Abstract
The Hapsidopareiidae is a group of “microsaurs” characterized by a substantial reduction of several elements in the cheek region that results in a prominent, enlarged temporal emargination. The clade comprises two markedly similar taxa from the early Permian of Oklahoma, Hapsidopareion lepton and Llistrofus pricei, which have been suggested to be synonymous by past workers. Llistrofus was previously known solely from the holotype found near Richards Spur, which consists of a dorsoventrally compressed skull in which the internal structures are difficult to characterize. Here, we present data from two new specimens of Llistrofus. This includes data collected through the use of neutron tomography, which revealed important new details of the palate and the neurocranium. Important questions within “Microsauria” related to the evolutionary transformations that likely occurred as part of the acquisition of the highly modified recumbirostran morphology for a fossorial ecology justify detailed reexamination of less well-studied taxa, such as Llistrofus. Although this study eliminates all but one of the previous features that differentiated Llistrofus and Hapsidopareion, the new data and redescription identify new features that justify the maintained separation of the two hapsidopareiids. Llistrofus possesses some of the adaptations for a fossorial lifestyle that have been identified in recumbirostrans but with a lesser degree of modification (e.g., reduced neurocranial ossification and mandibular modification). Incorporating the new data for Llistrofus into an existing phylogenetic matrix maintains the Hapsidopareiidae’s (Llistrofus + Hapsidopareion) position as the sister group to Recumbirostra. Given its phylogenetic position, we contextualize Llistrofus within the broader “microsaur” framework. Specifically, we propose that Llistrofus may have been fossorial but was probably incapable of active burrowing in the fashion of recumbirostrans, which had more consolidated and reinforced skulls. Llistrofus may represent an earlier stage in the step-wise acquisition of the derived recumbirostran morphology and paleoecology, furthering our understanding of the evolutionary history of “microsaurs.”
Highlights
The karst deposits near Richards Spur, Oklahoma preserve a diverse early Permian tetrapod assemblage that includes the recumbirostran “microsaurs” Cardiocephalus peabodyi (Carroll & Gaskill, 1978) and Nannaroter mckinziei (Anderson, Scott & Reisz, 2009)
Both taxa are endemic to early Permian deposits of Oklahoma, with Llistrofus known from Richards Spur and Hapsidopareion known from South Grandfield (Daly, 1973; Bolt & Rieppel, 2009)
We have presented new data regarding the hapsidopareiid Llistrofus pricei that have greatly improved our knowledge of its unusual anatomy
Summary
The karst deposits near Richards Spur, Oklahoma preserve a diverse early Permian tetrapod assemblage that includes the recumbirostran “microsaurs” Cardiocephalus peabodyi (Carroll & Gaskill, 1978) and Nannaroter mckinziei (Anderson, Scott & Reisz, 2009). That taxon is recognizable by a large, ventrally open temporal emargination, which results from a reduction in the jugal, the postorbital, and the squamosal This emargination is shared with another early Permian “microsaur,” Hapsidopareion, and unites them within the Hapsidopareiidae (Bolt & Rieppel, 2009). Both taxa are endemic to early Permian deposits of Oklahoma, with Llistrofus known from Richards Spur and Hapsidopareion known from South Grandfield (Daly, 1973; Bolt & Rieppel, 2009) This clade has sometimes included the early Permian taxon Saxonerpeton from Germany, despite the absence of an emargination in that taxon (Carroll & Gaskill, 1978). The earlier stages of “microsaur” evolution, both in general and with respect to the acquisition of the recumbirostran suite of characters, remain poorly understood
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