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During the Cretaceous period, a genus of sharks roamed the sea with rows of unusual teeth. Mostly large and rounded, these chompers were not made for slicing their prey, but for grinding and crushing shelled creatures.
However, since the presence of sharks in the fossil record consisted mainly of isolated teeth, scientists were left to speculate about what the rest of this ancient predator looked like since its creation. discovered 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 almost every element of the skeleton and an outline of the soft tissue of the specimen’s body. The discovery also reveals where the genus, known as Ptychodus, fell on the shark evolutionary tree, and other previously unknown characteristics of this “long-standing enigma,” according to a study published in April in the journal Sharks. Proceedings of the Royal Society B: Biological Sciences.
“The discovery of the skeletal remains in Mexico not only allows us to piece together these teeth that have long searched for a skeleton, but it also allows us as scientists to revise our previous hypotheses about their biology and relationships and see what we have achieved. 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 discovery also provides insight 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 end of the Cretaceous period. The deposits where the fossils were discovered — in Nuevo León, near the municipality of Vallecillo — 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, typically leaving archaeologists only teeth and few skeletal remains to find. But evidence suggests that the Nuevo León fossils ended up in mostly stagnant conditions that would have allowed for an oxygen-deficient zone, resulting in the preservation of the soft skeletons, Villalobos Segura 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. From these remains, the study authors determined that Ptychodus belonged to the order of sharks known as Lamniformes, or mackerel sharks, the same group as the extinct Otodus megalodon and the modern great white shark belong. Lamniformes also includes the modern species of megamouth, sand, goblin, and basking sharks, among others.
“Today’s sharks represent only a tiny portion of the astonishing biodiversity that has occurred throughout their entire evolutionary history (spanning nearly 400 million years)… studying shark fossils is crucial to fully understanding evolutionary phenomena related to current groups,” said the study co-author. Dr. Manuel Amadori, postdoctoral researcher in the department of paleontology at the University of Vienna, Austria, via email.
The existence of mackerel 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 in understanding the complex evolutionary history of mackerel sharks,” he added.
The body contour, which revealed the shape and location of the shark’s fins, also provides evidence that the prehistoric fish was not just a bottom dweller, as previously believed, but instead a swimming predator. quickly and that it could have hunted and eaten sea turtles and large ammonites instead of just eating 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 to the cause of Ptychodus’ extinction, as it puts the shark in competition with other Late Cretaceous marine predators with diets. similar.
“Without a complete specimen (hard evidence), what was known about Ptychodus beyond the teeth was largely scientific guesswork,” 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 was not involved in the study.
“The new specimens answer questions that go back more than 180 years, to the 1830s, when Louis Agassiz (one of the first renowned scientists and paleontologists) first coined the name Ptychodus”, which means rough or wrinkled toothEverhart added.
Huge shell-crushing sharks
The findings also suggested that the largest Ptychodus species may have been slightly smaller than previously thought, reaching a maximum length of 9.7 meters (almost 32 feet). Previous estimates of a species known as Ptychodus mortani put it at 11.2 meters (nearly 37 feet), but the revised size is still larger than that of modern shark predators, the authors note in the study. Great whites today reach 6 meters (20 feet) in length.
There are modern shell-destroying species, the largest being the zebra shark, which reaches a maximum length of just over 3.5 meters (12 feet) – not as gigantic as Ptychodus.
“The crushing teeth, coupled with the gigantic size, make Ptychodus a 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 this was joined together to form enormous 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 nearly 1.5 meters (about 5 feet) long, suggesting it was from a much smaller shark. This could be because the remains belonged to a younger shark, or because the genus Ptychodus included several species of different sizes, Villalobos Segura said.
According to the nonprofit Mindat.org Database, there are 22 species of Ptychodus currently known. Most species and individuals of Ptychodus were probably smaller than the largest Ptychodus mortani specimen discovered, but there may also be the possibility of larger species still being found, Everhart said.
Researchers can often tell different species of Ptychodus apart by varying the characteristics of their teeth, but the study authors were unable to identify which ones. species of Ptychodus the six fossils studied belonged to teeth due to excessive wear, Villalobos Segura said.
The researchers said they hope future research will reveal more about the ancient shark, including its diet and its role in past food chains and marine ecosystems.
“(The April study) is a comprehensive review of some remarkably complete fossils of the strange Cretaceous shark, Ptychodus,” said Dr. Bretton Kent, principal professor emeritus in the entomology department at the University of Maryland, who has studied and taught on 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 durophagous sharks (which consume hard-shelled organisms) are demersal, feeding on or near the bottom. And their bodies are often small and not particularly aerodynamic. So a gigantic, streamlined, high-speed durophagus that was much larger than a modern great white shark is quite remarkable,” Kent added, via email.
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