Dinosaur-era shark fossil solves centuries-old mystery

During the Cretaceous Period a genre of sharks roamed the seas with rows of unusual teeth. Mostly large and rounded, these teeth were not made to cut their prey but to crush and crush shelled creatures.

However, because the presence of these animals in the fossil record has consisted mostly of isolated teeth, scientists have been left to speculate about what the rest of this ancient predator looked like since its discovery in the 18th century.

Now, remains discovered in limestone quarries in northeastern Mexico are finally giving researchers a clearer idea of ​​what the shark looked like, including a fossil that shows nearly all of the skeletal elements and an outline of the specimen’s soft-tissue body.

The new findings also reveal where the genus, known as Ptychodus, fits into the shark evolutionary tree and other previously unknown features of this “long-standing enigma,” according to a study published in April in the journal Proceedings of the Royal Society B: Biological Sciences.

“The discovery of the skeletal remains in Mexico not only allows us to unite these long-sought teeth with a skeleton, but also allows us as scientists to revisit our previous hypotheses about their biology and relationships and see what we got right and what we got wrong,” study co-author Dr. Eduardo Villalobos Segura, an assistant professor in the department of paleontology at the University of Vienna, Austria, said in an email.

The fossils also provide insights into the evolutionary history of sharks found in our oceans today, experts say.

Ancient relative of the great white shark

Most Ptychodus species lived between 100 and 80 million years ago, during the Late Cretaceous period. The deposits in which the fossils were discovered — in Nuevo León, near the municipality of Vallecillo, Spain — date to approximately 93.9 million to 91.85 million years ago, Villalobos Segura said.

Because shark skeletons are made of cartilage, they do not fossilize well, often leaving archaeologists with only teeth and few skeletal remains to find. However, evidence suggests that the Nuevo León fossils ended up in predominantly inert conditions that would have allowed for an oxygen-deficient zone, resulting in the preservation of soft skeletons, the professor said.

In the study, researchers analyzed six fossils found at the site, including the complete specimen. Three other fossils were nearly complete and two were incomplete. Using these remains, the study authors determined that Ptychodus belonged to the order of sharks known as Lamniformes, or macro sharks, the same group to which the extinct Otodus megalodon and the modern great white belong. Lamniformes also includes modern species of megamouth sharks, sand sharks, goblin sharks and peregrine sharks, among others.

“Today’s sharks represent only a tiny fraction of the impressive biodiversity that has occurred throughout their entire evolutionary history (spanning nearly 400 million years) […] “Studying fossil sharks is crucial to fully understanding evolutionary phenomena related to present-day groups,” study co-author Dr. Manuel Amadori, a postdoctoral researcher in the department of paleontology at the University of Vienna in Austria, said in an email.

The existence of macro sharks with crushing teeth was unknown until now, Amadori said. “There is still much more to discover, but we can say that we have taken another important step towards understanding the complex evolutionary history of macro sharks,” he added.

The body outline, which revealed the shark’s shape and the location of its fins, also provides evidence that the prehistoric fish was not just a deep-sea dweller, as previously believed, but rather a fast-swimming predator that may have hunted and eaten sea turtles and large ammonites, rather than just mollusks found on the ocean floor, according to the study authors.

While the shark’s exact diet is still unknown, the researchers suggest that this revised hypothesis about what it ate could be a clue to the cause of Ptychodus’ extinction, as it puts the shark in competition with other Late Cretaceous marine predators with similar diets.

“Without a complete specimen (hard evidence), what was known about Ptychodus beyond the teeth was largely scientific conjecture,” Michael Everhart, associate curator of paleontology at the Sternberg Museum of Natural History in Hays, Kansas, and an expert on Late Cretaceous marine fossils, said in an email. He was not involved in the study.

“The new specimens answer questions that date back more than 180 years, to the 1830s, when Louis Agassiz — a renowned scientist and paleontologist of the time — first coined the name Ptychodus,” meaning rough or wrinkled tooth, Everhart added.

Giant Shell Crushing Sharks

The findings also suggest that the largest species of Ptychodus may have been slightly smaller than previously thought, reaching a maximum length of 32 feet (9.7 meters). Previous estimates for a species known as Ptychodus mortoni indicated a size of 37 feet (11.2 meters), but the revised length is still larger than modern shark predators, the study authors note. Modern great white sharks reach up to 20 feet (6 meters) in length.

There are modern species that crush shells, the largest being the zebra shark, which reaches a maximum length of just over 3.5 meters — nothing as gigantic as the Ptychodus.

“The crushing teeth, coupled with its gigantic size, make Ptychodus a very unique shark,” Amadori said. “[In the fossil record]some teeth are massive, polygonal and nearly flat, while others have strange, rounded protrusions or pointed cusps on their upper surfaces. All of these teeth were joined together to form large dental plates, which this ancient predator could have used to crush almost anything it encountered.”

A new fossil revealing the full side view of Ptychodus measured about 5 feet (1.5 meters) long, suggesting it was from a much smaller shark. That could be because the remains belonged to a younger animal, or because the genus Ptychodus included several species of different sizes, Villalobos Segura said.

According to the nonprofit database Mindat.org, 22 species of Ptychodus are currently known. Most species and individuals were likely smaller than the largest specimen of Ptychodus mortoni discovered, but there may also be the possibility of larger specimens yet to be found, Everhart said.

Researchers can often tell the difference between different Ptychodus species by varying features in their teeth, but the study authors were unable to identify which species the six fossils studied belonged to because the teeth were so worn, Villalobos Segura said.

The researchers hope that future research will reveal more about the ancient shark, including its diet and its role in marine food chains and past ecosystems.

“(The April study) is a comprehensive review of some remarkably complete fossils of the strange Cretaceous shark, Ptychodus,” said Dr. Bretton Kent, a professor emeritus in the department of entomology at the University of Maryland who has studied and taught about the diversification of elasmobranchs (sharks and rays). He was not involved in the study.

“Our current world can act as a set of blinders, limiting the scope of possible lifestyles we can imagine for extinct animals. […] Modern sharks that feed on hard-shelled organisms are demersal, feeding on or near the bottom. And their bodies are often small and not particularly streamlined. So a gigantic, streamlined, high-speed durophage that was much larger than a modern great white is quite remarkable,” Kent added in an email.