. The rather simple microstructure observed in P. ischigualastensis osteoderms suggests that these elements grew at a constant, low rate. Compared with proterochampsids, doswelliid osteoderms possess a more complex histology, which appears to be linked to variations in the growth rate during the osteoderm formation and also to the development of the external ornamentation. A comparison of our findings with the results of earlier studies on other archosauriforms (phytosaurs and pseudosuchians) reveals that the general osteoderm histology of doswelliids bears a closer resemblance to that of phytosaurs and pseudosuchians than the proterochampsid osteoderm microstructure. If all archosauriform osteoderms are homologous structures, the closer resemblance of doswellid osteoderm microstructures to that of phytosaurs and pseudosuchians is in agreement with the hypothesis that doswellids are more closely related to archosaurs than proterochampsids. J. Morphol. 276:385–402, 2015. © 2015 Wiley Periodicals, Inc.     

Osteoderm microstructure of “rauisuchian” archosaurs from South America

In this contribution we analyze and discuss the microanatomy and histology of postcranial osteoderms of a number of “rauisuchians” from different localities of South America (Argentina and Brazil). The studied sample includes osteoderms of Fasolasuchus tenax, Prestosuchus chiniquensis, Saurosuchus galilei and an undetermined rauisuchian from Brazil. The bone microanatomy of the osteoderms is variable: whereas some specimens have a rather compact structure, others show a diploe architecture with a central cancellous core bordered by two compact cortices. Both external and basal cortices are mainly composed of poorly vascularized, fine and coarse parallel-fibred bone and networks of interwoven and mineralized fiber bundles. The internal region of the non-remodeled specimens consists of a well-vascularized core in which the intrinsic fibers exhibit important variations (even in the same specimen), ranging from coarse, parallel-fibred to woven-fibred bone tissues. Lines of arrested growth (LAGs) are well recorded in both basal and external cortices. Differences in the bone microstructure (compact vs. diploe) could be related to the age, sex and reproductive status of the sampled individuals. Hence, age estimation based on the count of LAGs in rauisuchian osteoderms appears to be reliable only in the early stages of ontogeny. The bone microstructure suggests that rauisuchian osteoderms were originated through a mechanism that involves both intramembranous and metaplastic ossifications.     

Osteoderm histology of the Pampatheriidae (Cingulata,Xenarthra, Mammalia): Implications for systematics,osteoderm growth,and biomechanical adaptation

Pampatheres are extinct, large‐bodied cingulates, which share morphological characters with both armadillos and glyptodonts but are considered to be more closely related to the latter. The osteoderm histology of six pampathere taxa was examined and compared to the histology of other cingulate osteoderms. This study investigates the development and functional adaptation of pampathere osteoderms as well as the phylogenetic relationships of the Pampatheriidae within the Cingulata. We found that pampathere osteoderms share a uniform histological organization based on a basic diploe‐like structure. After initial stages of intramembranous growth, metaplastic ossification, that is, the direct incorporation and mineralization of pre‐existing protein fibers, plays an important role in osteoderm development and provides information on various kinds of soft tissue otherwise not preserved. The latest stages of osteoderm growth are dominated by periosteal bone formation especially in the superficial cortex. Movable band osteoderms show regular arrangements of incorporated fibers that may increase the resistance of particularly weak areas against strain. The histological composition of pampathere osteoderms is plesiomorphic in its basic structure but shows a number of derived features. A unique array of Sharpey's fibers that are incorporated into the bone matrix at sutured osteoderm margins is interpreted as a synapomorphy of pampatheres. The arrangement of dermal fibers in the deep and superficial cortexes supports the close relationship between pampatheres and glyptodonts. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Ontogenetic histology of Stegosaurus plates and spikes

