The idea of a shark with manta ray-like features may seem like something suitable for a low-budget science fiction movie. Yet paleontologists have reported discovering just such a creature in Cretaceous rock in Mexico. This strange shark combines a streamlined body with outstretched wing-like fins, an ancient creature not previously found 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 Karlsruhe State Museum in Germany. The fossil came to the attention of local paleontologist Margarito González González, who collected and prepared it by cutting stone from 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 milarcaethe six-foot-long fossil represents a type of filter-feeding shark that was not previously known. “My first thoughts upon seeing the fossil were that this unique morphology is totally new and unknown among sharks,” said Vullo, the lead author of the new study. Typically, fossil sharks are identified by teeth and the occasional stretch of the spine. Finding a complete skeleton, and one so strange, offers a rare opportunity to study the anatomy of this ancient swimmer.
Even though no teeth come out Aquilolamna have been found, Vullo and colleagues propose that it belongs to the same family of sharks, including great white sharks, mackerel sharks and basking sharks. However, the broad head and long, wing-like fins indicate that this was not a hunter. Aquilolamna was more likely a filter feeder, which opened its mouth to sift plankton and other small organisms out of the water.
A prehistoric oddity
Aquilolamna appears to combine features 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 rays and devil rays and form wide underwater wings.
This would make Aquilolamna one of the oldest known animals, moving in “underwater flight”, slowly flapping its fins like living manta rays.Aquilolamna may have swum relatively slowly with light movements of the caudal fin [tail fin] and the long pectoral fins mainly acted as an effective stabilizer, ”says Vullo.
This kind of body plan is completely unexpected for sharks, says Kenshu Shimada, a professor of paleobiology 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 they were thought to have evolved into much more modern forms.
Aquilolamna could be evidence that a wide variety of strange sharks survived much longer than thought. “The suggested body shape and lifestyle with filter feeding in the new study are quite compelling,” says Shimada.
Shark or something completely different?
But not all experts are convinced that this new creature was a manta-like shark. “There are many unusual features described by these authors, and I have some reservations about some of their interpretations, so I would like to see further investigation of this new, remarkable fossil,” said Allison Bronson, a paleontologist at Humboldt State. University in California.
While skin prints from Aquilolamna cited in the new study, they are not presented in sufficient detail for outside experts to determine whether the tissue is really fossilized skin or some other skin-like material, such as a bacterial mat. And while this fish is likely fed by sifting plankton or other small pieces from the water column, it may have tiny, pointed teeth that resemble modern filter-feeding sharks like the basking shark and megamouth. These teeth can be used to determine the evolutionary relationships of these sharks, but none have been found with the new fossil.
“It is a real shame that no teeth were preserved in the specimen allowing researchers to determine the exact taxonomic affinity of the new shark,” said Shimada.
The idea that this animal was a shark and a filter feeder will likely need to be corroborated by future finds and additional analysis. If this interpretation is correct, Aquilolamna searched the seas for plankton long before its modern relatives evolved to do the same. Perhaps this shark represents a specific way of filtering feed that evolved prior to the mass extinction at the end of the Cretaceous, which killed about 75 percent of all marine species. Other filter feeders, including the ancestors of megamouth, whale, and basking sharks, evolved after the world’s oceans bounced off.
If Aquilolamna was indeed a strange relative of basking sharks, there were probably other strange sharks or marine creatures that paleontologists have yet to discover. “The shark and ray fossil record is good” in terms of time periods covered, says Vullo, but “the body shape of many extinct species remains puzzling.” Perhaps some of the teeth that paleontologists have already found belonged to bizarrely shaped animals.
Even the famous giant shark Otodus megalodon has only been described using teeth and vertebrae – megalodon means ‘big tooth’ in Greek – leading to different interpretations of what the animal might have looked like. Exceptional fossils, such as that of Aquilolamna, show that many fossil sharks may have been much weirder than scientists ever expected.
“If we have the opportunity to discover complete skeletons in places like Vellecillo,” says Vullo, “we may be in for some surprises.”