Biology

Aquatic dinosaur may have been a shoreline stalker, not a fish-chaser

Aquatic dinosaur may have been...
Spinosaurus is now believed to have snatched fish from the shore, instead of chasing them down underwater
Spinosaurus is now believed to have snatched fish from the shore, instead of chasing them down underwater
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Spinosaurus is now believed to have snatched fish from the shore, instead of chasing them down underwater
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Spinosaurus is now believed to have snatched fish from the shore, instead of chasing them down underwater
Unlike Spinosaurus (pictured here) fully aquatic prehistoric reptiles such as the mosasaurs and plesiosaurs were not actually dinosaurs
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Unlike Spinosaurus (pictured here) fully aquatic prehistoric reptiles such as the mosasaurs and plesiosaurs were not actually dinosaurs

Just last year, scientists declared that Spinosaurus was the first dinosaur known to swim through the water, preying upon fish as it did so. A new study, however, suggests that it was probably more of a shore-based feeder.

Spinosaurus measured up to 18 meters in length (59 ft) and weighed at least 20 tons (18 tonnes). It lived in what is now North Africa, approximately 99 to 93.5 million years ago.

Based on an analysis of recently unearthed fossils, in 2020 it was determined that the creature had a flat paddle-like tail, similar to that of a crocodile. That feature, along with characteristics such as a long tooth-filled snout and short stumpy legs, led researchers to believe that it was adapted to an aquatic lifestyle in which it chased down, captured and ate fish.

Questioning this assumption, scientists from Queen Mary University of London and the University of Maryland have reviewed existing data regarding the creature's anatomy. They have concluded that while Spinosaurus was indeed a swimmer, it more likely fed like a stork or a heron, wading along the river's edge and snatching up fish that swam past beneath it. That said, it probably also fed on terrestrial prey, which it captured along the shoreline.

Unlike Spinosaurus (pictured here) fully aquatic prehistoric reptiles such as the mosasaurs and plesiosaurs were not actually dinosaurs
Unlike Spinosaurus (pictured here) fully aquatic prehistoric reptiles such as the mosasaurs and plesiosaurs were not actually dinosaurs

For one thing, the researchers point out that unlike a crocodile, Spinosaurus didn't have raised nostrils or eye sockets. This suggests that it couldn't easily breathe or look around when resting at the water's surface. The structure of its neck and inner ear also indicate that it evolved to move its head primarily up and down – as it would when grabbing fish passing beneath – as opposed to from side to side.

What's more, they state that Spinosaurus had fewer tail muscles than a crocodile, plus it wasn't as streamlined. This means it wouldn't have been as fast of a swimmer as a crocodile, which themselves don't usually chase after fish.

The new study additionally draws attention to the reptile's teeth, some of which are reportedly similar in shape to those of past and existing predators that crush hard-bodied prey. This factor, combined with the observation that Spinosaurus frequently shed and replaced teeth, could indicate that much of its diet consisted of crustaceans it dug up, or of slow armored fish that were easy to catch from above the surface.

Along with various other considerations, the scientists also make note of the fact that when relatively rare Spinosaurus fossils are discovered, the bones of multiple individuals are often found together in one area. This may indicate that instead of continuously swimming up and down the length of a river, the dinosaurs congregated in specific areas where the shoreline hunting was best, periodically swimming from one of those locations to another.

"The biology and ecology of Spinosaurus has been troubling paleontologists for decades," says the lead scientist, Queen Mary's Dr. David Hone. "Some recent studies have suggested that it was actively chasing fish in water but while they could swim, they would not have been fast or efficient enough to do this effectively. Our findings suggest that the wading idea is much better supported, even if it is slightly less exciting."

A paper on the research was recently published in the journal Palaeontologia Electronica.

Source: Queen Mary University of London

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