‘Chasmatosaurus ultimus,’ is NOT an Anisian Triassic Proterosuchid Archosauriform

‘Chasmatosaurus ultimus,’ a putative proterosuchid archosauriform from the Middle Triassic, is an indeterminate crown archosaur

Authors:

Liu et al

Abstract:

Proterosuchids represent the earliest known radiation of the highly diverse and important clade Archosauriformes (Gauthier et al., 1988; Nesbitt, 2011; Ezcurra et al., 2013). Huene (1908) erected Proterosuchidae as a monotypic Linnean family containing only Proterosuchus fergusi from the lowermost Triassic of South Africa, and several other species have been referred to this group in the subsequent century. However, phylogenetic analyses have provided only partial support for the existence of a diverse proterosuchid clade, and it is possible that some or even most putative proterosuchids form a series of outgroups to more derived archosauriforms rather than a monophyletic group (Ezcurra et al., 2010, 2013). Proterosuchids were historically classified alongside erythrosuchids in the Proterosuchia (e.g., Charig and Reig, 1970), but cladistic analyses (e.g., Benton, 1985; Gauthier et al., 1988; Gower and Sennikov, 1997; Ezcurra et al., 2010; Nesbitt, 2011) have uniformly found erythrosuchids to be phylogenetically closer to Archosauria (using a crown-group definition for the clade) than are any putative proterosuchids. Many of the species that have been assigned to Proterosuchidae require anatomical and taxonomic reevaluation (e.g., Ezcurra et al., 2010, 2013, 2014; Ezcurra, 2014), a research effort that is essential in order to clarify the phylogenetic relationships of the earliest archosauriforms and patterns of character evolution in taxa of this grade.

Following Proterosuchus, the next proterosuchid genera to be erected were Chasmatosaurus, established by Haughton (1924) for the new species C. vanhoepeni, and Elaphrosuchus, established by Broom (1946) for the new species E. rubidgei. A second species of Chasmatosaurus, C. alexanderi, was subsequently added (Hoffman, 1965). Chasmatosaurus vanhoepeni, C. alexanderi, and E. rubidgei were all based, like P. fergusi, on specimens from the Lower Triassic of South Africa, and more recently all of them have been suggested to represent subjective junior synonyms of P. fergusi (Welman, 1998). Occurrences of Chasmatosaurus have also been reported outside South Africa, including C. yuani (Young, 1936, 1963, 1978) from the Lower Triassic of China, Chasmatosaurus sp. from the Lower Triassic of India (Satsangi, 1964), and C. ultimus from the Middle Triassic of China (Young, 1958, 1964). The last of these species has the distinction of being one of only two putative proterosuchids from above the Lower–Middle Triassic boundary, the other being Sarmatosuchus otschevi from the Anisian of Russia (Gower and Sennikov, 1997, 2000; Ezcurra et al., 2013). However, the proterosuchid affinities of S. otschevi are questionable (Ezcurra et al., 2010, 2013), with S. otschevi having been recovered closer to erythrosuchids and archosaurs than to proterosuchids by Ezcurra et al. (2010). Thus, C. ultimus is the only generally accepted proterosuchid record younger than Early Triassic.

The holotype of C. ultimus is a partial skull including parts of the preorbital region preserved together with the anterior ends of the lower jaws, collected from the upper member of the Ermaying Formation of Shanxi Province in northern China, which is Anisian in age (Liu et al., 2013). This fragmentary specimen was originally assigned by Young (1958) to C. yuani, but later used by Young (1964) to erect C. ultimus based on differences in size and tooth curvature from the former species. Here we redescribe the specimen and show that ‘C. ultimus’ is not a proterosuchid but rather an indeterminate crown archosaur. An important implication of this revision is the absence of any confirmed record of proterosuchid archosauriforms from rocks of Middle Triassic age or younger.