Triassic stem caecilian supports dissorophoid origin of living amphibians.

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan-batrachian molecular divergence1 suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period2-6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls7-10, but the absence of pre-Jurassic period caecilian fossils11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial1,8,13,14. Here we report the geologically oldest stem caecilian-a crown lissamphibian from the Late Triassic epoch of Arizona, USA-extending the caecilian record by around 35 million years. These fossils illuminate the tempo and mode of early caecilian morphological and functional evolution, demonstrating a delayed acquisition of musculoskeletal features associated with fossoriality in living caecilians, including the dual jaw closure mechanism15,16, reduced orbits17 and the tentacular organ18. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology19, in combination with vicariance patterns driven by plate tectonics20. These fossils reveal a combination of features that is unique to caecilians alongside features that are shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.

A new non-mammalian eucynodont from the Chinle Formation (Triassic: Norian), and implications for the early Mesozoic equatorial cynodont record.

The Upper Triassic tetrapod fossil record of North America features a pronounced discrepancy between the assemblages of present-day Virginia and North Carolina relative to those of the American Southwest. While both are typified by large-bodied archosaurian reptiles like phytosaurs and aetosaurs, the latter notably lacks substantial representation of mammal relatives, including cynodonts. Recently collected non-mammalian eucynodontian jaws from the middle Norian Blue Mesa Member of the Chinle Formation in northeastern Arizona shed light on the Triassic cynodont record from western equatorial Pangaea. Importantly, they reveal new biogeographic connections to eastern equatorial Pangaea as well as southern portions of the supercontinent. This discovery indicates that the faunal dissimilarity previously recognized between the western and eastern portions of equatorial Pangaea is overstated and possibly reflects longstanding sampling biases, rather than a true biogeographic pattern.

The earliest equatorial record of frogs from the Late Triassic of Arizona.

Crown-group frogs (Anura) originated over 200 Ma according to molecular phylogenetic analyses, though only a few fossils from high latitudes chronicle the first approximately 60 Myr of frog evolution and distribution. We report fossils that represent both the first Late Triassic and the earliest equatorial record of Salientia, the group that includes stem and crown-frogs. These small fossils consist of complete and partial ilia with anteriorly directed, elongate and distally hollow iliac blades. These features of these ilia, including the lack of a prominent dorsal protuberance and a shaft that is much longer than the acetabular region, suggest a closer affinity to crown-group Anura than to Early Triassic stem anurans Triadobatrachus from Madagascar and Czatkobatrachus from Poland, both high-latitude records. The new fossils demonstrate that crown anurans may have been present in the Late Triassic equatorial region of Pangea. Furthermore, the presence of Early Jurassic anurans in the same stratigraphic sequence ( Prosalirus bitis from the Kayenta Formation) suggests that anurans survived the climatic aridification of this region in the early Mesozoic. These fossils highlight the importance of the targeted collection of microfossils and provide further evidence for the presence of crown-group representatives of terrestrial vertebrates prior to the end-Triassic extinction.

Incongruent ontogenetic maturity indicators in a Late Triassic archosaur (Aetosauria: Typothorax coccinarum).

Maximum individual body size in pseudosuchian archosaurs is not well constrained in the fossil record, but it may be influenced by a variety of factors including basal metabolic rate, evolutionary relationships, and environmental conditions. Body size varies among the Aetosauria in which estimated total length ranges between 1 m (e.g., Coahomasuchus kahleorum) and 5 m (e.g., Desmatosuchus spurensis). A new, very large specimen of the aetosaurian Typothorax coccinarum from Petrified Forest National Park in northeastern Arizona is nearly twice the size of all other known specimens of Typothorax and is the largest aetosaur specimen currently known worldwide. The specimen lacks co-ossified neurocentral sutures in the trunk vertebrae which may suggest that the individual had not yet reached skeletal maturity, yet smaller specimens of T. coccinarum exhibit partially or fully co-ossified neurocentral sutures in the same region. If body size correlates with skeletal maturity in aetosaurs, this discrepancy warns that timing of neurocentral suture co-ossification in aetosaurs may not be a reliable indicator of ontogenetic stage. Osteohistological observations of a trunk rib demonstrate that although PEFO 42506 shows a large body size, the specimen did not deposit an external fundamental system despite depositing as many as 19 growth lines, further indicating that it had not yet reached skeletal maturity. Thus, at least within Aetosauria, neurocentral suture co-ossification and skeletal maturity may correlate, whereas body size can be incongruent in comparison. Furthermore, this specimen indicates that non-desmatosuchin aetosaurs could exhibit large body sizes and suggests that some aetosaurs may have experienced indeterminate growth.

Osteology and relationships of Revueltosaurus callenderi (Archosauria: Suchia) from the Upper Triassic (Norian) Chinle Formation of Petrified Forest National Park, Arizona, United States.