Abstract: The dinosaur Stegosaurus is characterized by osteoderms of alternating plates and terminal paired spikes. Previous studies have described the histological features and possible functions of these osteoderms. However, ontogenetic changes are poorly documented. In this study, the ontogenetic changes of the osteoderms are examined using eight different ontogenetic skeletons (a juvenile, a subadult, a young adult, and five old adults based on the cortical histology of their body skeletons). The juvenile plate and subadult spike show thin cortex and thick cancellous bone. The young adult plates have an extensive vascular network, which is also seen in old adults. Old adult spikes are different from old adult plates in having a thick cortex and a large axial channel. The cortical histology, in both plates and spikes, show well‐vascularized bone tissue consisting of dense mineralized fibres in young adult forms. In old adult forms, the bone tissues in the spikes become more compact and are extensively remodelled. This might contribute to the structural reinforcement of the spikes. The plates in old adult forms also show extensive remodelling and lines of arrested growth, but only limited signs of compaction. The timing for acquisition of features seen in old adults is different between plates (an extensive vascular network in the young adult) and spikes (a thick cortex with a large axial channel in old adults). The result suggests that the timing for plate and spike functions is different. The extensive vascular networks seen in large plates suggest their function is for display and/or thermoregulation. The thick cortical bone of spikes of old adults suggests that spikes acquire a weapon function for defence ontogenetically late.     

Dermal armour histology of aetosaurs (Archosauria: Pseudosuchia), from the Upper Triassic of Argentina and Brazil

Cerda, I.A. & Desojo, J.B. 2010: Dermal armour histology of aetosaurs (Archosauria: Pseudosuchia), from the Upper Triassic of Argentina and Brazil. Lethaia, Vol. 44, pp. 417–428. One of the most striking features documented in aetosaurs is the presence of an extensive bony armour composed of several osteoderms. Here, we analyse the bone microstructure of these elements in some South American Aetosaurinae aetosaurs, including Aetosauroides scagliai. In general terms, Aetosaurinae osteoderms are compact structures characterized by the presence of three tissue types: a basal cortex of poorly vascularized parallel‐fibred bone tissue, a core of highly vascularized fibro‐lamellar bone, and an external cortex of rather avascular lamellar bone tissue. Sharpey’s fibres are more visible at the internal core, toward the lateral margins and aligned parallel to the major axis of the dermal plate. No evidence of metaplastic origin is reported in the osteoderms, and we hypothesize an intramembranous ossification for these elements. The bone tissue distribution reveals that the development of the osteoderm in Aetosaurinae starts in a position located medial to the plate midpoint, and the main sites of active osteogenesis occur towards the lateral and medial edges of the plate. The osteoderm ornamentation is originated and maintained by a process of resorption and redeposition of the external cortex, which also includes preferential bone deposition in some particular sites. Given that no secondary reconstruction occurs in the osteoderms, growth marks are well preserved and they provide very important information regarding the relative age and growth pattern of Aetosaurinae aetosaurs. □Aetosauria, Aetosauroides, Archosauria, bone microstructure, integumentary skeleton, osteoderm.     

Osteohistological characterization of notosuchian osteoderms: Evidence for an overlying thick leathery layer of skin

Osteoderms are mineralized structures embedded in the dermis, known for nonavian archosaurs, squamates, xenarthrans, and amphibians. Herein, we compared the osteoderm histology of Brazilian Notosuchia of Cretaceous age using three neosuchians for comparative purposes. Microanatomical analyses showed that most of them present a diploe structure similar to those of other pseudosuchians, lizards, and turtles. This structure contains two cortices (the external cortex composed of an outer and an inner layers, and the basal cortex) and a core in-between them. Notosuchian osteoderms show high bone compactness (>0.85) with varying degrees of cancellous bone in the core. The neosuchian Guarinisuchus shows the lowest bone compactness with a well-developed cancellous layer. From an ontogenetic perspective, most tissues are formed through periosteal ossification, although the mineralized tissues observed in baurusuchid LPRP/USP 0634 suggest a late metaplastic development. Histology suggests that the ossification center of notosuchian osteoderm is located at the keel. Interestingly, we identified Sharpey's fibers running perpendicularly to the outer layer of the external cortex in Armadillosuchus arrudai, Itasuchus jesuinoi, and Baurusuchidae (LPRP/USP 0642). This feature indicates a tight attachment within the dermis, and it is evidence for the presence of an overlying thick leathery layer of skin over these osteoderms. These data allow a better understanding of the osteohistological structure of crocodylomorph dermal bones, and highlight their structural diversity. We suggest that the vascular canals present in some sampled osteoderms connecting the inner layer of the external cortex and the core with the external surface may increase osteoderm surface and the capacity of heat transfer in terrestrial notosuchians.     

