The eagle shark's long, slender side fins are one of its "most striking features," says first author Romain Vullo. (Image by Wolfgang Stinnesbeck)
Smithsonian Magazine has a story about an interesting new shark fossil. The eagle shark, Aquilolomna milarcae, lived about 95 million years ago during the Cretaceous Period in what is now northeastern Mexico. It was found in 2012 by a quarry worker in the Vallecillo limestone quarry. The animal was about 5.4 feet long and 6 feet wide, due to its large wing-like pectoral fins. Paleontologists believe it was most likely a filter feeder. The description of this new species was published in the journal Science.
“My first thoughts on seeing the fossil were that this unique morphology is totally new and unknown among sharks,” says Vullo to National Geographic.
While its head and side fins are unusual, the eagle shark’s tail and tail fins resemble those of modern sharks. So the researchers suggest that the shark probably used its tail to propel itself forward and its long side fins for stabilization. Manta rays have a different strategy, flapping their wide side fins to propel themselves forward.
"One of the most striking features of Aquilolamna is that it has very long, slender pectoral [side] fins," writes Vullo in an email to Laura Geggel at Live Science, "This makes the shark wider than long," because it is just over six feet wide but only about 5.4 feet long.
The fossil didn’t show signs of a dorsal fin—the notorious sign of an approaching shark that sticks up above the water—or of pelvic fins, which are on the underside of the shark. It’s not yet clear whether the eagle shark lacked these fins, or if they just didn’t fossilize, per Live Science.
The biggest mystery surrounding the eagle shark comes from the lack of teeth in the fossil. Paleontologists rely on sharks’ teeth to identify them and figure out their evolutionary relationship to other ancient sharks. The eagle shark might have had tiny, pointed teeth like the basking shark and the megamouth—two modern filter-feeding sharks—or taken a different strategy.
“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,” says DePaul University paleobiologist Kenshu Shimada to National Geographic.
For now, the research team used the shape of the fossil’s vertebrae and the skeleton of its tail fin to classify it as a shark in the order Lamniformes, which includes filter-feeding sharks, mako sharks and the great white. Future fossilized finds and analysis of the eagle shark’s anatomy could help scientists understand the strange shapes of sharks in the distant past.
