The age and growth information of a ctenoid scale fossil from the Upper Cretaceous Nenjiang Formation in Songliao Basin, China.

The study of morphological characteristics and growth information in fish scales is a crucial component of modern fishery biological research, while it has been less studied in fossil materials. This paper presents a detailed morphological description and growth analysis of a fossil ctenoid scale obtained from the Upper Cretaceous Campanian lacustrine deposits in northeastern China. The morphological features of this fossil scale are well-preserved and consistent with the structures found in ctenoid scales of extant fish species and display prominent ring ornamentation radiating outward from the central focus, with grooves intersecting the rings. A comparative analysis of the morphological characteristics between the fossil ctenoid scale and those well-studied extant fish Mugilidae allows us to explore the applicability of modern fishery biological research methods to the field of fossil scales. The scale length, scale width, the vertical distance from the focus to the apex of the scale, and the total number of radii have been measured. The age of the fish that possessed this ctenoid scale has been estimated by carefully counting the annuli, suggesting an age equal to or more than seven years. The distribution of growth rings on the scale potentially reflects the warm paleoclimatic condition and fish-friendly paleoenvironment prevalent during that period. This paper, moreover, serves as a notable application of fishery biological methods in the examination of fossil materials.

Size and shape heterodonty in the early Permian synapsid Mesenosaurus efremovi.

Paleozoic synapsids represent the first chapter in the evolution of this large clade that includes mammals. These fascinating terrestrial vertebrates were the first amniotes to successfully adapt to a wide range of feeding strategies, reflected by their varied dental morphologies. Evolution of the marginal dentition on the mammalian side of amniotes is characterized by strong, size and shape heterodonty, with the late Permian therapsids showing heterodonty with the presence of incisiform, caniniform, and multicuspid molariform dentition. Rarity of available specimens has previously prevented detailed studies of dental anatomy and evolution in the initial chapter of synapsid evolution, when synapsids were able to evolve dentition for insectivory, herbivory, and carnivory. Numerous teeth, jaw elements, and skulls of the hypercarnivorous varanopid Mesenosaurus efremovi have been recently discovered in the cave systems near Richards Spur, Oklahoma, permitting the first detailed investigation of the dental anatomy of a Paleozoic tetrapod using multiple approaches, including morphometric and histological analyses. As a distant stem mammal, Mesenosaurus is the first member of this large and successful clade to exhibit a type of dental heterodonty that combines size and morphological (shape) variation of the tooth crowns. Here we present the first evidence of functional differentiation in the dentition of this early synapsid, with three distinct dental regions having diverse morphologies and functions. The quality and quantity of preserved materials has allowed us to identify the orientation and curvature of the carinae (cutting edges), and the variation and distribution of the ziphodonty (serrations) along the carinae. The shape-related heterodonty seen in this taxon may have contributed to this taxon's ability to be a successful mid-sized predator in the taxonomically diverse community of early Permian carnivores, but may have also extended the ecological resilience of this clade of mid-sized predators across major faunal and environmental transitions.

Exceptionally rapid tooth development and ontogenetic changes in the feeding apparatus of the Komodo dragon.

Dental developmental and replacement patterns in extinct amniotes have attracted a lot of attention. Notable among these are Paleozoic predatory synapsids, but also Mesozoic theropod dinosaurs, well known for having true ziphodonty, strongly serrated carinae with dentine cores within an enamel cap. The Komodo dragon, Varanus komodoensis, is the only extant terrestrial vertebrate to exhibit true ziphodonty, making it an ideal model organism for gaining new insights into the life history and feeding behaviours of theropod dinosaurs and early synapsids. We undertook a comparative dental histological analysis of this extant apex predator in combination with computed tomography of intact skulls. This study allowed us to reconstruct the dental morphology, ontogeny, and replacement patterns in the largest living lizard with known feeding behaviour, and apply our findings to extinct taxa where the behaviour is largely unknown. We discovered through computed tomography that V. komodoensis maintains up to five replacement teeth per tooth position, while histological analysis showed an exceptionally rapid formation of new teeth, every 40 days. Additionally, a dramatic ontogenetic shift in the dental morphology of V. komodoensis was also discovered, likely related to changes in feeding preferences and habitat. The juveniles have fewer dental specializations, lack true ziphodonty, are arboreal and feed mostly on insects, whereas the adults have strongly developed ziphodonty and are terrestrial apex predators with defleshing feeding behaviour. In addition, we found evidence that the ziphodont teeth of V. komodoensis have true ampullae (interdental folds for strengthening the serrations), similar to those found only in theropod dinosaurs. Comparisons with other species of Varanus and successive outgroup taxa reveal a complex pattern of dental features and adaptations, including the evolution of snake-like tongue flicking used for foraging for prey. However, only the Komodo dragon exhibits this remarkable set of dental innovations and specializations among squamates.

