Historical material of cf. Thoracosaurus from the Maastrichtian of Denmark provides new insight into the K/Pg distribution of Crocodylia

Abstract

Crocodylia are one of the distinctive groups that survived the K/Pg mass extinction event. New material from Upper Cretaceous–lower Palaeogene deposits is therefore important to understanding shifts in crocodylian distribution and ecology. This paper describes a fragmentary mandible of a marine crocodylian, from the lower Maastrichtian (Upper Cretaceous) of Møns Klint (Denmark). The specimen is referred to cf. Thoracosaurus (Gavialoidea). Although the material was collected in 1968, it has not yet been formally described. However, the specimen warrants further attention, given its palaeoecological and palaeobiogeographical significance. For instance, the Møns Klint “Thoracosaurus” is an indication of increased competition between large piscivores, and of added predation pressure on fish and cephalopod communities. This gavialoid therefore provides new insight into predator-prey interactions in the Danish Chalk palaeoenvironment. Regarding crocodylian distribution, the Møns Klint specimen represents the oldest European gavialoid record, with all other material deriving from uppermost Maastrichtian and Danian deposits. This indicates that Gavialoidea were present in the European “Chalk Sea” throughout the Maastrichtian, rather than being latest Maastrichtian arrivals. Furthermore, the Møns Klint “Thoracosaurus” represents one of few lower Maastrichtian gavialoids worldwide, the other two being from Mississippi and New Jersey (USA). The specimen thus extends the palaeogeographical range of lower Maastrichtian gavialoids across the proto-Atlantic. This reveals transatlantic distribution of Gavialoidea dating back to the earliest Maastrichtian, providing further evidence for distributional communication between marine vertebrate faunas of North America and Europe. These factors make the Møns Klint “Thoracosaurus” particularly valuable to understanding the distribution of K/Pg Crocodylia.