Shark fossil from the time of the dinosaurs solves centuries-old mystery

During the Cretaceous Perioda genre of sharks roamed the seas with rows of unusual teeth. Mostly large and rounded, these teeth weren’t made to cut their fangsbut to grind and crush creatures with shells.

However, because the presence of these animals in the fossil record has consisted mainly of isolated teeth, scientists have been left speculating about the appearance of the remains of this ancient predator since its discovery in the 18th century.

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

The news also reveals where the genus, known as Ptychodus, fits in the shark evolutionary tree and other previously unknown characteristics 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 teeth that have long searched for a skeleton, but it also allows us, as scientists, to review our previous hypotheses about their biology and relationships and see what we got right and what we got wrong. ,” said study co-author Dr. Eduardo Villalobos Segura, assistant professor in the department of paleontology at the University of Vienna, Austria, in an email.

The fossils also provide information about 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 back to approximately 93.9 to 91.85 million years ago, Villalobos Segura said.

Because shark skeletons are made of cartilage, they don’t fossilize well, often leaving archaeologists 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 commented.

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. From 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 shark belong. Lamniformes also includes the modern species of megamouth sharks, sand sharks, goblin sharks, and peregrine sharks, among others.

“Today’s sharks represent only an insignificant portion 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 current groups,” said study co-author Dr. Manuel Amadori, a postdoctoral researcher in the department of paleontology at the University of Vienna in Austria, in an email .

The existence of macro sharks with grinding 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 contour, which revealed the shark’s shape and fin location, also provides evidence that the prehistoric fish was not just a deep-sea dweller, as previously believed, but rather a fast-swimming predator that it could have hunted and eaten sea turtles and large ammonites, rather than just shellfish found on the ocean floor, according to the study authors.

Although the shark’s exact diet is still unknown, researchers suggest that this revised hypothesis about what it ate could be a clue as 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,” said Michael Everhart, associate curator of paleontology at the Sternberg Museum of Natural History in Hays, Kansas, and an expert on Late Cretaceous marine fossils, in an email. He did not participate in the study.

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

Giant shell-crushing sharks

The findings also suggested that the largest species of Ptychodus may have been slightly smaller than previously thought, reaching a maximum length of 9.7 meters. Previous estimates of a species known as Ptychodus mortani indicated a size of 11.2 meters, but the revised length is still greater than that of modern shark predators, the study authors note. Today’s great white sharks reach up to 6 meters.

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 Ptychodus.

“The grinding teeth, along with the 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 protuberances or pointed cusps on the upper surface. 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 1.5 meters long, suggesting it was from a much smaller shark. This 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 non-profit database Mindat.org, 22 species of Ptychodus are currently known. Most species and individuals were likely smaller than the largest Ptychodus mortani specimen discovered, but there may also be the possibility of larger representatives yet to be found, Everhart said.

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

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, professor emeritus in the entomology department at the University of Maryland, who has studied and taught on diversification elasmobranchs (sharks and rays). He was not involved in the study.

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

This content was originally created in English.

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