Ninety-nine million years ago, a 55-foot dinosaur chased the river deltas of North Africa. A candle on its back soared over the water while its crocodile-like jaws and curved claws made it difficult for fish the size of a car.
This was Spinosaurus, discovered in 1915. Since then, paleontologists have debated how this creature lived. Did it prowl the currents in search of prey, as suggested by recent research, or did it search for its prey in the shallow waters more like a huge wading bird? New evidence for this second explanation was published on Tuesday in Palaeontologia Electronica, challenging the hypothesis that scientists found a dinosaur that lived a mostly aquatic lifestyle.
The idea that spinosaurus spent most of its time underwater was fueled in recent years by the announcement in 2014 of Nizar Ibrahim, a paleontologist at the University of Portsmouth, England, and colleagues in the discovery of a partial predator skeleton . They argued that he had spent a great deal of time underwater, the first dinosaur known to do so. This idea was reinforced last year, when Dr. Ibrahim’s team announced the discovery of a spinel-like eel-like tail with fins.
The defense of a more aquatic spinosaurus is based on some genuinely bizarre characteristics, Ibrahim said. Unlike most predatory dinosaurs – including some relatives such as the narrow-snouted barionix – the spinosaurus had densely mineralized bones, exceptionally short hind limbs and a tail configuration that allowed for sinuous swimming movements.
“In fact, every part of the body that we examine has ‘water love’ on it,” said Ibrahim.
Other paleontologists have raised doubts about this hypothesis.
“True aquatic predation is a very challenging lifestyle,” said Thomas Holtz Jr., an expert on predatory dinosaurs at the University of Maryland and co-author of the new article. Aquatic predators such as otters, sea lions or sharks have compact, smooth bodies that reduce underwater drag.
According to David Hone, a paleontologist at Queen Mary University of London and co-author with Holtz, previous research by Ibrahim’s team showed that spinosaurus had a relatively small number of tail muscles, said Hone, even less than crocodiles, which they also tend to have problems with trawling and are not particularly effective in chasing prey underwater. With its tall sail and bulky limbs, the spinosaurus would have produced a tremendous drag on anything but the deepest waters, making rapid underwater pursuits extremely difficult.
Instead, the team points to a different set of characteristics. The spinosaurus had nostrils turned towards its long snout, like a heron, said Hone, instead of over its face, like a crocodile. He had wide feet, like a stork, as well as a head and neck adapted for a strong downward attack. Isotopic evidence of spinosaurus teeth also suggests that at least some individuals ate fully terrestrial prey, perhaps including other dinosaurs.
Without more spinosaurs, it is difficult to say what differences may have existed between animals of different ages or sexes, said Holtz. Several species of dinosaurs changed dramatically as they grew up: young tyrannosaurs were fast-chasing predators, while adults were clumsy giants. More fossils would help to determine “whether tail sails were primarily for display rather than locomotion (for example, if males had tall sails and females had more normal sails), or if different growth stages had more aquatic adaptations than others “he said in an email.
But, in light of the anatomical evidence available, said Hone, the spinosaurus seems more suited to peeking along the coast, wading like a heron to grab prey or swimming on the surface and diving to ambush fish below.
“The idea that spinosaurs hang from water is not controversial, ”he said. “We are not saying that you did not swim, or even relatively well. But the idea that this is an animal that looks like an otter is probably going too far. “
Dr. Ibrahim said that although he welcomes the debate, he and his team defend his earlier findings that spinosaurus it was better suited to underwater hunting than any other known dinosaur and probably targeted large, slow-moving fish, as well as land prey.
“In fact, there is nothing in this article that we have not considered before – which is not very surprising, considering that it does not contribute any new data, such as new fossil traces or quantitative analyzes and experiments,” he said.
Some paleontologists say that aspects of Dr. Ibrahim’s previous discoveries can be integrated into the hypothesis that spinosaurs were waders.
“The heron model at this point is the simplest explanation that fits the available data and, in our science, this is generally the best way forward,” said Serjoscha Evers, a spinosaurus expert at the University of Friborg in Switzerland, who reviewed the paper.