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'Winged' eagle shark soared through oceans 93 million years ago

An illustration of the newly described eagle shark, which lived in an ancient seaway 93 million years ago.
An illustration of the newly described eagle shark, which lived in an ancient seaway 93 million years ago. (Image credit: Oscar Sanisidro)

A bizarre shark with wing-like fins and a wide, gaping mouth soared through the seas of what is now Mexico about 93 million years ago, when dinosaurs still roamed the Earth, a new study finds.

This odd shark — dubbed Aquilolamna milarcae, or eagle shark of the Milarca Museum, where its fossil will go on display — looks remarkably like manta and devil rays, which also sport finned "wings." (Rays are closely related to, but are not, sharks.) This shark lived more than 30 million years before either of those creatures existed, the researchers said.

That's not the only similarity: This ancient shark was likely a filter feeder that gulped down tiny plankton-like critters when it was hungry, just like manta and devil rays do today. So, it's likely that the eagle shark lived in the same type of marine real estate that modern manta and devil rays do now, said study lead researcher Romain Vullo, a vertebrate paleontologist with the National Center for Scientific Research (CNRS) at Geosciences Rennes, in France.

Related: Image gallery: Mysterious lives of whale sharks 

A quarryman discovered the eagle shark specimen — a slab of limestone that preserved most of the shark's fossilized skeleton and imprints of its soft tissues — in Nuevo León, a state in northeastern Mexico, in 2012. When this shark was alive, that part of Mexico was covered by the Western Interior Seaway, a body of water that stretched from the Gulf of Mexico to the Arctic Ocean. 

This winged shark is unlike any shark alive today. "One of the most striking features of Aquilolamna is that it has very long, slender pectoral [side] fins," Vullo told Live Science in an email. "This makes the shark wider than long," with a "wingspan" of about 6.2 feet (1.9 meters) and a total body length of about 5.4 feet (1.65 meters). 

"Another interesting feature is that the head is short, with an indistinct snout and a wide mouth," Vullo added. "The other parts of the Aquilolamna, such as its tail and caudal [tail] fin, are like [those] in many modern sharks. This gives to Aquilolamna a unique chimeric appearance." 

The eagle shark's well-preserved fossil. Other fossils recovered with the shark include an ammonite (Pseudaspidoceras pseudonodosoides) and bony fishes, such as the needle fish (Rhynchodercetis regio).

The eagle shark's well-preserved fossil, along with the fossils of an ammonite (Pseudaspidoceras pseudonodosoides), and bony fishes, including the needle fish (Rhynchodercetis regio). (Image credit: Wolfgang Stinnesbeck)

Sharks, manta rays and other fish with skeletons made of cartilage are part of a group called elasmobranchs, which emerged about 380 million years ago. Modern plankton-eating elasmobranchs have two distinct body shapes — those with "traditional" shark bodies, such as the whale shark (the largest living fish in the world), and those with flattened bodies, including the manta and devil rays.

This newly analyzed shark has features from both of these body types. However, it's not a precursor species to rays, but rather an example of convergent evolution, where different groups independently evolved the same features. The newfound species' unusual remains reveal "an unexpected evolutionary experimentation with underwater flight among sharks," the researchers wrote in the study, published online Thursday (March 18) in the journal Science

Fast or slow?

The eagle shark was not a fast and fierce predator like today's great white shark (Carcharodon carcharias). 

"Aquilolamna was probably a relatively slow swimmer, comparable to other suspension-feeding elasmobranchs" that slowly swim through the water, guzzling down plankton, the researchers wrote in the study. It's likely that the eagle shark's long and slender pectoral fins acted as stabilizers, but they may have also helped propel the shark forward with slow flapping motions. The beast likely depended on its torpedo-shaped body and strong tail fin, waving side to side, to thrust it forward through the water.

The eagle shark's fossil doesn't have pelvic fins (located on sharks' undersides, near the tail) or a dorsal fin — the signature triangular fin that pokes ominously out of the water in most Hollywood shark movies. But it's not clear whether the shark didn't have these fins when it was alive, or whether they simply didn't fossilize. 

What's more, none of the shark's teeth were preserved, which makes it difficult to know what kind of shark it is, said Kenshu Shimada, a professor of paleobiology at DePaul University in Chicago and a research associate at the Sternberg Museum in Kansas, who wasn't involved with the study.

"Identification of fossil sharks generally relies on tooth characteristics," Shimada told Live Science in an email. "So, the authors of the new study tentatively placed the new fossil shark in a group called the Lamniformes based on the characteristics seen in its vertebrae and tail skeleton, which are less taxonomically diagnostic." Modern lamniform sharks include iconic animals, such as the goblin, megamouth, basking, mako and great white sharks, Shimada added. 

"This is indeed a remarkable discovery," but only the discovery of additional, well-preserved specimens, especially those with teeth, may shed light on the shark's true anatomy, as well as whether it really was a filter feeder, Shimada said.

It's unclear why A. milarcae went extinct, but this type of filter-feeding shark was probably dealt a grave blow by the 6-mile-wide (10 kilometers) asteroid that collided with Earth at the end of the Cretaceous period, about 65.5 million years ago. That mass extinction event, which killed the non-avian dinosaurs, also calcified "planktonic organisms resulting from an extreme acidification of surface oceans," which decimated ancient filter feeders' once-bountiful food buffet, the researchers wrote in the study.

Originally published on Live Science.