Evolution of the Permian and Triassic tetrapod communities of Eastern Europe

The Permo-Triassic terrestrial and freshwater tetrapod communities of Eastern Europe are reconstructed as food-webs. The Late Permian theriodont-dinocephalian community (Ocher, Mezen, Isheyevo) changes to a latest Permian theriodont-pareiasaur community (North Dvina, Vyazniki). After a major extinction, the Triassic thecodontian-dicynodont communities appear, a lystrosaurid one in the Early Triassic (Lower and ?Upper Vetluga), and a kannemeyerid one in the later Early Triassic (?Yarenga) and the Mid Triassic (Donguz, Bukobay). Similar stages are represented in the evolution of aquatic communities: the Late Permian temnospondyl community (Ocher, Isheyevo), the latest Permian chroniosuchian one (North Dvina, Vyazniki), the Lower and Middle Triassic new temnospondyl one (from Vetluga to Bukobay). The faunal changes in Eastern Europe are mirrored in other parts of the world, although there are some endemic Russian forms.     

Braincase,palatoquadrate and ear region of the plagiosaurid Gerrothorax pulcherrimus from the Middle Triassic of Germany

Abstract: The complete neurocranium plus palatoquadrate of the plagiosaurid temnospondyl Gerrothorax pulcherrimus from the Middle Triassic of Germany is described for the first time, based on outer morphological observations and micro‐CT scanning. The exoccipitals are strong elements with paroccipital processes and well‐separated occipital condyles. Anterolaterally, the exoccipitals contact the otics, which are mediolaterally elongated and have massive lateral walls. The otics contact the basisphenoid, which shows well‐developed sellar processes. Anteriorly, the basisphenoid is continuous with the sphenethmoid region. In its posterior portion, the sphenethmoid gives rise to robust, laterally directed laterosphenoid walls, a unique morphology among basal tetrapods. The palatoquadrate is extensively ossified. The quadrate portion overlaps the descending lamina of squamosal and ascending lamina of pterygoid anteriorly, almost contacting the epipterygoid laterally. The epipterygoid is a complex element and may be co‐ossified with otics and laterosphenoid walls. It has a broad, sheet‐like footplate and a horizontally aligned ascending process that contacts the laterosphenoid walls. The degree of ossification of the epipterygoid, however, is subject to individual variation obviously independent from ontogenetic changes. The stapes of Gerrothorax is a large, blade‐like element that differs conspicuously from the plesiomorphic temnospondyl condition. It has a prominent anterolateral projection which has not been observed in other basal tetrapods. Morphology of neurocranium and palatoquadratum of Gerrothorax most closely resembles that of the Russian plagiosaurid Plagiosternum danilovi, although the elements are less ossified in the latter. The extensive endocranial ossification of Gerrothorax is consistent with the general high degree of ossification in the exo‐ and endoskeleton of this temnospondyl and supports the view that a strong endocranial ossification cannot be evaluated as a plesiomorphic character in basal tetrapods.     

Developmental palaeontology of Reptilia as revealed by histological studies

Among the fossilised ontogenetic series known for tetrapods, only more basal groups like temnospondyl amphibians have been used extensively in developmental studies, whereas reptilian and synapsid data have been largely neglected so far. However, before such ontogenetic series can be subject to study, the relative age and affiliation of putative specimens within a series has to be verified. Bone histology has a long-standing tradition as being a source of palaeobiological and growth history data in fossil amniotes and indeed, the analysis of bone microstructures still remains the most important and most reliable tool for determining the absolute ontogenetic age of fossil vertebrates. It is also the only direct way to reconstruct life histories and growth strategies for extinct animals. Herein the record of bone histology among Reptilia and its application to elucidate and expand fossilised ontogenies as a source of developmental data are reviewed.     

