New *Ptychodus* Fossils Reveal a Giant Late Cretaceous Shark

An extraordinary discovery of several fully articulated shark fossils dating from the Late Cretaceous, 105 to 72 million years ago, is providing much-needed insight into the enigmatic shark family tree.

At the Vallecillo Lagerstätte fossil beds in Mexico, palaeontologists have uncovered a remarkable collection: several exceptionally preserved specimens of an extinct genus known as Ptychodus.

The fossils retain more than the sharks’ articulated skeletal remains. They also preserve some cartilaginous features, impressions of their complete bodies, and perhaps even organs. Such details show how the animals’ teeth and vertebrae related to the rest of their bodies, offering a new means of estimating their size and phylogenetic position.

The remains finally establish that Ptychodus was a shark within the Lamniform family, the group that includes great white sharks.

Ptychodus: a giant Lamniform shark

Ptychodus would have exceeded modern great whites in size, growing to an immense 9.7 metres in length. Its feeding method, however, was very different: it possessed crushing dental plates, enabling it to consume shelled animals that would otherwise have been difficult to eat.

"Our results support the view that lamniforms were ecomorphologically highly diverse and represented the dominant group of sharks in Cretaceous marine ecosystems," write a team of palaeontologists led by Romain Vullo of the French National Centre for Scientific Research.

"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."

Ever since its grinding teeth were first discovered in England in 1729, Ptychodus fossils have been puzzling scientists. Across the centuries that followed, most recovered remains have consisted of teeth and vertebrae: the only parts of a shark’s skeleton made from hard tissue. The rest is cartilage, which rarely lasts long enough to fossilise, leaving much open to interpretation.

Although incomplete, the handful of remains found worldwide supplied sufficient evidence to suggest the kind of animal that produced them.

Modern sharks with a comparable feeding approach, called durophagy, demonstrate that such a diet is possible. The lack of other remains despite the abundance of teeth in Late Cretaceous fossil beds indicates that it probably had a cartilaginous skeleton. Meanwhile, the rare Ptychodus vertebrae available match those of other sharks.

Yet the remains sharks typically leave behind cannot reveal with certainty what the animals looked like. Even our picture of the famous megalodon relies heavily on informed speculation.

Then, occasionally, a site such as the Vallecillo Lagerstätte is discovered.

Vallecillo fossils reveal the ancient predator’s form

A Lagerstätte is a fossil deposit known for exceptionally preserving remains, including soft tissues that would normally decay before fossilisation could occur. At the Vallecillo Lagerstätte, palaeontologists recovered six Ptychodus specimens that at last reveal the literal form of this prehistoric predator.

None of these fossils belonged to especially large individuals; the longest measured only a little over 2 metres in overall length. Crucially, though, they display the sharks in unprecedented anatomical detail. Researchers could therefore apply this anatomy to other recovered teeth and infer a maximum length of 9.7 metres.

Scientists now know how many vertebrae these sharks possessed, the dimensions of their eye sockets and heads, the number of fins, and the shape of their bodies. This made it possible to classify them confidently as lamniforms and establish that they were capable of swimming at high speeds.

Their body shape also differs from that of every living durophagous shark, underlining just how hard it is to reconstruct shark morphology from teeth alone. Indeed, Ptychodus may have been the largest durophagous shark ever to live, vastly bigger than any species alive today.

"The tachypelagic ecomorphotype revealed by the newly discovered material from Mexico challenges the widely held view that the durophagous genus Ptychodus was a group of bottom-dwelling sharks feeding mainly on shelled benthic invertebrates such as inoceramid bivalves," the researchers write.

"New evidence indicates that Ptychodus was an open-water, fast-swimming predator that preyed on well-armored pelagic organisms such as large ammonites and sea turtles, thus confirming the more active nektonic lifestyle previously suggested based on morphological features of vertebral centra and placoid scales."

We can only sincerely hope that such wonders never stop appearing.

The team’s study has been published in Proceedings of the Royal Society B: Biological Sciences.