New and historical specimens of burnetiamorph therapsids, with comments on ontogeny, biogeography, and bizarre structures
Date
2023-07
Authors
Sidor, Christian A.
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Abstract
For much of its history, the fossil record of burnetiamorph therapsids was both species poor and known from very few specimens. By
contrast, the past two decades have seen a dramatic increase in both measures of the group’s fossil record. Here I introduce two new
species, Bondoceras bulborhynchus gen. et sp. nov. and Pembecephalus litumbaensis gen. et sp. nov., from the Guadalupian middle
Madumabisa Mudstone Formation of Zambia and the Lopingian Usili Formation of Tanzania, respectively. Both are based on a series of
isolated skullcaps that preserve a unique combination of characters that suggest referral to subclades within Burnetiidae. Historical
burnetiamorph skullcaps collected in Tanzania in the early 1930s are figured, but these are kept in open nomenclature because not
enough information is preserved to warrant assigning them to Pembecephalus or another genus. A previously undescribed Russian
burnetiamorph is also figured, which is only the third specimen of the clade known from the northern hemisphere, but since the
specimen is possibly lost, it is also left unnamed. Specimens referred to Bondoceras and Pembecephalus suggest that while some
morphological features vary predictably with size in these burnetiamorphs, others do not, and that some features previously used to
assess ontogenetic status in burnetiamorphs are sometimes similarly problematic. Despite their wide range of shapes, surprisingly little
discussion of the function of burnetiamorph cranial adornments has taken place in the literature. Comparing the fossil records
of burnetiamorphs and other Permian synapsids to dinosaurs with similar adornments may shed light on the macroevolutionary
importance of these traits.