Unusual archosaur trackway and associated tetrapod ichnofauna from Irohalene member (Timezgadiouine formation, late Triassic, Carnian) of the Argana Basin, Western High Atlas, Morocco

Abstract

ABSTRACT An archosaur trackway consisting of 10 successive pes imprints has been recovered from the Late Triassic Irohalene Member of the Timezgadiouine Formation (Argana Basin, Morocco). Footprints are tetradactyl-pentadactyl, and show an elongated ‘heel’, probably enclosing the trace of digit V. The trackway lacks a manus impression, which could be due to bipedal progression of the trackmaker or be a preservational phenomenon. It is assigned to cf. Parachirotherium isp. based on similarities with the ichnogenus that has been described from the same stratigraphic unit. Other footprints of this assemblage are assigned to the ichnotaxa Grallator-Eubrontes (dinosauromorphs/crocodilian-stem archosaurs), Brachychirotherium isp. (crocodilian-stem archosaurs), Rhynchosauroides isp. (lepidosauromorphs/archosauromorphs), and indeterminate ichnotaxa. Associated with the footprints are numerous invertebrate traces such as Scoyenia gracilis. The ichnofauna of the Irohalene Member supports a cosmopolitan distribution of pentadactyl but functionally tridactyl chirotheres (Parachirotherium) and grallatorids across the Ladinian-Carnian boundary. Similar assemblages are known from the Germanic Basin and from Eastern North America. Furthermore, it documents the occurrence of very large Eubrontes trackmakers in the early Carnian. The depositional environment of the lower part of the Irohalene Member is interpreted as wet red beds of a flood basin transitional to a distal braid plain.