A re-evaluation of the notosuchian crocodyliform Eremosuchus elkoholicus from the lower Eocene of Algeria and the evolutionary and biogeographic history of sebecids

Abstract

Notosuchian systematics have been highly debated in recent decades, particularly the placement of sebecids and closely related species. As the only notosuchian lineage to have survived the Cretaceous–Paleogene mass extinction, reconciliation of conflicting views on the group’s relationships is required to better understand extinction selectivity. Here, we redescribe and newly diagnose Eremosuchus elkoholicus from the Lower Eocene El Kohol Formation of Algeria, known from the holotypic dentary and several referred remains. A new, smaller dentary is also considered to represent an immature individual of this species, providing a rare notosuchian ontogenetic series. Eremosuchus is incorporated into one of the largest notosuchian-focused character-taxon matrices yet to be compiled, comprising 450 characters and 130 taxa. Focus is placed on improved sampling of mandibular characters and putative sebecids, especially frequently neglected taxa from Europe and Africa. Phylogenetic analyses, incorporating continuous characters, consistently recover Eremosuchus elkoholicus as a sebecid, though its precise position within this clade is uncertain. Under equal weighting, Sebecidae is recovered as the sister taxon to all other notosuchians, whereas a monophyletic Sebecosuchia is retrieved using extended implied weighting. The latter weighting approach finds the early Paleogene South American species, Lorosuchus nodosus and Sahitisuchus fluminensis, within Peirosauria, which would indicate the survival of a second notosuchian lineage across the Cretaceous/Paleogene boundary. The taxonomic and spatiotemporal expansion of Sebecidae via the inclusion of fragmentary material from Africa and Europe hints at a more complicated biogeographic and evolutionary history of the clade, and it remains unclear whether sebecids originated in Gondwana or Europe.