Paleozoic cave system preserves oldest-known evidence of amniote skin.

The richest and most diverse assemblage of early terrestrial tetrapods is preserved within the infilled cave system of Richards Spur, Oklahoma (289-286 Mya1). Some of the oldest-known terrestrial amniotes2,3 are exquisitely preserved here because of early impregnation and encasement of organic material by oil-seep hydrocarbons within rapidly deposited clay-rich cave sediments under toxic anoxic conditions.4 This phenomenon has also afforded the preservation of exceedingly rare integumentary soft tissues, reported here, providing critical first evidence into the anatomical changes marking the transition from the aquatic and semiaquatic lifestyles of anamniotes to the fully terrestrial lifestyles of early amniotes. This is the first record of a skin-cast fossil (3D carbonization of the skin proper) from the Paleozoic Era and the earliest known occurrence of epidermal integumentary structures. We also report on several compression fossils (carbonized skin impressions), all demonstrating similar external morphologies to extant crocodiles. A variety of previously unknown ossifications, as well as what are likely palpebral ossifications of the deeper dermis layer of the skin, are also documented. These fossils also serve as invaluable references for paleontological reconstructions. Chromatographic analysis of extractable hydrocarbons from bone and cave samples indicates that the source rock is the Devonian age Woodford Shale. Hydrocarbons derived from ancient marine organisms interacting with geologically younger terrestrial vertebrates have therefore resulted in the oldest-known preservation of amniote skin proper.

Skeletal anatomy of the early Permian parareptile Delorhynchus with new information provided by neutron tomography.

Detailed description of the holotype skeleton of Delorhynchus cifellii, made possible through the use of neutron tomography, has yielded important new information about the cranial and postcranial anatomy of this early Permian acleistorhinid parareptile. Hitherto unknown features of the skull include a sphenethmoid, paired epipterygoids and a complete neurocranium. In addition, the stapes has been exposed in three dimensions for the first time in an early parareptile. Postcranial material found in articulation with the skull in this holotype allows for the first detailed description of vertebrae, ribs, shoulder girdle and humerus of an acleistorhinid parareptile, allowing for a reevaluation of the phylogenetic relationships of this taxon with other acleistorhinids, and more broadly among parareptiles. Results show that Delorhynchus is recovered as the sister taxon of Colobomycter, and 'acleistorhinids' now include Lanthanosuchus.

Neurosensory anatomy and function in Dimetrodon, the first terrestrial apex predator.

Dimetrodon is among the most recognizable fossil taxa, as well as the earliest terrestrial amniote apex predator. The neuroanatomy and auditory abilities of Dimetrodon has long been the subject of interest, but palaeoneurological analyses have been limited by the lack of three-dimensional endocast data. The first virtual endocasts reveal a strongly flexed brain with enlarged floccular fossae and a surprisingly well-ossified bony labyrinth clearly preserving the semicircular canals, along with an undifferentiated vestibule and putative perilymphatic duct. This first detailed palaeoneurological reconstruction reveals potential adaptations for a predatory lifestyle and suggests Dimetrodon was able to hear a wider range of frequencies than anticipated, potentially being sensitive to frequencies equal to or higher than many extant sauropsids, despite lacking an impedance matching ear. Ancestral state reconstructions support the long-standing view of Dimetrodon as representative of the ancestral state for therapsids, while underscoring the importance of validating reconstructive analyses with fossil data.

Neurosensory anatomy and function in Seymouria.

Seymouria is among the best-known stem amniotes and holds an important phylogenetic position for discussions of amniote evolution. Previous work has focused primarily on the osteology of Seymouria, with recent interest turning to the application of computed tomography (CT) to study the internal features. We utilized neutron CT to reconstruct the first virtual cranial endocast and the first complete otic endocasts of Seymouria, revealing previously unrecognized details of its palaeoneuroanatomy. The brain and inner ear of Seymouria are largely plesiomorphic relative to later-diverging crown amniotes, showing no indication of increased encephalization or braincase ossification. Our results also clarify the plesiomorphic condition for carotid artery morphology in amniotes, with Seymouria showing a similar condition to basal members of both the synapsid and sauropsid lineages. The reconstructed neuroanatomy also indicates that Seymouria did not possess any particular neuroanatomical specializations, despite the probable presence of an impedance matching hearing system.

