The femora of Drepanosauromorpha (Reptilia: Diapsida): Implications for the functional evolution of the thigh of Sauropsida.

The femora of diapsids have undergone morphological changes related to shifts in postural and locomotor modes, such as the transition from plesiomorphic amniote and diapsid taxa to the apomorphic conditions related to a more erect posture within Archosauriformes. One remarkable clade of Triassic diapsids is the chameleon-like Drepanosauromorpha. This group is known from numerous articulated but heavily compressed skeletons that have the potential to further inform early reptile femoral evolution. For the first time, we describe the three-dimensional osteology of the femora of Drepanosauromorpha, based on undistorted fossils from the Upper Triassic Chinle Formation and Dockum Group of North America. We identify apomorphies and a combination of character states that link these femora to those in crushed specimens of drepanosauromorphs and compare our sample with a range of amniote taxa. Several characteristics of drepanosauromorph femora, including a hemispherical proximal articular surface, prominent asymmetry in the proximodistal length of the tibial condyles, and a deep intercondylar sulcus, are plesiomorphies shared with early diapsids. The femora contrast with those of most diapsids in lacking a crest-like, distally tapering internal trochanter. They bear a ventrolaterally positioned tuberosity on the femoral shaft, resembling the fourth trochanter in Archosauriformes. The reduction of an internal trochanter parallels independent reductions in therapsids and archosauriforms. The presence of a ventrolaterally positioned trochanter is also similar to that of chameleonid squamates. Collectively, these features demonstrate a unique femoral morphology for drepanosauromorphs, and suggest an increased capacity for femoral adduction and protraction relative to most other Permo-Triassic diapsids.

Intraspecific variation in the cranial osteology of Diplometopon zarudnyi (Squamata: Amphisbaenia: Trogonophidae).

A snake-like body plan and burrowing lifestyle characterize numerous vertebrate groups as a result of convergent evolution. One such group is the amphisbaenians, a clade of limbless, fossorial lizards that exhibit head-first burrowing behavior. Correlated with this behavior, amphisbaenian skulls are more rigid and coossified than those of nonburrowing lizards. However, due to their lifestyle, there are many gaps in our understanding of amphisbaenian anatomy, including how their cranial osteology varies among individuals of the same species and what that reveals about constraints on the skull morphology of head-first burrowing taxa. We investigated intraspecific variation in the cranial osteology of amphisbaenians using seven individuals of the trogonophid Diplometopon zarudnyi. Variation in both skull and individual skull element morphology was examined qualitatively and quantitatively through three-dimensional (3D) models created from microcomputed tomography data. Qualitative examination revealed differences in the number and position of foramina, the interdigitation between the frontals and parietal, and the extent of coossification among the occipital complex, fused basioccipital and parabasisphenoid ("parabasisphenoid-basioccipital complex"), and elements X. We performed 3D landmark-based geometric morphometrics for the quantitative assessment, revealing shape differences in the skull, premaxilla, maxilla, frontal, and parietal. The observed intraspecific variation may be the result of different stages of ontogenetic development or biomechanical optimization for head-first burrowing. For example, variation in the coossification of the occipital region suggests a potential ontogenetic coossification sequence. Examination of these areas of variation across other head-first burrowing taxa will help determine if the variation is clade-specific or part of a broader macroevolutionary pattern of head-first burrowing.

Ecomorphometric Analysis of Diversity in Cranial Shape of Pygopodid Geckos.

Pygopodids are elongate, functionally limbless geckos found throughout Australia. The clade presents low taxonomic diversity (∼45 spp.), but a variety of cranial morphologies, habitat use, and locomotor abilities that vary between and within genera. In order to assess potential relationships between cranial morphology and ecology, computed tomography scans of 29 species were used for 3D geometric morphometric analysis. A combination of 24 static landmarks and 20 sliding semi-landmarks were subjected to Generalized Procrustes Alignment. Disparity in cranial shape was visualized through Principal Component Analysis, and a multivariate analysis of variance (MANOVA) was used to test for an association between shape, habitat, and diet. A subset of 27 species with well-resolved phylogenetic relationships was used to generate a phylomorphospace and conduct phylogeny-corrected MANOVA. Similar analyses were done solely on Aprasia taxa to explore species-level variation. Most of the variation across pygopodids was described by principal component (PC) 1(54%: cranial roof width, parabasisphenoid, and occipital length), PC2 (12%: snout elongation and braincase width), and PC3 (6%: elongation and shape of the palate and rostrum). Without phylogenetic correction, both habitat and diet were significant influencers of variation in cranial morphology. However, in the phylogeny-corrected MANOVA, habitat remained weakly significant, but not diet, which can be explained by generic-level differences in ecology rather than among species. Our results demonstrate that at higher levels, phylogeny has a strong effect on morphology, but that influence may be due to small sample size when comparing genera. However, because some closely related taxa occupy distant regions of morphospace, diverging diets, and use of fossorial habitats may contribute to variation seen in these geckos.

