Fossil experts say they have gained unprecedented insights into a type of enormous prehistoric shark, after finding complete skeletons of the creatures.
The samples, discovered in small quarries in north-eastern Mexico within the last decade, belong to Ptychodus – a creature that roamed the sea from about 105m to 75m years ago.
Ptychodus fossils have turned up before, but with its bones of cartilage, which do not mineralize well, were many isolated teeth, which were large and unusual.
As a result, it was difficult to determine exactly what Ptychodus looked like and where it sat on the evolutionary family tree.
“Its general appearance has so far remained a mystery due to the lack of more complete material during almost two centuries,” said Dr Romain Vullo, first author of the research from the University of Rennes.
“The discovery of the new specimens from Vallecillo, which reveal the body shape and anatomy of this extinct shark, solves this mystery.”
Prof Michael I Coates of the University of Chicago, who was not involved in the work, said the new fossils were superb.
“Ptychodus has long been a classic example of teeth in search of a body,” he said. “And here we have it, with thorough analyzes of where it sits in the shark family tree and a good stab at its ecomorphology—how it fits into Late Cretaceous marine ecosystems.”
Write in the journal Proceedings of the Royal Society BVullo and colleagues report how they studied six Ptychodus specimens, dating from about 93m years ago.
Among them was a complete specimen revealing a side view of Ptychodus, containing not only nearly all of its skeletal elements, but also teeth, preserved muscle remains, and a body outline complete with all of its fins.
A further three of the specimens were nearly complete, including a juvenile that was just over 56 cm long, while the remaining two specimens were incomplete or partial skeletons.
The team says the abundance of features preserved in the specimens, including the fin skeletal anatomy, allowed them to perform a fresh analysis of where Ptychodus sits on the evolutionary family tree.
The results show that it was a type of mackerel shark – a group that includes the extinct gigantic shark megalodon and the great white shark that inhabits oceans today.
The researchers add that as well as its overall body shape and proportions, a number of characteristics of Ptychodus, including the size, shape and position of its fins, as well as its thick spine, suggests that it swam fast, while its massive pavement-like teeth support previous conclusions that it fed on shelled creatures.
Collectively, the team says the findings suggest that Ptychodus hunted prey in open water, with its meals likely consisting of sea turtles and ammonites rather than creatures such as mussels that lived on the seafloor, as previously thought.
“Ptychodus it was generally thought to be morphologically similar to benthic sharks such as the modern nurse shark, but we now know that it resembled the extant basking shark, a fast-swimming pelagic form,” Vullo said.
While Ptychodus may have been the largest shark ever to subsist on such a diet, the new fossils suggest it had a maximum length of around 9.7 meters – larger than today’s great white but smaller than previous estimates suggested that it could have reached more than 10 meters In length.
The study also offers hints about the demise of Ptychodus, suggesting that it may have died out due to competition with other creatures, such as large aquatic reptiles, that fed on similar prey.
Patrick L Jambura, an expert on fossil fish at the University of Vienna, who was not involved in the study but works with three of the authors, said this was important, as more than a third of all sharks and rays threatened with extinction today.
“Ptychodus provides us with a mirror that shows us what will happen to large top predators like the great white shark if we, as their main competitor, do not rethink our actions,” he said.