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.

Mammalian tooth enamel functional sophistication demonstrated by combined nanotribology and synchrotron radiation FTIR analyses.

The teeth of limbed vertebrates used for capturing and processing food are composed of mineralized dentine covered by hypermineralized enamel, the hardest material organisms produce. Here, we combine scanning probe microscopy, depth sensing, and spectromicroscopy (SR-FTIR) to characterize the surface ultrastructural topography, nanotribology, and chemical compositions of mammal species with different dietary habits, including omnivorous humans. Our synergistic approach shows that enamel with greater surface hardness or thickness exhibited a more salient gradient feature from the tooth surface to the dentino-enamel junction (DEJ) one that corresponds to the in situ phosphate-to-amide ratio. This gradient feature of enamel covering softer dentine is the determining factor of the amazingly robust physical property of this unique biomaterial. It provides the ability to dissipate stress under loading and prevent mechanical failure. Evolutionary change in the biochemical composition and biomechanical properties of mammalian dentition is related to variations in the oral processing of different food materials.

Endocasts of the basal sauropsid Captorhinus reveal unexpected neurological diversity in early reptiles.

Captorhinids are a group of Paleozoic amniotes that represents one of the earliest-diverging clades of eureptiles. Although captorhinids are one of the best-known and most well-studied clades of early amniotes, their palaeoneuroanatomy has gone largely unexamined. We utilized neutron computed tomography to study the virtual cranial and otic endocasts of two captorhinid specimens. The neurosensory anatomy of captorhinids shows a mixture of traits considered plesiomorphic for sauropsids (no expansions of the cerebrum or olfactory bulbs, low degree of encephalization, low ossification of the otic capsule) and those considered more derived, including moderate cephalic and pontine flexures and a dorsoventrally tall bony labyrinth. The inner ear clearly preserves the elliptical, sub-orthogonal canals and the short, rounded vestibule, along with an unusually enlarged lateral canal and a unique curvature of the posterior canal. The reconstructed neurosensory anatomy indicates that captorhinids were sensitive to slightly higher frequencies than many of their contemporaries, likely reflecting differences in body size across taxa, while the morphology of the maxillary canal suggests a simple, tubular condition as the plesiomorphic state for Sauropsida and contributes to the ongoing discussions regarding the phylogenetic placement of varanopids. This study represents the first detailed tomographic study of the brain and inner ear of any basal eureptile. The new data described here reveal that the neuroanatomy of early sauropsids is far more complex and diverse than previously anticipated, and provide impetus for further exploration of the palaeoneuroanatomy of early amniotes.

An intriguing new diapsid reptile with evidence of mandibulo-dental pathology from the early Permian of Oklahoma revealed by neutron tomography.

The initial stages of diapsid evolution, the clade that includes extant reptiles and the majority of extinct reptilian taxa, is surprisingly poorly known. Notwithstanding the hypothesis that varanopids are diapsids rather than synapsids, there are only four araeoscelidians and one neodiapsid present in the late Carboniferous and early Permian. Here we describe the fragmentary remains of a very unusual new amniote from the famous cave deposits near Richards Spur, Oklahoma, that we recognize as a diapsid reptile, readily distinguishable from all other early amniotes by the unique dentition and lower jaw anatomy. The teeth have an unusual reeding pattern on the crown (long parallel ridges with rounded surfaces), with some teeth posteriorly tilted and strongly recurved, while a ventral protuberance forms the anterior terminus of the dentary. Overall, the lower jaw is unusually slender with a flattened ventral surface formed by the dentary and splenial anteriorly and the angular in the mid-region. The presence of a very slender triradiate jugal revealed through computed tomography confirms the existence of a large lower temporal fenestra, while the medial edge of the maxilla and the anatomy of the palatine confirm the presence of a large suborbital fenestra. Computed tomography of this new taxon reveals maxillary innervation that is characteristically reptile, not synapsid. Although no other definitively identifiable skull roof elements exist, the suborbital fenestra borders preserved on the palatine and maxilla supports the hypothesis that this is a diapsid reptile. Interestingly, the right dentary shows evidence of pathology, a rarely reported occurrence in Paleozoic amniotes, with several empty tooth sockets filled by bone. This small predator with delicate subthecodont implanted dentition provides strong evidence that diapsid reptiles were already diversifying rapidly in the early Permian, but likely were relatively rare members of terrestrial vertebrate assemblages.