Once known solely from dental material and thought to represent an early ornithischian dinosaur, the early-diverging pseudosuchian Revueltosaurus callenderi is described from a minimum of 12 skeletons from a monodominant bonebed in the upper part of the Chinle Formation of Arizona. This material includes nearly the entire skeleton and possesses a combination of plesiomorphic and derived character states that help clarify ingroup relationships within Pseudosuchia. A phylogenetic analysis recovers R. callenderi in a clade with Aetosauria and Acaenasuchus geoffreyi that is named Aetosauriformes. Key autapomorphies of R. callenderi include a skull that is longer than the femur, a complete carapace of dermal armor including paramedian and lateral rows, as well as ventral osteoderms, and a tail end sheathed in bone. Histology of the femur and associated osteoderms demonstrate that R. callenderi was slow growing and that the individuals from the bonebed were not young juveniles but had not ceased growing. A review of other material assigned to Revueltosaurus concludes that the genus cannot be adequately diagnosed based on the type materials of the three assigned species and that only R. callenderi can be confidently referred to Revueltosaurus.

Anatomy and systematics of the sauropodomorph Sarahsaurus aurifontanalis from the Early Jurassic Kayenta Formation.

Sarahsaurus aurifontanalis, from the Kayenta Formation of Arizona, is one of only three sauropodomorph dinosaurs known from the Early Jurassic of North America. It joins Anchisaurus polyzelus, from the older Portland Formation of the Hartford Basin, and Seitaad reussi, from the younger Navajo Sandstone of Utah, in representing the oldest North American sauropodomorphs. If it is true that sauropodomorphs were absent from North America during the Late Triassic, the relationship among these three dinosaurs offers a test of the mechanisms that drove recovery in North American biodiversity following the end-Triassic extinction event. Here we provide the first thorough description of Sarahsaurus aurifontanalis based on completed preparation and computed tomographic imaging of the holotype and referred specimens. With new anatomical data, our phylogenetic analysis supports the conclusion that Sarahsaurus aurifontanalis is nested within the primarily Gondwanan clade Massospondylidae, while agreeing with previous analyses that the three North American sauropodomorphs do not themselves form an exclusive clade. A revised diagnosis and more thorough understanding of the anatomy of Sarahsaurus aurifontanalis support the view that independent dispersal events were at least partly responsible for the recovery in North American vertebrate diversity following a major extinction event.

Microanatomy and paleohistology of the intercentra of North American metoposaurids from the Upper Triassic of Petrified Forest National Park (Arizona, USA) with implications for the taxonomy and ontogeny of the group.

Metoposaurids are temnospondyl amphibians that are commonly recovered from the Chinle Formation deposits of North America. Two species, Koskinonodon perfectus and Apachesaurus gregorii, are known from Petrified Forest National Park (PEFO), AZ. Small, elongate intercentra are the single diagnostic postcranial characteristic of the smaller A. gregorii. However, a poor understanding of the earliest life stages of Koskinonodon perfectus and other large metoposaurids makes it unclear whether the proportions of the intercentra are a diagnostic feature for species discrimination or whether they are influenced by ontogeny. Previous work on metoposaurid intercentra has shown that ontogenetic information can be extrapolated from histological analyses. Here, we perform an analysis of the microanatomy and the histology of metoposaurid intercentra from PEFO to determine their ontogenetic maturity and in turn whether elongate intercentra are a reliable taxonomic feature for distinguishing North American metoposaurids. Our findings suggest that the elongate intercentra are the result of ontogenetic variation within a single large-bodied metoposaurid taxon rather than interspecific variation between two metoposaurids of vastly different adult sizes. These findings have significant implications for the taxonomy of North American metoposaurids and subsequently for interpretations of the Chinle Basin paleoenvironment. Furthermore, this study provides the first histological characterization of North American metoposaurid intercentra, thereby improving the understanding of vertebral ontogeny within Metoposauridae and offering new insights into the ecology of large metoposaurids.

Loss of GABAergic interneurons in laminae I-III of the spinal cord dorsal horn contributes to reduced GABAergic tone and neuropathic pain after spinal cord injury.

Abstract In this study we explore if loss of GABAergic inhibitory interneurons in the superficial dorsal horn of the spinal cord contributes to reduced GABAergic tone and neuropathic pain following spinal cord injury (SCI). A moderate contusion injury to T11 resulted in the development of mechanical hyperalgesia and thermal hyperalgesia below the level of the lesion in gad1:GFP mice that were alleviated by IP administration of the GABA transporter antagonist tiagabine. Six weeks following SCI a decreased number of GFP(+) neurons were observed in the dorsal horn of SCI animals relative to sham mice. Tissue from a mouse 2 weeks post-SCI was subsequently observed to express activated caspase-3, indicative of apoptosis, co-localized to some GFP(+) GABAergic neurons. Glutamate decarboxylase (GAD)65 and GAD67 immunohistochemical staining was reduced in the dorsal horn of SCI animals. This observation was confirmed in Western blots showing reduced immunoreactivity for GAD67, as well as GABA transporter (GAT)1. Reversal of post-SCI neuropathic pain by tiagabine suggests that reduced GABAergic tone may contribute to hyperalgesia symptoms. This is supported by the subsequent observation that SCI reduced the number of GFP(+) inhibitory neurons, and the finding that some GABAergic GFP(+) neurons undergo cell death at a time point consistent with the development of neuropathic pain following SCI. Concordantly, reductions in both GAD65 and GAD67 and GAT1 immunoreactivity also support the observation of a loss of GABAergic inhibition and the associated spinal interneurons.