By working with a rare fossil found in northeastern Mexico in 2012, scientists have successfully reconstructed a 66 million-year-old shark that looked similar to modern-day manta rays, providing previously unknown insights into the morphology of cartilaginous fishes of the late Cretaceous.
The study, published March 18 in Science, describes a brand-new species, Aquilolamna milarcae, while reconstructing its body plan, a difficult task because complete shark fossils are exceedingly rare.
Though sharks are found in nearly all oceans and serve a variety of ecological functions, they are rarely discovered as complete fossils because their skeletons are not made of hard bones. Instead, shark skeletons are formed by calcite deposits held together by soft connective tissue. These skeletons are far more likely to fall apart after death than be preserved as a fossil.
"The fossil record of cartilaginous fishes is good and relatively complete but is mostly comprised of isolated teeth," said Romain Vullo, a CNRS researcher in geosciences at the University of Rennes. "Therefore, the body shape of many extinct species remains enigmatic."
However, it is possible to make determinations about the shape of a fossilized shark if the right fossil can be found. The rare specimen the researchers worked with came from the Vallecillo quarry in northern Mexico, a modern hotbed of fossil discovery. New species of octopus, fish and ammonites have been discovered there over the last two decades.
"At Vallecillo, many exquisitely preserved fossils of marine organisms, such as the specimen of Aquilolamna, are regularly discovered and collected during the exploitation of the laminated limestone," said Vullo.
The fossil of the new species was found relatively complete, compared with other sharks. The team collaborated with a paleoartist to create a reconstruction of the fossil, showing that the body of the shark was around 2 meters long and included two unusual pectoral fins extending from the side of the body, giving the animal a "chimeric" body shape somewhere between a modern shark and a manta ray.
However, this fossil is highly unusual for displaying these manta ray-like features some 30 million years before they appeared in manta rays or devilfish.
"Aquilolamna is bizarre because its unique morphology, characterized by wing-like pectoral fins, is totally new and unknown among sharks," said Vullo.
The anatomy of Aquilolamna leads the researchers to believe that it was not a ferocious predator like the ancient megalodon or the modern great white shark. Instead, they think that these sharks were peaceful suspension feeders, slowly coasting through the sea and scooping up plankton with their gaping mouths, using their long fins as stabilizers to swim like modern-day manta rays.
"The locomotion of manta rays is like an underwater flight, with flapping movements of their powerful pectoral fins," Vullo said. "In Aquilolamna, the long slender pectoral fins rather acted as the wings of a glider."
According to Vullo, the fact that these sharks appear to have occupied the same ecological niche as modern-day manta rays is a "striking example of convergent evolution," where species of different origins evolve into similar roles in the ecosystem by living in similar environments.
While the fossil reconstruction is mostly complete, the team hopes to find more specimens to help clarify how the teeth of Aquilolamna looked, allowing the researchers to better confirm their hypotheses about its lifestyle. More specimens would also help clarify what other species may have occupied this unusual group, which the group has termed "eagle sharks" on account of the winglike fins.
Meanwhile, the fossil represents a departure from current understanding of ancient sharks and another piece of a wider evolutionary puzzle about these poorly preserved animals.
"The discovery of Aquilolamna provides important information on the morphological and ecological diversity of extinct sharks, especially among Cretaceous sharks," Vullo said. "As fossil shark species are almost all known from isolated teeth, data from such complete skeletons are crucial for our understanding of the evolutionary history of sharks."
The study, "Manta-like planktivorous sharks in Late Cretaceous oceans," published March 18 in Science, was authored by Romain Vullo, University of Rennes; Eberhard Frey, State Museum of Natural History Karlsruhe; Christina Ifrim, SNSB–Jura-Museum; Margarito A. González González, Independent Researcher; Eva S. Stinnesbeck, Rhenish Friedrich Wilhelm University of Bonn; and Wolfgang Stinnesbeck, Heidelberg University.