Bone microstructures and mode of skeletogenesis in osteoderms of three pareiasaur taxa from the Permian of South Africa

The extinct parareptilian clade of pareiasaurs was in the past often presented to constitute a morphocline from larger, less armoured forms to smaller, well armoured forms, indicating that the osteoderm cover became an increasingly prominent aspect in the post‐cranial skeleton of these animals. Here, we describe microanatomical and microstructural aspects of osteoderms of the three pareiasaur taxa Bradysaurus, Pareiasaurus and Anthodon from the Permian of South Africa. A generalized mode of osteoderm formation, consistent with intramembraneous skeletogenesis, is hypothesized to be present in all pareiasaurs. Few characters are shared between pareiasaur dermal armour and turtle shell bones and osteoderms. Otherwise, there is strong evidence from microanatomy and histology (i.e. absence of structures that formed via metaplasia of dermal tissue) that indicates nonhomology between pareiasaur dermal armour and the armour of living eureptiles. Analysis with bone profiler revealed no clear connection between bone compactness and lifestyle in the amniote osteoderm sample.     

Novel data on aetosaur (Archosauria,Pseudosuchia) osteoderm microanatomy and histology: palaeobiological implications

One of the most striking features of aetosaurs is the possession of an extensive bony armour composed of dorsal, ventral and appendicular osteoderms. With the purpose of establishing the main histological changes during ontogeny and the degree of histological variation within the armour, we analysed the bone histology of dorsal (paramedian and lateral), ventral and appendicular osteoderms from different taxa from the Late Triassic of South America, including Aetosauroides scagliai, Aetobarbakinoides brasiliensis and Neoaetosauroides engaeus. Histological data support an intramembranous origin for osteoderms. Nevertheless, evidence for metaplastic ossification (i.e. structural fibres) at advanced ontogenetic stages, in at least some elements, is also present. A variant type of parallel fibred bone, which we have named ‘crossed parallel fibred bone’, is characterized for aetosaurs. In this pseudosuchian group, osteoderms exhibit very important microstructural changes during ontogeny, which can be useful for determining ontogenetic stages from isolated elements. Histological data suggest a relatively early onset of sexual maturity among aetosaurs. Microanatomical analysis from different taxa reveal that having high values of compactness is the plesiomorphic condition for Aetosauria. The notably increased compactness of the osteoderms does not appear to be related to size, ontogeny, sex or reproductive status of the individuals. Although a high degree of compactness of osteoderms and other bones has been considered as evidence for an aquatic lifestyle in vertebrates, such an inference contradicts the current concept of a fully terrestrial lifestyle in aetosaurs.     

New material of Pachyarmatherium from the late Pleistocene of northeastern Brazil: insights into its morphology and systematics

A study of recently collected and other undescribed material of Pachyarmatherium brasiliense from the State of Pernambuco, Brazil, was performed, and its type material was revised, improving our understanding of this species and providing additional information on its affinities. Two fused osteoderms representing the posteriormost osteoderm row of the pectoral buckler are described for the first time in the Pachyarmatherium. Several additional specimens included buckler, border, and at least one caudal osteoderm. The relationships of Pachyarmatherium have been subject to different interpretations. We propose that the presence of some osteoderm features suggests a closer affinity with Dasypodinae/cf. Dasypodini armadillos. A key feature supporting this conclusion is the presence of complementary small figures in adjoining buckler osteoderms of Pachyarmatherium, indicating that a single scale covered portions of three contiguous osteoderms. Other features observed are related to the histological microstructure, which includes a central cortex characterized by a poorly developed cancellous bone and a deep cortex with mineralized collagen fiber bundles that are oriented roughly parallel to the deep osteoderm surface. Likewise, these histological features support its exclusion from glyptodonts and pampatheres, which show a different histological arrangement in their osteoderms.     

On the use of osteoderm features in a phylogenetic approach on the internal relationships of the Chroniosuchia (Tetrapoda: Reptiliomorpha)

