Re-discovery of the Euparkeria bonebed locality (Mid-Triassic) in Aliwal North, South Africa, with an update of the taphonomy and depositional environment

Wolvaardt, Frederik P.
Smith, Roger M. H.
Arcucci, Andrea
Journal Title
Journal ISSN
Volume Title
Euparkeria capensis is an Early to Middle Triassic archosauriform reptile widely regarded as phylogenetically close to the base of the Archosauria. Fossils of this species are only known from a single locality in the townlands of Aliwal North, South Africa. The exact location was, until now, uncertain due to mis-reading of the field notes of the collector Mr Alfred ‘Gogga’ Brown and conflicting anecdotal information gleaned from local townsfolk. Careful transcription of the voluminous handwritten field notes, followed by archival research in the town museum, and ground-truthing of the targeted area, has led to the re-discovery of the Euparkeria type locality. Sedimentological facies analysis of the locality combined with taphonomic observations of the 38 fossil-bearing rock slabs collected by Brown, now housed in the Iziko South African Museum reveal the following scenario for the origin of the Euparkeria/ Mesosuchus bonebed: the bones are preserved within the base of two tabular massive sandstone beds in upper point-bar facies of a high sinuosity channel-fill. The stratigraphic position, sedimentology, and geometry of the sandstone beds with their interdigitation with overlying floodplain mudrocks is interpreted as part of an infilled chute-channel cutting across a point-bar of a meandering river. The taphonomic analysis of the main bonebed suggests that the initial concentration of tetrapod remains was controlled by a combination of a mass mortality event of a pack of Euparkeria, and a few Mesosuchus becoming overwhelmed by a flash flood, and the hydrodynamics of the small floating carcasses getting trapped within the confines of a downstream chute-channel. The re-discovered Euparkeria locality is stratigraphically positioned at the top of a locally significant sandstone marker bed informally named the Eldorado marker. This unit is generally accepted as the contact between the lower and middle Burgersdorp Fm. Biostratigraphically, the locality is positioned within the transition between the lower and middle subzones of the Cynognathus AZ, namely Langbergia-Garjainia below and Trirachodon- Kannemeyeria above. This is some 23mlower in the stratigraphy than previously thought, and the possible association with Langbergiatype burrow casts leads us to tentatively place it within the lower subzone.