During the Cretaceous period, a lineage of sharks roamed the seas with rows of unusual teeth. These teeth were usually large and round, and were not designed to cut their prey, but to grind and crush armored creatures.
However, because the sharks' fossil remains consisted largely of individual teeth, scientists are still left to speculate about what the rest of this ancient predator may have looked like since its discovery in the 18th century.
Now, remains unearthed from limestone quarries in northeastern Mexico are finally giving researchers a clearer picture of the shark's appearance, including a fossil that shows nearly all of the skeletal elements and an outline of the specimen's soft-tissue body. The find also reveals where the genus, known as Ptychodus, sat on 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 teeth that have been sought for a long time in a skeleton, but also allows us as scientists to revise our previous hypotheses about its 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.
According to experts, the discovery also provides insight into the evolutionary history of the sharks that live in our oceans today.
Ancient relative of the great white shark
Most species of Ptychodus lived between 100 and 80 million years ago during the late Cretaceous period. The deposits where the fossils were discovered - in Nuevo León near the municipality of Vallecillo - date to about 93.9 to 91.85 million years ago, Villalobos Segura said.
Because shark skeletons are made of cartilage, they don't fossilize well, leaving archaeologists with only teeth and few skeletal remains to find. But evidence suggests the Nuevo León fossils were exposed to mostly stagnant conditions, which would have allowed for a low-oxygen zone, resulting in the preservation of the soft-bodied 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. Using 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 includes the modern species of megamouth, sand, goblin and basking sharks, among others.
"Today's sharks represent only a negligible fraction of the astonishing biodiversity that has occurred throughout their entire evolutionary history (which spans almost 400 million years) ... studying fossil sharks is crucial to fully understand the 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 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 made a significant step forward in understanding the complex evolutionary history of mackerel sharks," he added.
The body outline, which revealed the shape and location of the shark's fin, also provides evidence that the prehistoric fish was not simply a bottom-dweller as previously thought, but instead a fast-swimming predator that may have hunted and eaten sea turtles and large ammonites rather than just mollusks found on the ocean floor, the study authors said. While the shark's exact diet remains unknown, the researchers suggest that this revised hypothesis about what it ate could hint at what caused Ptychodus's extinction, as it puts the shark in competition with other Late Cretaceous marine predators with a similar diet.
"Without a complete specimen (hard evidence), what was known about Ptychodus, other than the teeth, was largely scientific guesswork," 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 go back more than 180 years, to the 1830s, when Louis Agassiz (an early famous scientist and paleontologist) first coined the name Ptychodus," which means rough or wrinkled tooth, Everhart added.
Huge 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 (nearly 32 feet). Previous estimates for a species known as Ptychodus mortoni put it at 11.2 meters (nearly 37 feet), but the revised size is still larger than modern top shark predators, the authors note in the study. Great white sharks today reach lengths of up to 6 meters (20 feet).
There are modern species that crush shells. The largest is the zebra shark, which reaches a maximum length of just over 3.5 meters (12 feet), making it nowhere near as gigantic as the Ptychodus.
"The crushing teeth combined with its gigantic size make Ptychodus a very unique shark," Amadori said. "(In the fossil record) some of the teeth are massive, polygonal and almost flat, while others have strange, rounded projections or pointed spikes on their upper surfaces. All of these teeth were connected together to form enormous tooth plates, which this ancient predator could have used to crush almost anything it came across."
A new fossil revealing the complete side view of Ptychodus was nearly 1.5 meters (about 5 feet) long, suggesting it came 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 currently 22 known species of Ptychodus. Most species and individuals of Ptychodus were likely smaller than the largest Ptychodus mortoni specimen discovered, but there is also the possibility that even larger species have yet to be found, Everhart said.
Often, researchers can tell different species of Ptychodus apart by distinguishing different features of the teeth, but the study authors could not determine which species of Ptychodus the six fossils they studied belonged to because the teeth were too worn down, Villalobos Segura said.
The researchers hope that future research will reveal more about the ancient shark, including its diet and its role in past marine food webs and ecosystems.
"(The April study) is a comprehensive review of some remarkably complete fossils of the strange Cretaceous shark, Ptychodus," said Dr. Bretton Kent, an associate professor emeritus in the department of entomology at the University of Maryland who has studied and lectured on the diversification of cartilaginous fishes (sharks and rays). He was not involved in the study.
"Our current world can act like a set of blinders, limiting the range of possible lifestyles we can imagine for extinct animals. ... Modern durophagous sharks (those that eat hard-shelled organisms) are demersal, feeding on or near the bottom. And their bodies are often small and not particularly streamlined. So a giant, streamlined, fast durophage that was much larger than a modern white shark is quite remarkable," Kent added in an email.
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