Permian hypercarnivore suggests dental complexity among early amniotes.

The oldest known complex terrestrial vertebrate community included hypercarnivorous varanopids, a successful clade of amniotes with wide geographic and temporal distributions. Little is known about their dentition and feeding behaviour, but with the unprecedented number of specimens of the varanopid Mesenosaurus from cave deposits in Oklahoma, we show that it exhibited serrations on the tooth crowns, and exceptionally rapid rates of development and reduced longevity relative to other terrestrial amniotes. In contrast, the coeval large apex predator Dimetrodon greatly increased dental longevity by increasing thickness and massiveness, whereas herbivores greatly reduced tooth replacement rates and increased dental longevity. Insectivores and omnivores represented the primitive condition and maintained modest replacement rates and longevity. The varied patterns of dental development among these early terrestrial amniotes reveal a hidden aspect of dental complexity in the emerging diverse amniote community, very soon after their initial appearance in the fossil record.

Dental anatomy and replacement patterns in the early Permian stem amniote, Seymouria.

The exquisite preservation of maxillary and mandibular fragments of Seymouria has allowed us to examine for the first time in detail the dental anatomy and patterns of development in this stem amniote. The results obtained through histological examination show that Seymouria has pleurodont implantation with ankylosis of the tooth to the labial side of the jawbone. The dentary and maxillary teeth exhibit similar dental characteristics, such as the attachment bone (alveolar bone) and cementum rising above the jawbone on the base of the tooth, and smooth carinae extending lingually toward the tooth apex. Additionally, the clear presence of plicidentine, infolding of dentine into the pulp cavity, was found within the tooth root extending into the tooth crown. Lastly, the tooth replacement pattern is alternating, illustrating that Seymouria retains the classic primitive condition for tetrapods, a pattern that is continued in amniotes. Our results provide an important basis for comparison with other stem amniotes and with amniotes.

Neurosensory anatomy of Varanopidae and its implications for early synapsid evolution.

Varanopids are a group of Palaeozoic terrestrial amniotes which represent one of the earliest-diverging groups of synapsids, but their palaeoneurology has gone largely unstudied and recent analyses have challenged their traditional placement within synapsids. We utilized computed tomography (CT) to study the virtual cranial and otic endocasts of six varanopids, including representative taxa of both mycterosaurines and varanodontines. Our results show that the varanopid brain is largely plesiomorphic, being tubular in shape and showing no expansion of the cerebrum or olfactory bulbs, but is distinct in showing highly expanded floccular fossae. The housing of the varanopid bony labyrinth is also distinct, in that the labyrinth is bounded almost entirely by the supraoccipital-opisthotic complex, with the prootic only bordering the ventral portion of the vestibule. The bony labyrinth is surprisingly well-ossified, clearly preserving the elliptical, sub-orthogonal canals, prominent ampullae, and the short, undifferentiated vestibule; this high degree of ossification is similar to that seen in therapsid synapsids and supports the traditional placement of varanopids within Synapsida. The enlarged anterior canal, together with the elliptical, orthogonal canals and enlarged floccular fossa, lend support for the fast head movements indicated by the inferred predatory feeding mode of varanopids. Reconstructed neurosensory anatomy indicates that varanopids may have a much lower-frequency hearing range compared to more derived synapsids, suggesting that, despite gaining some active predatory features, varanopids retain plesiomorphic hearing capabilities. As a whole, our data reveal that the neuroanatomy of pelycosaur-grade synapsids is far more complex than previously anticipated.

Osteology, relationships and functional morphology of Weigeltisaurus jaekeli (Diapsida, Weigeltisauridae) based on a complete skeleton from the Upper Permian Kupferschiefer of Germany.

