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During the Cretaceous Period, a genus of sharks roamed the seas with unusual rows of teeth. Large and mostly round, these kippers were not meant to slither through their prey, but to grind and crush scaly creatures.
However, since sharks in the fossil record were mostly detached teeth, scientists were left to speculate what the rest of this ancient predator looked like since its discovery in the 18th century.
Now, remains unearthed from limestone quarries in northeastern Mexico are giving researchers a clearer idea of what the shark looked like, including one fossil that shows nearly every skeletal feature and outline of the specimen’s soft-tissue body. The discovery also shows where the genus, known as Ptychodus, sat 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 Proceedings of the Royal Society B: Biological Sciences.
“The discovery of the skeletal remains in Mexico not only allows us to identify these long-sought skeleton teeth, but also allows us as scientists to revise our previous hypotheses regarding their biology and relationships and look what we found. 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 sheds light on the evolutionary history of sharks found in our oceans today, experts say.
An 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 where the fossils were found – in Nuevo León near the municipality of Vallecillo – are about 93.9 to 91.85 million years ago, said Villalobos Segura.
Because shark skeletons are made of cartilage, they don’t fossilize well, archaeologists usually leave only teeth and few skeletal remains to be found. But evidence suggests that the Nuevo León fossils ended up in mostly stagnant conditions that would have allowed an oxygen-deficient zone, which would have preserved the soft skeletons, said Villalobos Segura.
In the study, the researchers analyzed six fossils found at the site, including the entire specimen. Three other fossils were nearly complete, and two were incomplete. With these remains, the study authors determined that Ptychodus belonged to the order of sharks known as Lamniformes, or mackerel sharks, the same group that includes the extinct Otodus megalodon and the modern great white shark. Lamniformes also include modern species of megamouth, sand, goblin and sun sharks, among others.
“Today’s sharks represent a minimal part of the amazing biodiversity that occurred during their entire evolutionary history (which spanned almost 400 million years) … the study of fossil sharks is essential to the evolutionary phenomena to fully understand existing groups,” said the study’s coauthor. Dr. Manuel Amadori, a postdoctoral researcher in the department of paleontology at the University of Vienna in Austria, in an email.
Until now, mackerel sharks with crushed teeth were not known to exist, Amadori said. “There is still much more to discover, but we can say that we have taken another important step forward in understanding the complex evolutionary history of mackerel sharks,” he said.
The outline of the body, which revealed the shape and position of the shark’s fin, also provides evidence that the prehistoric fish was not just a bottom dweller as previously believed, but instead, a fast swimming predator that could prey on sea turtles. hunt and eat large ammonites instead of eating molluscs found only on the ocean floor, according to the study’s authors. Although the exact diet of the shark is still unknown, the researchers suggest that this revised hypothesis about what it ate could be a clue to the cause of the Ptychodus extinction, since 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 mostly scientific guesswork,” said Michael Everhart, assistant curator of paleontology at the Sternberg Museum of Natural History in Hays, Kansas, and an expert on Late cretaceous marine fossils, in email. He was not involved in the study.
“The new specimens will answer questions that go back 180+ years to the 1830s when Louis Agassiz (soon-to-be famous scientist and paleontologist) first coined the name Ptychodus,” meaning rough or wrinkled tooth, which Everhart said.
Giant shell-pressing sharks
The results also indicated that the largest species of Ptychodus may have been slightly smaller than previously thought, reaching a maximum length of 9.7 meters (almost 32 feet). Previous estimates for a species called Ptychodus mortoni put it at 11.2 meters (almost 37 feet), but the revised size is still larger than modern shark apex predators, the authors note in the study. Big bays today reach up to 6 meters (20 feet) in length.
There are modern shell-breaking species, the largest of which is the zebra shark, which reaches a maximum length of just over 3.5 meters (12 feet) – nowhere near as huge as Ptychodus.
“Ptychodus is a very unique shark because of the crushing of the teeth along with the enormous size,” Amadori said. “(In the fossil record) some teeth are huge, polygonal and almost flat, while others have odd-looking, rounded or pointed cusps on the upper surface. All of these were joined to form huge tooth plates, which this prehistoric predator could have used to crush almost anything it encountered.”
A new fossil showing the full side view of Ptychodus was nearly 1.5 meters (about 5 feet) long, suggesting it was from a much smaller shark. This could be because the remains belong to a younger shark, or because the Ptychodus genus included different species of different sizes, Villalobos Segura said.
According to the non-profit database Mindat.org, there are currently 22 known species of Ptychodus. Most of the Ptychodus species and individuals were probably smaller than the largest Ptychodus mortoni specimen found, but it is possible that even larger species could be found, which Everhart said.
Often, researchers can tell different species of Ptychodus by different features in the teeth, but the study authors were unable to identify which species of Ptychodus belonged to the six fossils studied because the teeth were too small. spent, said Villalobos Segura.
The researchers said they hope future research will reveal more about the ancient shark, including its diet and role in past food chains and marine ecosystems.
“(The April study) is a comprehensive review of some extremely complete fossils of the strange Cretaceous shark, Ptychodus,” said Dr. Bretton Kent, principal lecturer emeritus in the Department of Entomology at the University of Maryland, who has studied and lectured. on 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 range of possible lifestyles we have for extinct animals. … Modern durophagous sharks (which eat hard-shelled organisms) are demersal, feeding on or near the bottom. And their bodies are often small and not particularly streamlined. So a huge, streamlined, high-speed durophage that was much larger than a modern great white shark is remarkable,” Kent added, in an email.
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