Abstract: Chroniosuchians are an enigmatic Permian to Triassic group of crocodile‐like basal tetrapods. Their conspicuous dorsal osteoderm systems include most of the group’s yet documented postcranial morphological variability but have hardly been considered in cladistic approaches. Aiming at the clarification of the internal relationships of the Chroniosuchia, we have carried out a parsimony analysis including, among others, 23 morphological and osteohistological osteoderm characters and 12 chroniosuchian taxa. According to the most parsimonious trees, taxa usually referred to Chroniosuchidae form a paraphyletic succession with Madygenerpeton pustulatus and Chroniosaurus dongusensis as the basalmost chroniosuchians and Uralerpeton tverdochlebovae as the sister group of Bystrowianidae (hypothesis A). However, the concurrent hypothesis of a basal split into monophyletic subtaxa Chroniosuchidae and Bystrowianidae (hypothesis B) is only slightly less parsimonious and supported by an alternative analysis which includes embolomeres as the only reptiliomorph outgroup. Searching for the better hypothesis, we compare the respective order of branching to the order of first occurrences in the fossil record, demonstrating that hypothesis A provides a better stratigraphic fit than hypothesis B. Accordingly, the last common ancestor of the yet known chroniosuchians had a series of broad complexly interlocking ‘chroniosuchid’ osteoderms that served as a protection carapace apart from supporting the trunk during terrestrial locomotion. The later evolution of chroniosuchian carapaces was marked by a stepwise increase in flexibility and size reduction, which resulted in a loss of protective function and in a reduction in trunk support function. The flexibility increase is paralleled by the evolution of the Crocodylomorpha whose extant members do not possess as extensively interlocking osteoderm systems as some of their Mesozoic relatives.     

The histological structure of glyptosaurine osteoderms (Squamata: Anguidae), and the problem of osteoderm development in squamates

Glyptosaurinae, a fossil clade of anguid lizards, possess robust osteoderms, with granular ornamentation. In this study, the structural and histological features of these osteoderms were described in order to reconstruct their developmental pattern and further document the degree of homology that could exist between vertebrate integumentary skeletons. Glyptosaurine osteoderms have a diploe architecture and display an unusually complex structure that includes four tissue types: a core of woven‐fibered bone intensely remodeled; a peripheral formation of the same tissue containing dense bundles of long Sharpey fibers; a thick basal layer of lamellar bone; and a superficial layer of a non‐osseous material that belongs to the category of hypermineralized tissues such as ganoine, or enameloid and enamel tissues. The growth pattern of glyptosaurine osteoderms involved appositional processes due to osteoblast activity. In early growth stages, osseous metaplasia might have also been involved, but this possibility is not substantiated by histological observations. The superficial layer of the osteoderms must have resulted from epidermal contribution, a conclusion that would support previous hypotheses on the role of epidermal‐dermal interactions in the formation of squamate osteoderms. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Osteoderm morphology and development in the nine-banded armadillo, Dasypus novemcinctus (Mammalia, Xenarthra, Cingulata)

Among modern mammals, armadillos (Xenarthra, Cingulata) are the only group that possesses osteoderms, bony inclusions within the integument. Along the body, osteoderms are organized into five discrete assemblages: the head, pectoral, banded, pelvic, and tail shields. The pectoral, banded, and pelvic shields articulate to form the carapace. We examined osteoderm skeletogenesis in the armadillo Dasypus novemcinctus using serial and whole-mount histochemistry. Compared with the rest of the skeleton, osteoderms have a delayed onset of development. Skeletogenesis begins as condensations of osteoblasts secreting osteoid, localized within the papillary layer of the dermis. Osteoderm formation is asynchronous both within each shield and across the body. The first osteoderms to mineralize are situated within the pectoral shield of the carapace, followed by elements within the banded, head, pelvic, and tail shields. In general, within each shield ossification begins craniomedially and proceeds caudally and laterally, except over the head, where the earliest elements form over the frontal and parietal bones. The absence of cartilage precursors indicates that osteoderms are dermal elements, possibly related to the all-encompassing vertebrate dermal skeleton (exoskeleton). The mode of development of D. novemcinctus osteoderms is unlike that described for squamate osteoderms, which arise via bone metaplasia, and instead is comparable with intramembranously derived elements of the skull.     

