T.rex-size crocodiles ruled the ancient seas

An over 22-foot-long crocodile that ripped prey to death and another huge croc that sucked prey to its doom were at the top of the European marine food chain 150 million years ago, a new study finds.

The enormous prehistoric crocodiles, Plesiosuchus and Dakosaurus, were such ravenous carnivores that their methods have been compared to today's killer whales and a famous, iconic, meat-loving dinosaur.

"The skulls of these two sea croc species have some similarities to T. rex," lead author Mark Young of the University of Edinburgh told Discovery News. "The largest known skull of Plesiosuchus manselii was approximately four feet, three inches long, putting it in the size range of adult T. rex skulls."

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For the study, published in PLoS ONE, Young and his colleagues analyzed fossils for the two crocodiles, which were unearthed in sites from England to Germany. In their U.K. home, the crocs once dwelled in the shallow seas that covered England. At the same time, Archaeopteryx was flying over Europe and giant dinosaurs, such as Diplodocus and Allosaurus, were stomping around North America.

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The researchers determined that Plesiosuchus was the largest known species of metriorhynchid, meaning sea crocodile.

"It was bigger than living salt water crocodiles and great white sharks," Young said.

Its teeth functioned like those of today's killer whales, based on shape and wear. This ripper crocodile probably bit into both large and small prey, which it would grab, kill and gulp.

Perhaps even more unusual was the sucker croc, Dakosaurus. The skull and jaw characteristics of this nearly 15-foot-long ancient crocodile suggest that it was a suction feeder, making it the first known suction-feeding crocodilian.

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This way of eating "involves being able to rapidly open the mouth wide, and generating negative pressure," Young said. "This sucks a prey item into the mouth."

"We think that Plesiosuchus specialized in eating other marine reptiles, and Dakosaurus was a generalist," co-author Lorna Steel of the Natural History Museum in London said, "probably eating fish and whatever else it could get hold of, perhaps including the small metriorhynchid Geosaurus." The latter looked like a barracuda.

It's suspected that modern killer whales can also suck in victims. Young explained that juvenile killer whales in captivity are known to generate negative pressures with their mouths.

Both prehistoric crocodiles, therefore, fed very similar to modern killer whales. These animals are not related, since killer whales are mammals. The researchers instead believe that the similarities exemplify what's known as convergent evolution.

"Convergence is the evolution of a similar body plan, feeding mechanism (or other characteristic or behavior) in two different and not closely related groups, in this case crocodiles and mammals," Young said.

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"The continual evolution of these morphologies in distantly related groups could be telling us something about the limits and optimal method of underwater feeding in vertebrates," he added. "For example, the shearing, tooth-to-tooth occlusion of Dakosaurus is today found in false killer whales. But over the past 10 million years, numerous species of fossil sperm whales also evolved this feeding mechanism."

As for how the two different, yet equally formidable, crocodiles got along, there is no evidence that either species attacked the other. The scientists believe the Dinosaur-Era English seas had niche partitioning, whereby various predators fed on different animals, so the hunters never had to compete with one another for food. Today's ocean ecosystems are organized in much the same way, with animals such as whales, sharks and dolphins living in the same overall area.

Young and his colleagues would next like to delve deeper into understanding the feeding mechanics of the two crocs and other marine predators.