DOI: 10.15468/t6x92v
title: Redescription of Digalodon rubidgei, an emydopoid dicynodont (Therapsida, Anomodontia) from the Late Permian of South Africa

description: This dataset contains the digitized treatments in Plazi based on the original journal article Kammerer, C. F., Angielczyk, K. D., Fröbisch, J. (2015): Redescription of Digalodon rubidgei, an emydopoid dicynodont (Therapsida, Anomodontia) from the Late Permian of South Africa. Fossil Record 18 (1): 43-55, DOI: 10.5194/fr-18-43-2015, URL: http://dx.doi.org/10.5194/fr-18-43-2015Abstract. The Late Permian dicynodont Digalodon rubidgei Broom and Robinson, 1948, is redescribed based on reanalysis of the holotype and newly recognized referable specimens. Digalodon can be diagnosed by the presence of a long “beak” sharply demarcated from the caniniform process; an extremely tall zygomatic ramus of the squamosal, with a thickened, “folded-over” dorsal margin; raised parietal “lips” along the lateral edges of the pineal foramen; and a broad posterolateral expansion of the parietal, excluding the postorbital from the back of the skull roof. Inclusion of Digalodon in a recent analysis of anomodont phylogeny recovers it as a kistecephalian emydopoid, specifically as the sister taxon to the clade containing the remaining kistecephalians. Four definite specimens of Digalodon are known, but several additional specimens lacking tusks, the swollen pineal “lips”, and a thickened zygoma may represent sexually dimorphic females or juveniles. Specimens of Digalodon are restricted to the central portion of the Karoo Basin, in the area around Graaff-Reinet, and are part of a characteristic fauna probably representing a limited time span.1 IntroductionEmydopoidea is one of the three major groups of “advanced” dicynodont therapsids (therochelonians sensu Kammerer and Angielczyk, 2009). Unlike the other therochelonians, which are primarily large-bodied (&gt; 1 m body length) animals, emydopoids are typically small, with only a single genus (Dicynodontoides) attaining a skull length greater than 15 cm (Angielczyk et al., 2009). Four families of emydopoids are known: the basal Emydopidae (containing Emydops) and the three clades united in Kistecephalia – Kingoriidae (Dicynodontoides and Kombuisia), Myosauridae (Myosaurus), and the fossorial Cistecephalidae (Cistecephalus, Cistecephaloides, and Kawingasaurus). Emydopoid taxonomy and anatomy have been extensively studied in the past few decades and their phylogenetic relationships are generally well understood (Cox, 1959, 1972; Cluver, 1978; Hammer and Cosgriff, 1981; Angielczyk et al., 2005, 2009; Fröbisch, 2007; Fröbisch and Reisz, 2008). Emydopoids are common in the early Late Permian of South Africa, with hundreds of specimens recovered from the Tropidostoma and Cistecephalus assemblage zones (AZs) of the Karoo Basin (Smith et al., 2012). In the latest Permian (Dicynodon AZ), however, emydopoids have traditionally been considered a rare faunal component, part of a general pattern of extinction of small dicynodonts (e.g., Emydops, but also including Diictodon and Pristerodon) well before the terminal Permian extinction (Smith and Botha-Brink, 2014). Despite the extinction of these well-known genera, there are various enigmatic, emydopoid-like small dicynodonts from the Late Permian that have been historically understudied and may influence this supposed pattern.Broom and Robinson (1948) described Digalodon rubidgei based on a single skull (RC 76) collected in the Dicynodon AZ of Ferndale, Graaff-Reinet. The new taxon was diagnosed primarily by characters of the intertemporal region. For example, the lengthy intertemporal portion of the frontals in RC 76 was contrasted with the condition in “all typical Dicynodonts” [sic] in which “the frontals lie mainly between the orbits” (Broom and Robinson, 1948:402). It should be noted that the frontal morphology of RC 76 is actually very similar to that of the common Permian dicynodont Pristerodon, and a long contribution of the frontal to the intertemporal region is also present in myosaurids and cistecephalids. In their defense, Broom and Robinson (1948:404) considered at least Myosaurus and Cistecephalus to be atypical dicynodonts, “much specialized and degenerate types ”, and even mentioned that they “may have evolved from a dicynodont like Digalodon ”. Broom and Robinson (1948) also distinguished Digalodon from Dicynodon (sensu lato, including Diictodon, Oudenodon, and various other taxa – see Kammerer et al., 2011) by its broad parietal exposure in the intertemporal region. They noted (correctly) that this represents a primitive character common in small-bodied dicynodonts like Pristerodon and most emydopoids.Following its initial description, Digalodon was largely ignored by subsequent workers, appearing primarily in comprehensive lists of Karoo vertebrates or therapsid taxa (e.g., Haughton and Brink, 1954; Romer, 1956; Lehman, 1961). Two attempts were made to refer the mysterious type specimen of D. rubidgei to a better-known dicynodont taxon. Cluver and King (1983) noted a few similarities between Digalodon and Aulacephalodon, leading King (1988) to consider them tentative synonyms (presumably considering the much smaller Digalodon a juvenile of the latter genus). Brink (1986) listed Digalodon rubidgei as a junior synonym of Dicynodontoides parringtoni (i.e., Dicynodontoides recurvidens) without comment. Most recently, Angielczyk et al. (2009) reconsidered the synonymy of Digalodon and Dicynodontoides, and concluded that Digalodon represents a distinct taxon possibly related to Emydops. Furthermore, they referred two additional specimens to D. rubidgei (RC 469 and USNM 22941), but noted that these referrals were tentative pending a full redescription of the species.Here, we completely redescribe the cranial morphology of Digalodon rubidgei based on the holotype. Additionally, we address the status of additional skulls that may be referable to Digalodon and consider the geographic and stratigraphic range of this taxon and its phylogenetic position.Institutional Abbreviations – B, Bremner Collection, Graaff-Reinet Museum, Graaff-Reinet, South Africa; BP, Evolutionary Studies Institute (formerly Bernard Price Institute for Palaeontological Research), University of the Witwatersrand, Johannesburg, South Africa; RC, Rubidge Collection, Wellwood, Graaff-Reinet District, South Africa; USNM, National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA.