Think of « shark » and maybe imagine a slender, streamlined, torpedo-like fish. But a fossil shark unearthed in Mexico was wider than it was long thanks to its extraordinarily long fins.
The shark, called Aquilolamna milarcae, was 1.7 meters long but had a fin span of about 1.9 meters.
It cruised the oceans, flowing sideways like a glider, and seventh plankton from the ocean water flowing through its gaping mouth.
In an article published today in Science, a team of paleontologists said aquilolamna sharks were born about 93 years ago Lived millions of years until they became extinct 66 million years ago during a mass extinction.
The fossil shows the first time that a body plan with super-long pectoral fins has been found in sharks, according to Romain Vullo, a paleontologist at the University of Rennes in France and lead author of the paper.
« If you replace the pectoral fins with regular fins, it looks like a regular shark, » said Dr. Vullo.
While the long pectoral fins on either side of Aquilolamna’s body are reminiscent of the broad, flaky fins on manta rays, the animals are not related.
During this event, the intense climate change acidified the oceans, which are each other eat away at the calcium carbonate skeletons and shells of some plankton species.
As the plankton died, so did the animals that fed on them – such as Aquilolamna.
Plankton eventually recovered. And without Aquilolamna roaming the oceans, manta rays and other plankton eaters were able to develop freely about 30 million years later, said Dr. Vullo.
« [Body] shapes can adapt to any food source that may be in abundance, » said Professor Long, who was not involved in the study.
The fossil was placed in a limestone slab in the Discovered northern Mexico not far from the US border.
In the late Cretaceous period, 66 to 100 million years ago, this area was flooded by an ocean.
At this point in time, fish species with a cartilaginous skeleton had been around for some time like sharks. They first appeared about 380 million years ago.
Although cartilage is strong and flexible, it is not as hard as bones or teeth. This means that it is less likely to petrify.
« So you need very special conditions to preserve cartilaginous fish because their skeleton is not mineralized like bony animals, » said Dr. Vullo.
« It was quite deep – probably between 200 and 300 meters – and the bottom was soft and muddy. »
As the eons went by, that slushy sediment turned into limestone and that patch of seabed turned out in the water.
In 2012, a quarry worker split this slab of rock and the old fish was brought back to light.
A single fossil can tell paleontologists a lot about where the animal lived, what it ate and how it behaved.
The shape of the tail of Aquilolamna is typical of sharks, whose feeding speed is not sufficient.
Its blunt head with a wide mouth in front resembles the plankton-eating, filter-eating sharks that are on the ocean today.
Unlike rays that flap their wings to move through the water, Aquilolamna mainly used its pectoral fins as stabilizers and its tail to propel itself, said Dr. Vullo.
The long pectoral fins on ancient Aquilolamna and today’s manta rays are a great example of convergent evolution, said Will White, ichthyologist at CSIRO.
« The pectoral fins are such a unique structure. No other animal has something like that. And they are completely different groups – they did not evolve from each other.
« But at two different points in evolutionary history, these organisms basically developed this particular trait completely independently of each other. »
Von In today’s sharks, the enigmatic megamund shark (Megachasma pelagios) has fairly long pectoral fins, added Dr. White, but they’re nowhere near as big as Aquilolamna’s.
They could be buried a few millimeters in the limestone but the plate is too big to scan and find out.
That’s because these hard, mineralized teeth, unlike their relatively soft skeleton, are light petrify – and there are more of them.
Sharks are constantly growing and replacing old teeth. Some species go through up to 20,000 teeth in the course of their life.
Dr. However, Vullo and his team suspect that teeth from the same family of longfin sharks have already been discovered.
In 1990, a trio of paleontologists revealed teeth to a group of fish called Cretomanta.
They lived at the same time as Aquilolamna and their teeth became found in Texas – not far from Aquilolamna’s discovery site.
And as soon as the pandemic allows, Dr. Vullo to search for Aquilolamna teeth, through existing fossils and by revisiting the site in Mexico.
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Ref: https://www.abc.net.au
