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Embolomeres have been considered candidate stem-amniotes by many authors, but recent work has shown that anatomical and phylogenetic support for this hypothesis may be ambiguous. Newer phylogenetic approaches focus on the anatomy of the endochondral bones surrounding the brain (neurocranium) and associated dermal investing bones, so accurate description of the neurocranium of tetrapods, and particularly embolomeres, is critical to assess the phylogenetic context of amniote origins. I here amend prior descriptions of the neurocranium of the abundant Early Permian embolomere, Archeria crassidisca , with a description of a previously unreported partial braincase. This specimen reveals features broadly associated with stem-tetrapods of the fin-to-limb transition and recently described in aïstopod stem-tetrapods, including a cranial notochordal remnant, a possible buccohypophyseal canal, and anteroventrally projecting saccular fossae that extend deep into the basioccipital. I present an amended description and reconstruction of the neurocranium and a phylogenetic analysis integrating these codings, which recovers embolomeres more robustly within the tetrapod stem. I review characters previously used to assign embolomeres to the amniote stem and find this evidence is ambiguous or incorrect. I then outline strong character support for a stem-tetrapod position of embolomeres, and implications for the ancestral body plan of crown-group tetrapods. [ABSTRACT FROM AUTHOR]

The group of Permo-Carboniferous tetrapods known as Recumbirostra have recently been hypothesized to represent the earliest radiation of fossorial reptiles. Therefore, understanding the anatomy and diversity of this clade is essential to understanding the origin and early evolution of amniotes. Here, we redescribe the diminutive 'microsaur' Odonterpeton triangulare from the Moscovian-age deposit of Linton, Ohio, revealing new details on the structure of the cranium and palate, including the presence of a conspicuous transverse flange of the pterygoid, which is a feature traditionally associated with early amniotes. Phylogenetic analysis supports the placement of Odonterpeton triangulare within Recumbirostra as sister taxon to the recently described Joermungandr bolti from the slightly older Mazon Creek, Illinois, Lagerstätte. Together, these two diminutive recumbirostrans are placed in an Odonterpetidae clade. Furthermore, we provide a discussion of newly recognized anatomical features in recumbirostrans and discuss major features of recumbirostran phylogeny. We designate a new recumbirostran clade, Chthonosauria (Brachystelechidae + Molgophidae), which is supported by a number of shared derived cranial features. [ABSTRACT FROM AUTHOR]

Lysorophians are a group of early tetrapods with extremely elongate trunks, reduced limbs, and highly reduced skulls. Since the first discovery of this group, general similarities in outward appearance between lysorophians and some modern lissamphibian orders (specifically Urodela and Gymnophiona) have been recognized, and sometimes been the basis for hypotheses of lissamphibian origins. We studied the morphology of the skull, with particular emphasis on the neurocranium, of a partial growth series of the lysorophian Brachydectes newberryi using x-ray micro-computed tomography (μCT). Our study reveals similarities between the braincase of Brachydectes and brachystelechid recumbirostrans, corroborating prior work suggesting a close relationship between these taxa. We also describe the morphology of the epipterygoid, stapes, and quadrate in this taxon for the first time. Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium. Morphology previously considered indicative of taxonomic diversity within Lysorophia may reflect ontogenetic rather than taxonomic variation. The highly divergent morphology of lysorophians represents a refinement of morphological and functional trends within recumbirostrans, and is analogous to morphology observed in many modern fossorial reptiles. [ABSTRACT FROM AUTHOR]

Recumbirostran ‘microsaurs,’ a group of early tetrapods from the Late Carboniferous and Early Permian, are the earliest known example of adaptation to head-first burrowing in the tetrapod fossil record. However, understanding of the diversity of fossorial adaptation within the Recumbirostra has been hindered by poor anatomical knowledge of the more divergent forms within the group. Here we report the results of μCT study of Quasicaecilia texana, a poorly-known recumbirostran with a unique, broad, shovel-like snout. The organization of the skull roof and braincase of Quasicaecilia is found to be more in line with that of other recumbirostrans than previously described, despite differences in overall shape. The braincase is found to be broadly comparable to Carrolla craddocki, with a large presphenoid that encompasses much of the interorbital septum and the columella ethmoidalis, and a single compound ossification encompassing the sphenoid, otic, and occipital regions. The recumbirostran braincase conserves general structure and topology of braincase regions and cranial nerve foramina, but it is highly variable in the number of ossifications and their extent, likely associated with the reliance on braincase ossifications to resist compression during sediment compaction and mechanical manipulation by epaxial and hypaxial musculature. Expansion of the deep ventral neck musculature in Quasicaecilia, autapomorphic among recumbirostrans, may reflect unique biomechanical function, and underscores the importance of future attention to the role of the cervical musculature in contextualizing the origin and evolution of fossoriality in recumbirostrans. [ABSTRACT FROM AUTHOR]