The toothy beast was a marine crocodile that looked part shark and part sinister dolphin. Its scientific name is Tyrannoneustes lythrodectikos, or "Tyrant Swimmer".
"Tyrannoneustes lythrodectikos is the oldest known metriorhynchid macrophage - an animal that was adapted to feeding on large-bodied prey," lead author Mark Young of the University of Edinburgh's School of Biological Sciences told Discovery News.
He explained that the term "metriorhynchid" refers to a group of marine crocodiles that were superficially similar to living dolphins.
"They lacked bony armour, had flipper-like forelimbs and had a tail fluke," he said.
This animal evolved from related species that were opportunistic predators of small, fast moving prey.
These marine hunters had narrow snouts and multiple teeth, but the teeth weren't serrated like those of Tyrant Swimmer, which also could open its mouth very wide.
Young and his colleagues studied the remains of Tyrant Swimmer, found in the Oxford Clay Formation, a Jurassic marine sedimentary rock formation underlying much of southeast England.
The remains have been in storage for some time at the Hunterian Museum in Glasgow.
"Tyrannoneustes is known from shallow marine deposits across Europe (England, France and Poland)," Young said.
"During the Middle Callovian 165 million years ago, much of Europe was covered by a shallow sea, creating a chain of large to small islands. Tyrannoneustes lived in this shallow sea, along with numerous other marine reptiles," Young added.
As its name suggests, Tyrant Swimmer would have been swift in the water, so it likely could have out-swam possible predators and used the swimming prowess to capture its own prey.
As of now, no stomach contents for the Tyrant Swimmer have been located, so what it precisely ate remains a mystery.
Young suggested that evolved into an even more stealthy group of marine predators with very large and numerous teeth and mouths that could open extremely wide.
The study was published in the Journal of Systematic Paleontology.