Comparative paleohistology in osteoderms of Pleistocene Panochthus sp. Burmeister, 1886 and Neuryurus sp. (Xenarthra,Glyptodontidae)

Xenarthran osteoderms are integumentary bones with high fossilization potential presenting a high degree of morphological and histological diversity. Here, new data on the osteoderms histology of two glyptodonts, Panochthus and Neuryurus are presented. The poor spatial organization of the mineralized fibers and a large trabecular area in the middle zone identified in Neuryurus indicate a different bone pattern than the one found in Panochthus, which is mainly characterized by a middle zone with less spongiosa. Through the Bone Profiler program, the degree of compactness of the specimens was obtained, with about 70% for Neuryurus sp. and approximately 90% for Panochthus sp., showing the difference in bone pattern. These values confirm the visible difference in the histological patterns of these taxa, especially in the middle zone. This work demonstrates the microstructural variation studied in osteoderms and shows the importance of paleohistology as a starting point for a better understanding of extinct taxa.     

Cranial morphology of the plagiosaurid Gerrothorax pulcherrimus as an extreme example of evolutionary stasis

Schoch, R.R. & Witzmann, F. 2011: Cranial morphology of the plagiosaurid Gerrothorax pulcherrimus as an extreme example of evolutionary stasis. Lethaia, Vol. 45, pp. 371–385. The plagiosaurid Gerrothorax pulcherrimus from the Triassic of Greenland and Germany is represented by skulls ranging from 4 to 12 cm in length and sheds light on ontogeny, individual variation, and variation in time and space. Ontogeny was remarkably stable in G. pulcherrimus, with the smallest known specimens resembling the adults closely in most features. A true ontogenetic change is evident in the ornament of dermal bones, in that the smallest specimens have ridges whereas in the successively larger ones, pustules spread over increasingly larger areas. The skull becomes proportionally longer, and the adductor chambers relatively narrower. The positive allometry of both the orbits and the interpterygoid vacuities suggests that the eye supporting musculature – rather than the jaw adductors – increased proportionally during growth. Individual, not age‐related variation in the dermal skull roof affects partial fusion of parietals, presence and extent of the interfrontoparietal, and the morphological pattern of the posterior skull table. The ventral surface of the basal plate of the parasphenoid ranges from smooth over poorly to heavily ornamented or dentigerous. Considering the impressive longevity of more than 35 Myr, the morphological changes of G. pulcherrimus are minor. Our ecological interpretation for G. pulcherrimus is that it relied on the permanent presence of water, but was flexible with respect to the size and nature of the water body as well as to changes in salinity. The unparalleled extent of evolutionary stasis may therefore be based on the ecological flexibility of this morphologically so tightly constrained temnospondyl. □Ecological flexibility, ontogeny,Temnospondyli, Triassic, variation.     

Skull morphology and phylogenetic relationships of a new Middle Triassic plagiosaurid temnospondyl from Germany,and the evolution of plagiosaurid eyes

A partial skull from the Lower Keuper (Middle Triassic) of Germany is recognized as belonging to a new genus and species of plagiosaurid temnospondyl. It is readily identified by the following autapomorphies: (1) extremely large orbits medially extended to give very thin interorbital region and cheek; (2) posterior skull table abbreviated, with splint‐like supratemporals, postparietals and parietals; (3) supraorbital lateral line sulcus absent on frontal, with blind ending on parietal, and continued at the anterior margin of the postorbital; (4) occiput sloping posteriorly, with subtympanic fossa exposed in ventral view; (5) cultriform process and basicranial suture extremely narrow; (6) mandibular and maxillary teeth very long with crowns markedly curved inwards. The new taxon shares the following derived character states with Plagiosternum: (1) the pentagonal shape of orbits; (2) the slender interorbital region and cultriform process; (3) the small‐scale, polygonal pit‐and‐ridge ornament on posterior skull table, with single prominent tubercles rising from ridges. Phylogenetic analysis finds plagiosaurids to be monophyletic within a broad range of temnospondyls, nesting within a clade of short‐skulled stereospondyls. Plagio‐suchus is the most basal plagiosaurid, and Plagiosternum forms the sister group to the new taxon. The biological significance of the large orbits of plagiosaurids and their relationships to the eyeballs is discussed. The only type of eyeball that would be feasible for all known plagiosaurids would be a small spherical structure possibly situated near the anterior edge of the orbit. For those plagiosaurs with extremely large shallow orbits like Plagiosternum and Megalophthalma, a possible adaptation would be a lens‐less eye comprising a flat retinal plate across the entire orbit.