By Matt Walker
Editor, Earth News
A nurse shark may be similar in shape but not size to the prehistoric fossil
The fossilised remains of a gigantic 10m-long predatory shark have been unearthed in Kansas, US.
Scientists dug up a gigantic jawbone, teeth and scales belonging to the shark which lived 89 million years ago.
The bottom-dwelling predator had huge tooth plates, which it likely used to crush large shelled animals such as giant clams.
Palaeontologists already knew about the shark, but the new specimen suggests it was far bigger than previously thought.
The scientists who made the discovery, published in the journal Cretaceous Research, last week also released details of other newly discovered giant plankton-eating fish that swam in prehistoric seas for more than 100 million years.
But this new fish, called Ptychodus mortoni, is both bigger and more fierce, having a taste for flesh rather than plankton.
It may even have been the largest shellfish-eating animal ever to have roamed the Earth.
Dr Kenshu Shimada of DePaul university in Chicago, Illinois, US found the fossilized remains of the shark in rocks known as the Fort Hays Limestone in Kansas.
"Kansas back then was smack in the middle of an inland sea known as the Western Interior Seaway that extended in a north-south direction across North America," says Dr Shimada.
The jawbone fragment came from a huge fish (diagram courtesy of Dr Shimada)
Along with a piece of jaw, Dr Shimada and colleagues uncovered a piece of jaw, teeth and scales.
"Although it represents a fraction of the entire body of the shark, the jaw fragment is gigantic. The estimated jaw length was almost 1m long, and that would suggest that the shark was likely at least 10m in length," says Dr Shimada.
Due to the lack of a complete skeleton, it is difficult to gauge the physical appearance of the shark.
But Dr Shimada suspects it had a body shaped much like that of a modern nurse shark (Ginglymostoma cirratum), with a broad rounded head and stout body.
However, its teeth and lifestyle would have been very different.
Hundreds of robust teeth line the upper and lower parts of the shark's mouth, forming large slab-like plates capable of crushing shellfish.
"This in turn suggests that P. mortoni was probably a sluggish bottom-dwelling shark, rather than an actively fast swimmer," says Dr Shimada.
Fossils of this and other closely-related species have long been known.
"While there have been many teeth and a few incomplete skeletal remains of P. mortoni in museum collections, the significance of this new specimen is that it contains one of the largest teeth of this species that were found with a gigantic jaw fragment.
"The size of the jaw fragment in fact supports the contention that P. mortoni was likely a gigantic animal," says Dr Shimada.
The scientists have dated the fossil at 88.7 million years old.
At that time, a variety of animals, such as giant clams, other sharks, bony fishes, and predatory marine reptiles called mosasaurs and plesiosaurs inhabited the same water.
Some, including certain mosasaurs would also have grown to gigantic proportions, reaching lengths of 10m or more.
Dr Shimada excavates the jawbone
Why P. mortoni became so huge is still a mystery.
"The emergence of large ptychodontids roughly coincides with the timing of when many other kinds of organisms, including clams as well as sharks and bony fishes, became bigger," explains Dr Shimada.
"Clearly, the food resources must have been abundant enough in the marine ecosystem to support such large organisms.
"Becoming big does have advantages such as deterring predators and being able to travel faster, but it does come with disadvantages as well, most notably needing more food for energy."
Ptychodus mortoni's tooth, close up
Another specimen of P. mortoni has been found alongside another type of meat-eating shark called Squalicorax, with some scientists suggesting that the meat-eating shark may have been scavenging on the body of its larger relative.
Last week, Dr Shimada and colleagues published details in the journal Science of how a dynasty of large plankton-eating fish roamed the oceans between 66 and 172 million years ago.
These fish died out with the dinosaurs.
Once they had vanished from the ecosystem, mammals and cartilaginous fish such as manta rays, basking sharks and whale sharks began to adapt to fill a similar ecological role.