BACKGROUND: Weigeltisauridae is a clade of small-bodied diapsids characterized by a horned cranial frill, slender trunk and limbs, and a patagium supported by elongated bony rods. Partial skeletons and fragments are definitively known only from upper Permian (Lopingian) rocks in England, Germany, Madagascar and Russia. Despite these discoveries, there have been few detailed descriptions of weigeltisaurid skeletons, and the homologies of many skeletal elements-especially the rods supporting the patagium-remain the subject of controversy. MATERIALS & METHODS: Here, we provide a detailed description of a nearly complete skeleton of Weigeltisaurus jaekeli from the upper Permian (Lopingian: Wuchiapingian) Kupferschiefer of Lower Saxony, Germany. Briefly addressed by past authors, the skeleton preserves a nearly complete skull, postcranial axial skeleton, appendicular skeleton, and patagial supports. Through comparisons with extant and fossil diapsids, we examine the hypotheses for the homologies of the patagial rods. To examine the phylogenetic position of Weigeltisauridae and characterize the morphology of the clade, we integrate the material and other weigeltisaurids into a parsimony-based phylogenetic analysis focused on Permo-Triassic non-saurian Diapsida and early Sauria (61 taxa, 339 characters). RESULTS: We recognize a number of intriguing anatomical features in the weigeltisaurid skeleton described here, including hollow horns on the post-temporal arch, lanceolate teeth in the posterior portion of the maxilla, the absence of a bony arch connecting the postorbital and squamosal bones, elongate and slender phalanges that resemble those of extant arboreal squamates, and patagial rods that are positioned superficial to the lateral one third of the gastral basket. Our phylogenetic study recovers a monophyletic Weigeltisauridae including Coelurosauravus elivensis, Weigeltisaurus jaekeli, and Rautiania spp. The clade is recovered as the sister taxon to Drepanosauromorpha outside of Sauria (=Lepidosauria + Archosauria). CONCLUSIONS: Our anatomical observations and phylogenetic analysis show variety of plesiomorphic diapsid characters and apomorphies of Weigeltisauridae in the specimen described here. We corroborate the hypothesis that the patagial ossifications are dermal bones unrelated to the axial skeleton. The gliding apparatus of weigeltisaurids was constructed from dermal elements unknown in other known gliding diapsids. SMNK-PAL 2882 and other weigeltisaurid specimens highlight the high morphological disparity of Paleozoic diapsids already prior to their radiation in the early Mesozoic.

Thecodont tooth attachment and replacement in bolosaurid parareptiles.

Permian bolosaurid parareptiles are well-known for having complex tooth crowns and complete tooth rows in the jaws, in contrast to the comparatively simple teeth and frequent replacement gaps in all other Paleozoic amniotes. Analysis of the specialized dentition of the bolosaurid parareptiles Bolosaurus from North America and Belebey from Russia, utilizing a combination of histological and tomographic data, reveals unusual patterns of tooth development and replacement. The data confirm that bolosaurid teeth have thecodont implantation with deep roots, the oldest known such example among amniotes, and independently evolved among much younger archosauromorphs (including dinosaurs and crocodilians) and among synapsids (including mammals). High-resolution CT scans were able to detect the density boundary between the alveolar bone and the jawbone, as confirmed by histology, and revealed the location and size of developing replacement teeth in the pulp cavity of functional teeth. Evidence provided by the paratype dentary of Belebey chengi indicates that replacement teeth are present along the whole tooth row at slightly different stages of development, with the ontogenetically more developed teeth anteriorly, suggesting that tooth replacement was highly synchronized. CT data also show tooth replacement is directly related to the presence of lingual pits in the jaw, and that migration of tooth buds occurs initially close to these resorption pits to a position immediately below the functional tooth within its pulp cavity. The size and complex shape of the replacement teeth in the holotype of Bolosaurus grandis indicate that the replacement teeth can develop within the pulp cavity to an advanced stage while the previous generation remains functional for an extended time, reminiscent of the condition seen in other amniotes with occluding dentitions, including mammals.

Lateralized Feeding Behavior in a Paleozoic Reptile.

Lateralized behaviors have been reported in a variety of extant vertebrates, including birds and reptiles [1-3] and non-human mammals [4-6]. However, evidence of lateralized behaviors in extinct vertebrates is rare, primarily because of the difficulty of identifying such behaviors with confidence in fossils. In rare instances, paleontologists can infer asymmetry in predatory or foraging behavior, including predation scars on trilobites [7], directionality of invertebrate traces [8], and even behavioral asymmetry in fossil non-human primates [9, 10]. Because lateralized behaviors have been linked to hemispheric (brain) lateralization in some vertebrates [11-15], evidence of lateralized behaviors in ancient vertebrates might yield clues about the evolutionary origins of vertebrate brain lateralization. Here, we show the earliest evidence of lateralized behavior in a fossil reptile based on repeatable observations of tooth wear in a large sample of intact jaws. The patterns of dental wear along the tooth rows of nearly one hundred jaws of the small, early Permian (289 million years ago) reptile Captorhinus aguti indicate that it exhibited lateralized behavior, preferring to feed using the right side of the jaw. Discovery of such a feeding behavior in this ancient, terrestrial, and omnivorous animal provides direct evidence of the deep history of directional behavior among amniotes and may indicate an early origin of brain lateralization.