Multi-year dynamics of ranavirus, chytridiomycosis, and co-infections in a temperate host assemblage of amphibians.

Chytridiomycosis and ranavirosis are 2 emerging infectious diseases that have caused significant global amphibian decline. Although both have received much scrutiny, little is known about interactions between the 2 causative agents Batrachochytrium dendrobatidis (Bd) and ranavirus (Rv) at the individual host and population levels. We present the first longitudinal assessment of Bd, Rv, and co-infections of a temperate amphibian assemblage in North America. From 2012 to 2016, we assessed the temporal oscillations of Bd, Rv and co-infection dynamics in a sample of 729 animals representing 13 species. Bd, Rv, and co-infected amphibians were detected during all 5 yr. Bd, Rv, and co-infection prevalence all varied annually, with the lowest instances of each at 2.1% (2013), 7.9% (2016), and 0.6% (2016), respectively. The highest Bd, Rv, and co-infection prevalence were recorded in 2012 (26.8%), 2016 (38.3%), and 2015 (10.3%), respectively. There was no association between Bd or Rv infection prevalence and co-infection, either when assessing the entire amphibian assemblage as a whole (odds ratio 1.32, 95% CI: 0.83-2.1, p = 0.29) or within species for amphibians that were more numerically represented (n > 40, p > 0.05). This suggests neither Bd nor Rv facilitate host co-infections within the sampled host assemblage. Instead, the basis for co-infections is the spatiotemporal distribution of both pathogens. Despite lack of interplay between Bd and Rv in this population, our study highlights the importance of considering numerous pathogens that may be present within amphibian habitats in order to properly anticipate interactions that may have direct bearing on disease outcomes.

Deep-time evolution of regeneration and preaxial polarity in tetrapod limb development.

Among extant tetrapods, salamanders are unique in showing a reversed preaxial polarity in patterning of the skeletal elements of the limbs, and in displaying the highest capacity for regeneration, including full limb and tail regeneration. These features are particularly striking as tetrapod limb development has otherwise been shown to be a highly conserved process. It remains elusive whether the capacity to regenerate limbs in salamanders is mechanistically and evolutionarily linked to the aberrant pattern of limb development; both are features classically regarded as unique to urodeles. New molecular data suggest that salamander-specific orphan genes play a central role in limb regeneration and may also be involved in the preaxial patterning during limb development. Here we show that preaxial polarity in limb development was present in various groups of temnospondyl amphibians of the Carboniferous and Permian periods, including the dissorophoids Apateon and Micromelerpeton, as well as the stereospondylomorph Sclerocephalus. Limb regeneration has also been reported in Micromelerpeton, demonstrating that both features were already present together in antecedents of modern salamanders 290 million years ago. Furthermore, data from lepospondyl 'microsaurs' on the amniote stem indicate that these taxa may have shown some capacity for limb regeneration and were capable of tail regeneration, including re-patterning of the caudal vertebral column that is otherwise only seen in salamander tail regeneration. The data from fossils suggest that salamander-like regeneration is an ancient feature of tetrapods that was subsequently lost at least once in the lineage leading to amniotes. Salamanders are the only modern tetrapods that retained regenerative capacities as well as preaxial polarity in limb development.

Skeletal Morphogenesis of Microbrachis and Hyloplesion (Tetrapoda: Lepospondyli), and Implications for the Developmental Patterns of Extinct, Early Tetrapods.

The ontogeny of extant amphibians often is used as a model for that of extinct early tetrapods, despite evidence for a spectrum of developmental modes in temnospondyls and a paucity of ontogenetic data for lepospondyls. I describe the skeletal morphogenesis of the extinct lepospondyls Microbrachis pelikani and Hyloplesion longicostatum using the largest samples examined for either taxon. Nearly all known specimens were re-examined, allowing for substantial anatomical revisions that affect the scoring of characters commonly used in phylogenetic analyses of early tetrapods. The palate of H. longicostatum is re-interpreted and suggested to be more similar to that of M. pelikani, especially in the nature of the contact between the pterygoids. Both taxa possess lateral lines, and M. pelikani additionally exhibits branchial plates. However, early and rapid ossification of the postcranial skeleton, including a well-developed pubis and ossified epipodials, suggests that neither taxon metamorphosed nor were they neotenic in the sense of branchiosaurids and salamanders. Morphogenetic patterns in the foot suggest that digit 5 was developmentally delayed and the final digit to ossify in M. pelikani and H. longicostatum. Overall patterns of postcranial ossification may indicate postaxial dominance in limb and digit formation, but also more developmental variation in early tetrapods than has been appreciated. The phylogenetic position and developmental patterns of M. pelikani and H. longicostatum are congruent with the hypothesis that early tetrapods lacked metamorphosis ancestrally and that stem-amniotes exhibited derived features of development, such as rapid and complete ossification of the skeleton, potentially prior to the evolution of the amniotic egg.

