Articles | Volume 20, issue 1
Research article
21 Feb 2017
Research article |  | 21 Feb 2017

A re-examination of the enigmatic Russian tetrapod Phreatophasma aenigmaticum and its evolutionary implications

Neil Brocklehurst and Jörg Fröbisch

Abstract. 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 up-to-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.

Short summary
Phreatophasma is a mysterious species from the middle Permian of Russia. Since its description in 1954, it has been assigned to many different groups: therapsids (the most diverse of middle Permian terrestrial vertebrates), caseids (one of the earliest herbivore lineages), and seymouriamorphs (a Permian amphibian clade). Using recent knowledge of morphology and state-of-the art analyses, we confirm that Phreatophasma is one of the youngest members of Caseidae, the second member to enter Russia.