A team of paleontologists describes a new species of shark characterized by large, wing-shaped pectoral fins. This fish evolved 93 million years ago in a sea that, in its time, covered present-day Mexico.
Isolated in a limestone slab that had preserved most of its fossilized skeleton and indentations from its soft tissues, the specimen was discovered in 2012 in Nuevo León, a state in northeastern Mexico. There is 93 million years ago, this part of the globe was covered by the Western Interior Seaway, a body of water that stretched from the Gulf of Mexico to the Arctic Ocean.
A unique chimerical appearance
This shark represents a new species named Aquilolamna milarcae or eagle shark from the Milarca museum where his fossil will soon be on display. Physically, this fish was distinguished by its large pectoral fins, giving it the appearance of a manta ray (the rays will appear thirty million years later).
However, it is not a precursor species, but rather a example of convergent evolution (different groups independently develop the same characteristics). The unusual remains of this new species reveal “an unexpected evolutionary experiment of underwater flight in sharks“, Write the authors of the study published in the journal Science.
As for measurements, imagine a “wingspan” of 1.9 m for a total body length of about 1.65 m.
This is not the only similarity with stingrays. This ancient shark probably filtered its food as well. It is therefore likely that these fish evolved in the same type of marine environment.
“Another interesting feature is that its head was short, with an indistinct muzzle and a wide mouth.“, Explains Romain Vullo, from CNRS in Rennes. “The other parts of the Aquilolamna, such as its caudal fin (tail), are like [celles] many modern sharks. This gives this fish a unique chimerical appearance“.
The eagle shark was not not a fast and fierce predator like today’s great white shark. “Aquilolamna was probably a relatively slow swimmer, comparable to other elasmobranchs“, Underlines the researcher, adding that his long and slender pectoral fins acted“like stabilizers“. Also, the animal probably depended on its torpedo-shaped body and strong tail fin to propel itself forward.
Probably extinct at the end of the Cretaceous
The fossil of this shark did not show pelvic fins (located under the body, near the tail) or a dorsal fin. However, it is not known if this species had not developed any or if these remains were simply not fossilized.
The excavations have also revealed no teeth. “However, the identification of fossil sharks is generally based on the characteristics of the teeth.“, Says Kenshu Shimada of DePaul University in Chicago. “So, the authors of the new study tentatively placed the new fossil shark in a group called Lamniformes, based on features seen in its vertebrae and tail skeleton.“.
Recall that modern lamniform sharks include iconic animals such as the bigmouth shark (Megachasma pelagios), the basking shark (Cetorhinus maximus), the mako shark or shortfin mako shark (Isurus oxyrinchus) and great white sharks ( Carcharodon carcharias).
Finally, we still do not know when and why this species disappeared. For the authors, these sharks probably went extinct 66 million years ago, along with non-avian dinosaurs. This mass extinction event had indeed led to a very sharp increase in the rate of ocean acidification.