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The 95-million-year-old fossil has paleontologists wondering if other ancient sharks could have also had peculiar body shapes.
Published March 18, 2021
6 min read
The idea of a shark with manta ray-like features might seem like something fit for a low-budget sci-fi movie. Yet paleontologists have reported discovering just such a creature in the Cretaceous-period rock of Mexico. This strange shark combines a streamlined body with expansive wing-like fins, an ancient creature unlike anything found before in the fossil record.
In 2012, an unknown quarry worker found a strange set of bones in 95-million-year-old rock layers near Vallecillo, Mexico, says Romain Vullo, a paleontologist at the State Museum of Karlsruhe in Germany. The fossil came to the attention of local paleontologist Margarito González González, who collected and prepared it by chipping rock off the preserved skeleton. Photos of the shark started making waves at paleontological conferences, and the specimen was described in a study published today in the journal Science.
Named Aquilolamna milarcae, the six-foot-long fossil represents a kind of filter-feeding shark unlike any previously known. “My first thoughts on seeing the fossil were that this unique morphology is totally new and unknown among sharks,” says Vullo, the lead author of the new study. Most of the time, fossil sharks are identified by teeth and the occasional piece of the spinal column. To find a complete skeleton, and one so strange, presents a rare opportunity to study the anatomy of this ancient swimmer.
Even though no teeth from Aquilolamna have been found, Vullo and colleagues propose that it belongs in the same family of sharks that includes great white, mako, and basking sharks. The broad head and long, wing-like fins hint that this was no hunter, though. Aquilolamna was more likely a filter feeder, opening its mouth to sift plankton and other small organisms out of the water.
A prehistoric oddity
Aquilolamna appears to combine characteristics of both sharks and manta rays, the latter of which would not evolve until millions of years later. The body of Aquilolamna is long and tube-like, similar to many sharks that cruise the oceans today. But the expanded pectoral fins are reminiscent of manta and devil rays, forming broad underwater wings.
This would make Aquilolamna one of the oldest known animals to move via “underwater flight,” slowly flapping its fins much like living manta rays. “Aquilolamna may have swum relatively slowly with slight movements of its caudal fin [tail fin] and the long pectoral fins mainly acted as an effective stabilizer,” Vullo says.
This type of body plan is completely unexpected for sharks, says Kenshu Shimada, a paleobiology professor at DePaul University in Chicago. Older sharks from before the time of the dinosaurs had a wide variety of different body shapes, but by the Cretaceous period, they were thought to have evolved into much more modern-looking forms.
Aquilolamna could be evidence that a broad variety of strange sharks continued to exist for much longer than thought. “The proposed body form and filter-feeding lifestyle in the new study are quite compelling,” Shimada says.
Shark or something else entirely?
But not all experts are convinced that this new creature was a manta-like shark. “There are a lot of unusual features described by these authors, and I have some reservations about some of their interpretations, so I would be excited to see further investigations of this new, remarkable fossil,” says Allison Bronson, a paleontologist with Humboldt State University in California.
While skin impressions from Aquilolamna are mentioned in the new study, they are not shown in enough detail for outside experts to determine whether the tissue is really fossilized skin or some other material that resembles skin, like a bacterial mat. And even though this fish likely fed by sifting plankton or other small morsels from the water column, it may have had tiny, pointed teeth similar to modern filter-feeding sharks such as the basking shark and megamouth. These teeth can be used to determine the evolutionary relationships of these sharks, but none were found with the new fossil.
“It is truly unfortunate that no teeth were preserved in the specimen that could have allowed researchers to determine the exact taxonomic affinity of the new shark,” Shimada says.
The idea that this animal was a shark and a filter feeder will likely need to be confirmed by future finds and additional analysis. If this interpretation is correct, Aquilolamna was straining the seas for plankton long before its modern relatives evolved to do the same. Perhaps this shark represents one particular way to filter feed that evolved prior to the mass extinction at the end of the Cretaceous period that killed off roughly 75 percent of all marine species. Other filter feeders, including the ancestors of megamouth, whale, and basking sharks, evolved after the world’s oceans had bounced back.
If Aquilolamna was indeed an odd relative of basking sharks, there were probably even more strange sharks or marine creatures that paleontologists have yet to uncover. “The fossil record of sharks and rays is good” in terms of time periods covered, Vullo says, but “the body shape of many extinct species remains enigmatic.” Perhaps some teeth that paleontologists have already found belonged to bizarrely shaped animals.
Even the famous giant shark Otodus megalodon has only been described from teeth and vertebrae—megalodon means “great tooth” in Greek—leading to varying interpretations of what the animal could have looked like. Exceptional fossils, like that of Aquilolamna, hint that many fossil sharks may have been far stranger than scientists ever expected.
“When we have the opportunity to discover complete skeletons in localities such as Vellecillo,” Vullo says, “we can have some surprises.”