Early Jurassic dinosaur fetal dental development and its significance for the evolution of sauropod dentition.

Rare occurrences of dinosaurian embryos are punctuated by even rarer preservation of their development. Here we report on dental development in multiple embryos of the Early Jurassic Lufengosaurus from China, and compare these to patterns in a hatchling and adults. Histology and CT data show that dental formation and development occurred early in ontogeny, with several cycles of tooth development without root resorption occurring within a common crypt prior to hatching. This differs from the condition in hatchling and adult teeth of Lufengosaurus, and is reminiscent of the complex dentitions of some adult sauropods, suggesting that their derived dental systems likely evolved through paedomorphosis. Ontogenetic changes in successive generations of embryonic teeth of Lufengosaurus suggest that the pencil-like teeth in many sauropods also evolved via paedomorphosis, providing a mechanism for the convergent evolution of small, structurally simple teeth in giant diplodocoids and titanosaurids. Therefore, such developmental perturbations, more commonly associated with small vertebrates, were likely also essential events in sauropod evolution.

Histological skeletochronology indicates developmental plasticity in the early Permian stem lissamphibian Doleserpeton annectens.

Doleserpeton annectens is a small-bodied early Permian amphibamiform, a clade of temnospondyl amphibians regarded by many workers to be on the lissamphibian stem. Most studies of this taxon have focused solely on its anatomy, but further exploration of other aspects of its paleobiology, such as developmental patterns, is critical for a better understanding of the early evolutionary history of lissamphibians. Here, we present a histological analysis of growth patterns in D. annectens that utilizes 60 femora, the largest sample size for any Paleozoic tetrapod. We identified pervasive pairs of closely spaced lines of arrested growth (LAGs), a pattern that indicates a marked degree of climatic harshness and that would result in two cessations of growth within a presumed single year. We documented a wide degree of variation compared to previous temnospondyl skeletochronological studies, reflected in the poor correlation between size and inferred age, but this observation aligns closely with patterns observed in extant lissamphibians. Furthermore, sensitivity analyses conducted by subsampling our dataset at more typical sample sizes for paleontological studies produced a wide range of results. This includes biologically improbable results and exceptionally well-fit curves that demonstrate that low sample size can produce potentially misleading artifacts. We propose that the weak correlation between age and size represents developmental plasticity in D. annectens that typifies extant lissamphibians. Detection of these patterns is likely only possible with large sample sizes in extinct taxa, and low sample sizes can produce false, misleading results that warrant caution in drawing paleobiological interpretations from such samples.

Postcranial anatomy and histology of Seymouria, and the terrestriality of seymouriamorphs.

Seymouria is the best known of the seymouriamorphs, a group of Permo-Carboniferous reptiliomorphs with both terrestrial and aquatic taxa. The majority of research on Seymouria has focused on cranial anatomy, with few detailed descriptions or illustrations of the postcrania. We utilized neutron computed tomography (nCT) and histological sampling to provide updated, detailed figures that clarify details of the postcranial anatomy and to assess the development and histology of Seymouria through specimens from the early Permian Richards Spur locality. The correlation of morphological and histological data indicate rapid metamorphosis in this terrestrially capable stem amniote, with the youngest specimen being postmetamorphic despite being distinctly younger than premetamorphic individuals of Discosauriscus, the only other seymouriamorph to have been histologically sampled. The microanatomical data (e.g., semi-open medullary cavity) also substantiate the hypothesis that Seymouria was terrestrial based on interpretation of external features, although the persistence of a modestly developed medullary spongiosa in comparison to either Discosauriscus or to other co-occurring terrestrial tetrapods suggests additional nuances that require further exploration. In the absence of clearly recognizable postmetamorphic stages in several seymouriamorph taxa, it is difficult to determine the evolutionary trajectory of terrestriality within the clade. Our analysis provides the first histological characterization of the life history of Seymouria and highlights the need for further study of seymouriamorph ontogeny.