Some 170 million years ago, deadly Jurassic crocodiles ruled the oceans. But researchers think they have unlocked the key to their successful reign of terror -- by mimicking the shape and senses of whales and dolphins.

The extinct beasts -- known as thalattosuchians -- could reach up to 10 meters in size and evolved from their land-living ancestors by adapting their limbs into flippers, developing fluked tails for swimming and streamlining their bodies, making them formidable, fast-swimming predators.

The teleosauroid Machimosaurus rex, the largest semi-aquatic thalattosuchian, could reach up to 10 meters in size, and is thought to have fed on hard prey like turtles due to its teeth, paleontologist Julia Schwab told CNN.

Plesiosuchus, the largest marine thalattosuchian that lived in the open ocean, measured about 6.8 meters and hunted anything from squid-like creatures to fish and other marine reptiles.

By studying the crocodile's skulls, researchers also found that they had adapted part of the inner ear as they adjusted to life in the oceans.

Paleontologists from the University of Edinburgh analyzed computerized axial tomography (CAT) scans of more than a dozen fossil skulls to study the vestibular system -- the three looping semicircular canals of the inner ear which control balance.

Researchers found that as the creatures entered their semi-aquatic phase, the canals started to become fatter and smaller like those of whales and dolphins.

This shape, experts say, made the creature's sensory system less sensitive, and better suited to life in the oceans.

"Sensory organs such as the inner ear are key to understand how ancient animals lived. We found that marine crocodile relatives have a very unique inner ear shape, similar to other water-living reptiles and today's whales," Julia Schwab, a PhD student in the University of Edinburgh's School of GeoSciences who led the study, said in a statement.

On land, animals need a sensitive sense of balance to cope with gravity and complex landscapes, but in the ocean, buoyancy keeps animals afloat.

The adaptation came in response to the crocodile's new deep water environment, rather than driving them into it, scientists say.

"The ancient aquatic crocs developed unusual inner ears after modifying their skeletons to become better swimmers. Whales also changed their ears in a similar way, but did it soon after entering the water," Dr Steve Brusatte, senior author on the study, said.

In a paper, published in the journal Proceedings of the National Academy of Sciences, experts noted that a similar change occurred independently in whales, leading experts to believe that each species mimicked each other's changes.

"It seems like the crocs and whales took similar, but different, evolutionary routes from land to water," Brusatte added.

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