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| An illustration of the recently described eagle shark, which lived 93 million years ago in an ancient seaway. (Photo courtesy of Oscar Sanisidro) |
It appeared to be a hybrid of a shark and a manta ray.
According to a new study, a strange shark with wing-like
fins and a wide, gaping mouth soared through the waters of what is now Mexico
about 93 million years ago, when dinosaurs still roamed the Earth.
This strange shark, called Aquilolamna milarcae, or eagle
shark of the Milarca Museum, where its fossil will be displayed, resembles
manta and devil rays, which have finned wings as well. (Rays are related to
sharks but are not sharks.)
According to the researchers, this shark lived more
than 30 million years before either of those creatures existed.
This ancient shark was probably a filter feeder, like manta
and devil rays today, and ate small plankton-like critters when they were
hungry. The eagle shark lived in the same kind of marine real estate that
modern manta and devil rays do now, according to study lead researcher Romain
Vullo.
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, the Western Interior Seaway, a body of water that
extended from the Gulf of Mexico to the Arctic Ocean, covered that part of
Mexico. This winged shark is unlike any other shark on the planet today.
Aquilolamna has very long, slender pectoral [side] fins, which is one of the
most striking features, according to Vullo. With a "wingspan" of
about 6.2 feet (1.9 meters) and a total body length of about 5.4 feet, the
shark is broader than long (1.65 meters).
Another intriguing feature is the short head, which has an
indistinct snout and a broad mouth, according to Vullo. The Aquilolamna's other
components, such as its tail and caudal [tail] fin, are similar to those of
many contemporary sharks. Aquilolamna has a distinct chimeric appearance as a
result of this.
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| A well-preserved eagle shark fossil, as well as ammonite (Pseudaspidoceras pseudonodosoides) and bony fish fossils, including the needle fish (Rhynchodercetis regio). (Photo courtesy of Wolfgang Stinnesbeck) |
Sharks, manta rays, and other fish with cartilage skeletons
belong to the elasmobranch family, which first appeared about 380 million years
ago. Modern plankton-eating elasmobranchs are divided into two groups: those
with "traditional" shark bodies, such as the whale shark (the world's
largest living fish), and those with flattened bodies, such as manta and devil
rays.
Both of these body types are represented in this newly
discovered shark. It is, however, not a ray precursor species, but rather an
example of convergent evolution, in which different groups evolved the same
characteristics independently. The researchers wrote in the study, which was
published online Thursday (March 18) in the journal Science, that the
discovered species' unusual remains reveal "an unexpected evolutionary
experimentation with underwater flight among sharks."
Fast
or slow?
The eagle shark, unlike today's great white shark, was not a
fast and vicious predator (Carcharodon carcharias).
According to the researchers, Aquilolamna was probably a
slow swimmer, similar to other suspension-feeding elasmobranchs that slowly
swim through the water while gulping down plankton. The long and slender
pectoral fins of the eagle shark likely acted as stabilizers, but they could
also have helped propel the shark forward with slow flapping motions. The
beast's torpedo-shaped body and powerful tail fin, which waved side to side,
likely propelled it forward through the water.
The eagle shark's fossil lacks pelvic fins (located near the
tail on sharks' undersides) and a dorsal fin — the signature triangular fin
that pokes ominously out of the water in most Hollywood shark films. However,
it's unclear whether the shark had these fins when it was alive or whether they
simply did not fossilize.
Furthermore, none of the shark's teeth were preserved, making it difficult to determine what kind of shark it is, according to Kenshu Shimada.
Shimada Told that tooth features are used to identify fossil
sharks. As a result of the features seen in its vertebrae and tail skeleton,
which are less taxonomically diagnostic, the authors of the new study
tentatively placed the new fossil shark in a group called the Lamniformes.
Shimada added that modern lamniform sharks include well-known species such as
the goblin, megamouth, basking, mako, and great white sharks.
This is an incredible find, but only the discovery of more
well-preserved specimens, especially those with teeth, can reveal the shark's
true anatomy and whether it was a filter feeder, according to Shimada.
It's unclear why A. milarcae went extinct, but the
6-mile-wide (10-kilometer) asteroid that collided with Earth at the end of the
Cretaceous period, about 65.5 million years ago, probably dealt a fatal blow to
this type of filter-feeding shark. According to the researchers, the mass
extinction event that killed non-avian dinosaurs also calcified planktonic organisms
as a result of excessive acidification of surface oceans, decimating ancient
filter feeders' once-abundant food buffet.
Originally Published By Live Science.
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