A new notosuchian crocodyliform from the Paleocene 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–Paleogene extinction, 66 Ma, which eliminated large-bodied
species (~>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 Paleocene 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 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 ~300 kg for
Tewkensuchus, one of the largest known notosuchians. Phylogenetic
optimization of notosuchian body size change reconstructs a
Cretaceous–Paleogene boundary-crossing sebecoidean lineage with an
estimated mass between 332–443 kg. This provides the first support for
survival of a large-bodied terrestrial vertebrate lineage across the K-Pg
boundary.