Abstract
A single species of the dicynodontoid dicynodont Dicynodon is currently recognized from the late Permian Usili Formation of Tanzania: Dicynodon huenei Haughton, 1932. Restudy of the known Tanzanian materials of D. huenei demonstrates that they represent two distinct morphotypes, here considered separate taxa. The holotype of D. huenei is not referable to Dicynodon and instead is transferred to the genus Daptocephalus (but retained as a valid species, Daptocephalus huenei comb. nov.). A number of published dicynodontoid specimens from the Usili Formation, however, are referable to Dicynodon, and are here recognized as a new species (Dicynodon angielczyki sp. nov.) Dicynodon angielczyki can be distinguished from its South African congener Dicynodon lacerticeps by the presence of an expansion of the squamosal and jugal beneath the postorbital bar and a curved, posterolateral expansion of the squamosal behind the temporal fenestra. Inclusion of Dicynodon angielczyki and D. huenei in a phylogenetic analysis supports their referral to Dicynodon and Daptocephalus (respectively). These results indicate higher basinal endemism in large late Permian dicynodonts than previously thought, a sharp contrast to the cosmopolitanism in the group in the earliest Triassic.
Highlights
The Usili Formation is a sedimentary unit of late Permian age exposed in the Ruhuhu Basin at the southwestern edge of Tanzania (Wopfner, 2002; Sidor & Nesbitt, 2018)
The Ruhuhu Basin has been recognized as fossiliferous since the initial geological surveys of Stockley (Stockley, 1932; Stockley & Oates, 1931), who collected a number of therapsid fossils in what is considered the Usili Formation and sent them to the South African Museum (Cape Town, South Africa) for study
Only D. huenei is considered valid today: Dicynodon tealei and M. rugosus are considered rhachiocephalids of dubious validity (Brink, 1986; Kammerer, Angielczyk & Fröbisch, 2011) and P. stockleyi is considered a probable synonym of Endothiodon bathystoma, a taxon better known from the Karoo Basin of South Africa (Cox & Angielczyk, 2015)
Summary
The Usili Formation is a sedimentary unit of late Permian age exposed in the Ruhuhu Basin at the southwestern edge of Tanzania (Wopfner, 2002; Sidor & Nesbitt, 2018). The left mandibular ramus has been separated from the main block and fully prepared (Fig. 12) This specimen can be identified as a dicynodontoid on the basis of an enlarged labial fossa and the uniformly rugose palatine pad flush with the surrounding palate (as opposed to the condition in geikiids, where the palatine pad is smoother and flush with the rest of the palate anteriorly but raised and extremely rugose posteriorly), and can be recognized as Dicynodon angielczyki rather than Daptocephalus huenei based on the relatively narrow median pterygoid plate bearing a well-developed crista oesophagea. The Dicynodon clade is supported by two characters (continuous character 14, shape of mandibular fenestra, and discrete state character 51, postorbital contribution to intertemporal bar relatively flat) and the Daptocephalus clade is supported by four characters (continuous characters 8, median pterygoid plate width, and 11, relative area of internal nares, and discrete state characters 92, ventral surface of median pterygoid plate smooth and flat, and 111, absence of central circular depression on occipital condyle) (refer to Supplementary Information for lists of all character states)
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