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During the Cretaceous Period, a genus of sharks roamed the seas with unusual rows of teeth. Generally large and round, these chompers are not intended for slicing prey, but for grinding and crushing shelled creatures.
However, since the presence of sharks in the fossil record mostly consists of isolated teeth, scientists have been left to speculate about what this ancient predator looked like since it was discovered in the 18th century.
Now, remains unearthed from a limestone quarry in northeastern Mexico finally provide a clearer picture of what the shark looked like, including one fossil that shows nearly all of the specimen’s skeletal and soft-tissue body outline. The discovery also reveals where the genus, known as Ptychodus, sits on the shark evolutionary tree, and other previously unknown properties of a “long-standing enigma,” according to a study published in April in the journal Proceedings of the Royal Society B: Science Biology.
“The discovery of skeletal remains in Mexico not only allows us to put together these teeth that have been searching for bones for a long time, but also allows us as scientists to change our previous hypotheses about biology and their relationships and see what we get. right and what is wrong,” he 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.
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 deposit where the fossil was found – in Nuevo Le贸n near the municipality of Vallecillo – dates from about 93.9 to 91.85 million years ago, Villalobos Segura said.
Because shark bones are made of cartilage, they don’t fossilize well, usually leaving archaeologists with only teeth and a few skeletal remains to find. But evidence shows the Nuevo Le贸n fossils ended up in a mostly stagnant state that would have allowed oxygen-deficient zones, resulting in the preservation of soft bones, Villalobos Segura said.
In the study, the researchers analyzed six fossils found at the site, including complete specimens. Three other fossils are nearly complete, and two are incomplete. With these remains, the authors of the study determined that Ptychodus belonged to the order of sharks known as Lamniformes, or mackerel sharks, a group similar to the extinct Otodus megalodon and modern great white sharks. Lamniformes also include modern species of megamouth, sand, goblin and basking sharks, among others.
“Today’s sharks only represent the smallest part of the amazing biodiversity that has occurred in the entire history of evolution (almost 400 million years) … the study of fossil sharks is very important to understand the evolutionary phenomena related to today’s groups,” said co-authors of the study. Dr. Manuel Amadori, a postdoctoral researcher in the department of paleontology at the University of Vienna in Austria, in an email.
The presence of mackerel sharks with crushed teeth was unknown until now, Amadori said. “There is still much to discover, but we can say that we have taken another important step towards understanding the complex evolutionary history of mackerel sharks,” he said.
The outline of the body, which shows the shape of the shark and the location of the fins, also provides evidence that the prehistoric fish did not live on the bottom as previously believed, but were fast-swimming predators that probably hunted and ate sea turtles and large ammonites instead of just eating molluscs found at the bottom of the sea, according to the authors of the study. Although the shark’s exact diet is still unknown, the researchers suggest that this revised hypothesis of what it ate may be a clue to the extinction of Ptychodus, as the shark competed with other Late Cretaceous marine predators with a similar diet.
“Without a complete specimen (strong evidence), what is known about Ptychodus beyond the teeth is largely scientific speculation,” said Michael Everhart, adjunct curator of paleontology at the Sternberg Museum of Natural History in Hays, Kansas, and an expert on the Late. Cretaceous marine fossils, in email. He did not participate in the study.
“The new specimen answers a question that goes back 180+ years to the 1830s when Louis Agassiz (the famous scientist and paleontologist) first coined the name Ptychodus,” which means rough or wrinkled teeth, Everhart added.
Massive shell breaking shark
The findings also suggest that the largest Ptychodus species may have been smaller than previously thought, reaching a maximum length of 9.7 meters (nearly 32 feet). Previous estimates of the species known as Ptychodus mortoni put it at 11.2 meters (almost 37 feet), but the revised size is still larger than the modern apex predator shark, the authors noted in the study. Great whites are now up to 6 meters (20 feet) long.
There are species of modern shell-breakers, the largest of which is the Zebra shark, which can reach a maximum length of 3.5 meters (12 feet) – not unlike the giant Ptychodus.
“The crushed teeth together with the enormous size make Ptychodus a very unique shark,” Amadori said. “(In the fossil record) some teeth are large, polygonal and almost flat, while others have strange protuberances, rounded or pointed cusps on the upper surface. All these are combined into a very large tooth plate, which could have been used by predators in the past to crushing almost everything it encountered.
A new fossil showing a full side view of Ptychodus was nearly 1.5 meters (about 5 feet) long, suggesting it was from a smaller shark. This could be due to the remains of younger sharks, or because the genus Ptychodus includes different species of different sizes, Villalobos Segura said.
According to the nonprofit Mindat.org database, there are currently 22 known species of Ptychodus. Most Ptychodus species and individuals are likely smaller than the largest Ptychodus mortoni specimen ever found, but it’s possible there are larger species yet to be discovered, Everhart said.
Often, researchers can distinguish different species of Ptychodus by different features in their teeth, but the study authors were unable to identify the six Ptychodus species they studied because their teeth were too damaged, said Villalobos Segura.
Researchers say they hope future research will reveal more about the ancient shark, including its diet and role in past marine food chains and ecosystems.
“(The April study) is a comprehensive review of some exceptionally complete fossils of the strange Cretaceous shark, Ptychodus,” said Dr. Bretton Kent, professor emeritus in the department of entomology at the University of Maryland, who has studied and taught. on the diversification of elasmobranchs (sharks and rays). He did not participate in the study.
“The world today can act like a set of blinders, limiting the scope of the lifestyle that can be imagined 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 especially streamlined. It’s a very large, sleek, high-speed durophage that’s much larger than the modern great white shark,” Kent added, in an email.
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