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

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.

Methods
We explored the phylogenetic relationships of the genus Ptychodus Agassiz, 1834 within the elasmobranchs using a modified version of Jambura et al.’s (2023) data matrix. In order to provide a broader context of these relations, several Palaeozoic, Mesozoic, Cenozoic, and Recent chondrichthyan taxa were additionally included as outgroups, with Pucapampella and Doliodus serving as the root. To accommodate the inclusion of these groups, several modifications and additional characters from previous works were added to the present data matrix (e.g., Allis 1923; Holmgren 1940, 1941; Patterson 1965; Schaeffer 1981; Maisey 1980, 1984, 1985, 2001; Compagno 1990a; Nishida 1990; Shirai 1996;  Goto 2001). The resulting data matrix was assembled in Mesquite 3.81 (Madisson and Madisson 2023) and includes 221 characters. Analysis was carried out in TNT v.1.6 (Goloboff and Morales 2023), under Goloboff et al.’s (2021) protocol, considering the inclusion of inapplicable characters. A traditional parsimony search with step matrices (smatrix &) was conducted with the command (mult) using TBR (tree bisection and reconnection) as the algorithm for branch permutations for 10000 iterations holding ten trees for each iteration. This protocol was repeated ten times using the command loop with different random seeds to evaluate if the tree space was adequately explored by the parameters.