DOI: 10.15468/8tj7p8
title: A re-examination of the enigmatic Russian tetrapod Phreatophasma aenigmaticum and its evolutionary implications

description: This dataset contains the digitized treatments in Plazi based on the original journal article Brocklehurst, Neil, Fröbisch, Jörg (2017): A re-examination of the enigmatic Russian tetrapod Phreatophasma aenigmaticum and its evolutionary implications. Fossil Record 20 (1): 87-93, DOI: 10.5194/fr-20-87-2017, URL: http://dx.doi.org/10.5194/fr-20-87-2017Abstract. Phreatophasma aenigmaticum is a mysterious tetrapod from the earliest middle Permian of Russia, represented by a single femur. At various times since its original description it has been considered a therapsid synapsid, a pelycosaurian-grade synapsid from the family Caseidae, and most recently a seymouriamorph amphibian. Using upto-date knowledge of the postcranial morphology and evolution of early synapsids, the specimen is re-evaluated and subjected to cladistic analysis. Seymouriamorph and therapsid affinities are rejected, and a caseid affinity is supported based on the deep intertrochanteric fossa; the widely spaced distal condyles; the short, robust femoral shaft; and the lack of a longitudinal ridge enclosing the posterior margin of the intertrochanteric fossa. When included in two cladistic matrices, the first a global analysis of basal synapsids and the second devoted to caseids, Phreatophasma is found to occupy a basal position within caseids, retaining plesiomorphic characters such as the lack of compression of the anterior condyle of the femur and the almost identical distal extent of the two condyles. The recognition of Phreatophasma as a basal caseid has great implications for the evolution and biogeography of this family. This is only the second example of a caseid from the palaeo-temperate region of Russia, and it is not closely related to the first (Ennatosaurus tecton), implying at least two distinct dispersal events from the palaeoequatorial to temperate latitudes. It also implies that a number of plesiomorphic characteristics of caseids, including small body size and a relatively long femur, were retained as late as the middle Permian, a time when caseids were otherwise represented by large herbivorous taxa.1 IntroductionThe early–middle Permian transition was a crucial period in the evolution of early synapsids. During the Cisuralian, terrestrial faunas were dominated by a paraphyletic grade of six synapsid families known as pelycosaurs. However, at the start of the Guadalupian these declined in diversity, possibly due to a mass extinction event (Sahney and Benton, 2008; Brocklehurst et al., 2013), and the Therapsida (the clade containing mammals) became more diverse and abundant. This transition coincides with a shift in the location of greatest sampling from the palaeoequatorial localities of North America and western Europe to the palaeo-temperate localities of South Africa and Russia. There is only a brief period of temporal overlap between the Russian and North American localities in the latest Kungurian and Roadian (Benton, 2012).Two of the six pelycosaur families are known to have survived into the middle Permian and dispersed into the palaeo-temperate latitudes. Species of Varanopidae are known from both Russia and South Africa (Dilkes and Reisz, 1996; Reisz et al., 1998; Modesto et al., 2011; Reisz and Modesto, 2007; Botha-Brink and Modesto, 2009), as late as the Pristerognathus Assemblage Zone (latest Capitanian) of the Karoo Supergroup (Modesto et al., 2011). Thus, Varanopidae represents not only the geologically longest living pelycosaur family but also the geographically most widespread. The second such family is Caseidae. Although this family was most diverse in the San Angelo and Chickasha formations of North America (Olson, 1962, 1965, 1968; Brocklehurst et al., 2013), a substantial species richness of caseids was recently also reported from Europe (Berman et al., 2004; Reisz et al., 2011; Romano and Nicosia, 2015; Spindler et al., 2015), and the Russian species Ennatosaurus tecton appears to have been quite abundant in the Mezen Faunal Complex (Efremov, 1956).A possible second Russian caseid has been tentatively put forward: Phreatophasma aenigmaticum, from the middle Permian Golyusherma Group. First described as a therapsid from the family Phreatosuchidae (Efremov, 1954), it was reassigned to Caseidae by Olson (1962), although the lack of material made this assignment extremely uncertain. Phreatophasma is represented by only a single femur and has not received substantial attention since Olson’s monograph on the family Caseidae (Olson, 1968). A brief note by Golubev (2000) cast doubt on its caseid affinities and suggested it might be a seymouriamorph, although no discussion of specific morphological characters was supplied.Now, however, our knowledge of the postcranial morphology of early synapsids has substantially increased (Benson, 2012), as has our knowledge of the evolution and phylogeny of caseids (Reisz, 2005; Maddin et al., 2008; Reisz and Fröbisch, 2014; Spindler et al., 2015; Romano and Nicosia, 2015; Brocklehurst et al., 2016a). Thus, following a re-examination and phylogenetic analysis of the specimen, we are able to confirm the assignment of Phreatophasma to Caseidae.