The fossil shark Ptychodus was first identified 190 years ago, but in subsequent centuries of paleontological research it has been difficult to achieve a comprehensive view of the ancient fish. So far. In paper published last week in the Proceedings of the Royal Society B, a team of researchers described an exceptionally well-preserved Ptychodus, fossilized from nose to tip of tail.
In their study, paleontologists analyzed six nearly complete Ptychodus specimens excavated from Vallecillo, Mexico, in the past decade. The specimens reveal the components of the shark’s skeleton, as well as the preserved outline of their body. The team used the fossils to determine new information about shark anatomy and their place in the shark family tree.
“This new study provides important information about affinities [evolutionary relations] and the paleoecology of PtychodusRomain Voulot, a paleontologist at the University of Rennes in France and lead author of the report, said in an email to Gizmodo. “Until now, this Cretaceous shark was known only from isolated teeth, fangs [sets of teeth]and several skeletal elements such as vertebrae.
“The complete specimens from Mexico reveal this Ptychodus was a fast-swimming open-water shark (similar in shape to the living porpoise) that probably used its grinding teeth to feed mostly on ammonites and sea turtles,” Vullo added.
In 2021, Vullo was the lead author of a report describing Aquilolamna milarcaea a strange-looking lamniform shark from the Cretaceous era which was excavated from the same clearing of eastern Mexico. In the recent paper, the team also classified Ptychodus as a lamniform – mackerel shark – and to suggest that the animal’s extinction may have occurred due to competition with mosasaursan extinct group of giant marine reptiles.
But the reality may be more complicated, as Tyler Greenfield, a paleontologist at the University of Wyoming, explained to Gizmodo. Instead of being a mackerel shark, Greenfield suggests Ptychodus belongs to a completely different category.
“Sharks in the order Lamniformes have specific patterns of tooth size and shape, the cavities in the jaws that hold the rows of teeth, and the cartilaginous structures inside the vertebrae that Ptychodus there is none,” Greenfield, who is not affiliated with the new paper, wrote in an email. “These features were overlooked by the authors of the new paper, and they instead used certain features of the skull and jaws that are not unique to lamniforms to classify Ptychodus.
Greenfield added that based on the similarities between Ptychodus both Squalicorax and Ptychocorax (two other species of ancient sharks), including the shark family Ptychodus and that comprising the last two species must be placed in a separate order, the Anacoraciformes, or crow-sharks.
“Anacoraciformes was named by other authors before me, but has not since been used as valid, nor has it included ptychodontids until now,” Greenfield said, adding that teeth built for crushing shelled prey likely evolved outside of Lamniformes. “Overall, my hypothesis seeks to build a more accurate picture of the relationships and diversity of prehistoric sharks,” Greenfield said.
One might assume that such immaculately preserved fossils would settle aspects of shark phylogeny rather than complicate it. But regardless of how the dust settles with respect to Ptychodus‘ classification, it is refreshing – and indeed, incredibly fortunate – that paleontologists have such well-preserved specimens to use in making their decisions.
| More ▼: Two bull sharks swam up the Mississippi River all the way to St. Louis
