A Partially Articulated Cynodont Encased in a Putative Burrow Structure from the Cynognathus Subzone C

Date
2015-10
Authors
Afonso, Nadia
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Abstract
A sedimentary structure containing a fossilized therapsid, and bioglyphs on the surface morphology, was found during a field expedition to Cynognathus subzone-C in the Eastern Cape, South Africa. A combination of surface scanning, petrographic thin sections, bone mapping and anatomical comparison were used to determine the deposit type and taxonomic identity of the encased therapsid, and examine the implications for biostratigraphy and faunal assemblage of the Cynognathus Assemblage Zone. The deposit is hypothesized to be a portion of a cynodont burrow (burrow margin) that was constructed in fluvial sediment near a river bank. This is the first account of a burrow in subzone-C. Pattern and directionality analysis of the bioglyphs suggest that the bioglyphs are scratch marks made by the burrower during excavation. The scratch marks are mediolaterally narrow, with some exhibiting indentation marks, indicating that the tracemaker had mediolaterally narrow unguals at the distal phalanx (claws). Anomodonts and cynodonts were common burrowers during the Triassic, however comparisons of Thrinaxodon and Lystrosaurus scratch marks to the bioglyphs on this deposit suggests that the bioglyphs were likely constructed by a cynodont as anomodont unguals are laterally wider, and are unable to create mediolaterally narrow markings. The tracemaker is hypothesized to be closely related to Thrinaxodon based on scratch mark comparison. However, bioglyph published literature is limited, and therefore the tracemaker cannot be identified. The therapsid was identified as Diademodon based on cranial and dental morphology. There is limited published literature on Diademodon constructing and/or exploiting burrows, however the taphonomy suggests that the Diademodon was near the entrance or in the tunnel of burrow nearing/during death. This is the first account of Diademodon encased in a burrow.
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Keywords
cynodonts , Triassic , Karoo , ichnology
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