Re-discovery of the Euparkeria bonebed locality (Mid-Triassic) in Aliwal North, South Africa, with an update of the taphonomy and depositional environment
Date
2023-07
Authors
Wolvaardt, Frederik P.
Smith, Roger M. H.
Arcucci, Andrea
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Abstract
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.