A new notosuchian crocodyliform from the Early Palaeocene of Patagonia and the survival of a large-bodied terrestrial lineage across the K-Pg mass extinction.

Sebecid notosuchians are the only terrestrial crocodyliforms to survive the Cretaceous-Palaeogene extinction, 66 Ma, which eliminated large-bodied species (above approximately 5 kg) in terrestrial ecosystems. Early sebecid evolution is unclear due to the scarcity of remains from both sides of the boundary. We present the stratigraphically earliest post-extinction notosuchian record, from the lower Palaeocene Salamanca Formation of Patagonia. Tewkensuchus salamanquensis n. gen. n. sp. has unique features, including a skull roof with elevated lateral margins, and an accessory peg and socket articulation between the postorbital and posterior palpebral. Our phylogenetic analysis allies Tewkensuchus with a clade of predatorial crocodyliforms from the Eocene of Europe (and possibly of Africa, as Eremosuchus may also belong to this clade). This clade forms the sister taxon of South American sebecids. We name Sebecoidea for this more inclusive clade of Eurogondwanan notosuchians and suggest that its spatial distribution reflects earlier diversification and dispersal events, which are only partially known. We estimate a body mass of around 300 kg for Tewkensuchus, one of the largest known notosuchians. Phylogenetic optimization of notosuchian body size change reconstructs a Cretaceous-Palaeogene boundary-crossing sebecoidean lineage with an estimated mass between 332 and 443 kg. This provides the first support for the survival of a large-bodied terrestrial vertebrate lineage across the K-Pg boundary.