Ninety-nine million years ago, a 55-foot dinosaur chased the river deltas of North Africa. A sail on its back loomed over the water as its crocodile-like jaws and curved claws did the work of wagon-size fish.
This was Spinosaurus, which was discovered in 1915. Paleontologists have since debated how this creature lived. As recent research has shown, did it move through currents in search of prey, or did it look for its quarry in the lowlands more like a giant wader? New evidence to support this second explanation was published Tuesday in Palaeontologia Electronica, challenging a hypothesis that scientists had found a dinosaur that led a primarily aquatic lifestyle.
The idea that Spinosaurus spent most of its time underwater was fueled in recent years by the announcement by Nizar Ibrahim, a paleontologist at the University of Portsmouth in England, and colleagues about the discovery of a partial skeleton of the predator in 2014. They argued it spent much of its time underwater, the first known dinosaur to do so. That idea was strengthened last year when Dr. Ibrahim’s team announced the discovery of a fin-like, eel-like Spinosaurus tail.
The case of a more aquatic Spinosaurus is based on some really bizarre features, said Dr. Ibrahim. Unlike most predatory dinosaurs – including some relatives like the slender-snouted baryonyx – the Spinosaurus had densely mineralized bones, unusually short hind legs, and a tail configuration that enabled tortuous, skull-topping movements.
“Really, every part of the body we looked at has written ‘water-loving’ everywhere,” said Dr. Ibrahim.
Other paleontologists have expressed doubts about this hypothesis.
“Real predators in the water are a very challenging lifestyle,” said Thomas Holtz Jr., a predatory dinosaur specialist at the University of Maryland and co-author of the new paper. 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 in London and co-author with Dr. Holtz, earlier studies by Dr. Ibrahim’s team demonstrated that Spinosaurus had a relatively small number of tail muscles, said Dr. Hone, even less so than crocodiles, which also have drag problems and are not particularly effective at tracking prey underwater. With its tall sail and bulky limbs, the Spinosaurus would have created tremendous drag in anything but deepest water, making quick subaquatic car chases extremely difficult.
Instead, the team points to other characteristics. Spinosaurus had nostrils on its long snout, like a heron, said Dr. Hone instead of sitting on his face like a crocodile. It had broad feet like a stork, and a head and neck suitable for a strong downward blow. Isotopic evidence from Spinosaurus teeth also suggests that at least some individuals ate completely terrestrial prey, possibly other dinosaurs as well.
Without more Spinosaurus remains, it is difficult to tell what differences there might have been between animals of different ages or sexes, said Dr. Holtz. Some types of dinosaurs changed drastically as they grew: young tyrannosaurs quickly chased predators, while adults were cumbersome giants. More fossils would help determine “whether the stern sails were designed for display rather than locomotion (for example, when men had tall sails and women more normal sails) or whether different stages of growth had more aquatic adaptations than others,” he said in an email.
But given the anatomical evidence available, Dr. Hone, Spinosaurus seems better suited for trudging along the coast, wading like a heron to snatch prey, or swimming on the surface and ambushing fish.
“The idea of spinosaurs hanging out by the water is not controversial,” he said. “We’re not saying that it didn’t swim or that it swam relatively well. But the idea that this is an otter-like animal probably goes way too far. “
Dr. Ibrahim said that while welcoming the debate, he and his team stand by their previous findings that Spinosaurus is better suited for underwater hunting than any other known dinosaur and is likely to target large, slow-moving fish as well as terrestrial prey.
“There’s really nothing in this article that we haven’t considered before – which isn’t too surprising when you consider that it doesn’t provide new data like new fossil remains or quantitative analysis and experimentation,” he said.
Some paleontologists say that aspects of Dr. Ibrahim’s earlier findings can be integrated into the hypothesis that Spinosaurus were waders.
“The heron model at this point is the simplest explanation that fits the available data, and in our science this is often the best way to go,” said Serjoscha Evers, a spinosaur specialist at the University of Friborg in Switzerland who wrote the paper.