Comparative skull morphology of uropeltid snakes (Alethinophidia: Uropeltidae) with special reference to disarticulated elements and variation.

Uropeltids form a diverse clade of highly derived, fossorial snakes that, because of their phylogenetic position among other alethinophidian lineages, may play a key role in understanding the early evolution of cranial morphology in snakes. We include detailed osteological descriptions of crania and mandibles for eight uropeltid species from three nominal genera (Uropeltis, Rhinophis, and Brachyophidium) and emphasize disarticulated elements and the impact of intraspecific variation on previously proposed morphological characters used for phylogenetic analysis. Preliminary analysis of phylogenetic relationships strongly supports a clade composed exclusively of species of Plectrurus, Uropeltis, and Rhinophis. However, monophyly of each of those genera and Melanophidium is not upheld. There is moderate support that Sri Lankan species (e.g., Rhinophis and Uropeltis melanogaster) are monophyletic with respect to Indian uropeltids. Previously proposed characters that are phylogenetically informative include the shape of the nasals, length of the occipital condyle, level of development of the posteroventral process of the dentary, and participation of the parietal in the optic foramen. Additionally, thirty new features that may be systematically informative are identified and described, but were not verified for their utility. Such verification must await availability of additional disarticulated cranial material from a larger sample of taxa. All characters require further testing through increased focus on sources and patterns of intraspecific variation, inclusion of broader taxonomic samples in comparative studies, and exploration of skeletal development, sexual dimorphism, and biogeographic patterns. Additionally, trends in the relative enlargement of the sensory capsules, reduction in cranial ossification and dentition, fusion of elements, and the appearance of novel morphological conditions, such as the structure and location of the suspensorium, may be related to fossoriality and miniaturization in some uropeltid taxa, and may complicate analysis of relationships within Uropeltidae and among alethinophidian snakes.

A redescription of Carrolla craddocki (Lepospondyli: Brachystelechidae) based on high-resolution CT, and the impacts of miniaturization and fossoriality on morphology.

Some recent morphological analyses have brought into question the monophyly of Lissamphibia (frogs, salamanders, and caecilians). In these analyses, brachystelechid "microsaurs" are found to be sister group to caecilians. To test this hypothesis, the holotype specimen of the brachystelechid Carrolla craddocki was submitted to high-resolution X-ray computed tomography to gain insight into the nature of the morphology supporting the potential relationship between brachystelechids and caecilians. This analysis enabled us to conduct a detailed description of the internal anatomy such as the braincase and otic capsule endocast (the first of its kind for a lepospondyl), and new information regarding the architecture of the skull. Our results suggest brachystelechid cranial morphology is strongly influenced by miniaturization (enlarged sensory organs, anterior placement of the jaw articulation, and combination of both reduced- and hyper-ossifications) and burrowing habits (co-ossified braincase with broad, sloping occipital surface, overlapping joints between skull roof bones, and well-ossified anterior braincase). Characteristics of brachystelechids that appear unrelated to size-reduction and burrowing are the diamond-shaped skull and possible pedicellate dentition. We provide a revised diagnosis for Carrolla and identify possible new characters within the anatomy of the braincase and inner ear. Several characters currently uniting caecilians and "microsaurs" are among those associated with either miniaturization or burrowing, demonstrating that future efforts should continue to focus on fine details of anatomy minimally affected by these influences to contribute to the resolution of the question of the origin of caecilians.

Amphibian development in the fossil record.

Ontogenetic series of extinct taxa are extremely rare and when preserved often incomplete and difficult to interpret. However, the fossil record of amphibians includes a number of well-preserved ontogenetic sequences for temnospondyl and lepospondyl taxa, which have provided valuable information about the development of these extinct groups. Here we summarize the current knowledge on fossil ontogenies of amphibians, their potential and limitations for relationship assessments, and discuss the insights they have provided for our understanding of the anatomy, life history, and ecology of extinct amphibians.