Now, they believe it would have been less like an otter, and more like a huge, flightless stork or heron.
Rather than hunting fish in the water, the massive dinosaur would have likely caught prey out of the water from a position on the shoreline, researchers from Queen Mary University of London and the University of Maryland found in a study published Tuesday.
Spinosaurs were a group of large-bodied theropods that were bigger than both Tyrannosaurus rex and Giganotosaurus, growing to around 15 meters (49.2 feet) in length. These dinos lived during the Cretaceous period -- between 145.5 and 65.5 million years ago.
The study compared Spinosaurus fossils with skulls and skeletons of dinosaurs and reptiles that lived both in and out of the water, concluding that a wading, heronlike behavior was the most likely.
This behavior has been debated since the Spinosaurus was first discovered in 1915.
"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," lead study author David Hone, senior lecturer at Queen Mary University of London, said in a news statement.
"Our findings suggest that the wading idea is much better supported, even if it is slightly less exciting."
'A bizarre animal'
Spinosaurus has proven hard to understand because it's "a bizarre animal even by dinosaur standards, and unlike anything alive today," said study coauthor Tom Holtz, a paleontologist and principal lecturer at the University of Maryland's department of geology, in a statement.
"We sought to use what evidence we have to best approximate its way of life. And what we found did not match the attributes one would expect in an aquatic pursuit predator in the manner of an otter, sea lion, or short-necked plesiosaur."
The researchers compared Spinosaurus to crocodiles in order to illustrate its relatively poor adaptation to aquatic life.
Crocodiles are excellent in water compared to land animals, Hone said, but they can't actively chase fish.
"If Spinosaurus had fewer muscles on the tail, less efficiency and more drag then it's hard to see how these dinosaurs could be chasing fish in a way that crocodiles cannot," Hone said.
There remains a lot to learn about Spinosaurus, said Hone, who explained that there is a lack of good fossils.
There is still some debate as to whether there were distinct species of Spinosaurus that preferred different environments, or a single species that adapted, Hone told CNN.
The best way to start solving some of these mysteries is to carry out more digs, said Hone, who added that there is good Spinosaurus material in Egypt, Morocco and possibly Algeria.
The study was published in the journal Palaeontologia Electronica.