Vertebrate scratch traces from the Middle Triassic Burgersdorp Formation of the main Karoo Basin, South Africa: Sedimentological and ichnological assessment

摘要

Containing one of the richest late Early to early Middle Triassic continental biotas globally, the Burgersdorp Formation of the main Karoo Basin in South Africa hosts a diverse vertebrate fossil assemblage. Comparatively, trace fossils in this unit are lesser known, and thus how the Burgersdorp biotas behaved and interacted with their palaeoenvironment remain enigmatic. Here we report on two Middle Triassic continental ichnofossil localities dominated by tetrapod scratch traces from the Anisian part (subzones B and C) of the Burgersdorp Formation in the main Karoo Basin (Eastern Cape, South Africa). Analyses of the trace fossils and associated sedimentary facies aim to identify the possible trace makers, their behaviours and the local depositional conditions in this southern part of Pangea. We establish the palaeoenvironmental context for the ichnofossils combining ichnological data with the sedimentological records of the trace fossil sites and the nearby type area of the Burgersdorp Formation. To facilitate comparisons with similar trace fossils in the future, we quantify the morphometric parameters of these Burgersdorp trace fossils in digital models. Our results show that bilobated, branching and cross-cutting furrow casts with abundant scratch traces: (a) were generated within crevasse splay deposits adjacent to laterally migrating river channels in a semi-arid setting, and (b) can be interpreted as the claw-scratched ventral walls of a therapsid burrow system with a 3D network architecture. This well-developed vertebrate burrow network complex from the Middle Triassic of South Africa is among a handful of similarly complex vertebrate trace fossils in the global pre-Jurassic ichnological record. Collectively, these trace fossils indicate that the Triassic, and in particular the Middle Triassic, was an important period in the evolutionary history of the complex vertebrate burrowing.