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Dryad

Exceptionally preserved shark fossils from Mexico elucidate the long-standing enigma of the Cretaceous elasmobranch Ptychodus

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Mar 20, 2024 version files 12.36 MB

Abstract

The fossil fish Ptychodus Agassiz, 1834, characterized by a highly distinctive grinding dentition and an estimated gigantic body size, has remained one of the most enigmatic extinct elasmobranchs (i.e. sharks, skates, and rays) for nearly two centuries. This widespread Cretaceous taxon is common in Albian to Campanian deposits from almost all continents. However, specimens mostly consist of isolated teeth or more or less complete dentitions, whereas cranial and post-cranial skeletal elements are very rare. Here we describe newly discovered material from the early Late Cretaceous of Mexico, including complete articulated specimens with preserved body outline, which reveals crucial information on the anatomy and systematic position of Ptychodus. Our phylogenetic and ecomorphological analyses indicate that ptychodontids were high-speed (tachypelagic) durophagous lamniforms (mackerel sharks), occupying a specialized predatory niche previously unreported among both extant and extinct elasmobranchs. Our results support the view that lamniforms were ecomorphologically highly diverse and represented the dominant group of sharks in Cretaceous marine ecosystems. Ptychodus may have fed predominantly on nektonic hard-shelled prey items such as ammonites and sea turtles rather than on benthic invertebrates, and its extinction during the Campanian, well before the end-Cretaceous crisis, might have been related to competition with emerging blunt-toothed globidensine and prognathodontine mosasaurs.