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'Balloon head' dolphin discovered

The fossil's concave shape suggests it fit below a dome-like forehead

A new type of dolphin with a short, spoon-shaped nose and high, bulbous forehead has been identified from a fossil found in the North Sea.

The Platalearostrum hoekmani was named after Albert Hoekman, the Dutch fisherman who in 2008 trawled up a bone from the creature's skull.

Up to six metres in length, the dolphin lived two to three million years ago.

The so-called rostrum bone and a model of the dolphin are on display at the Natural History Museum Rotterdam.

As museum researchers Klaas Post and Erwin Kompanje write in the museum's journal Deinsea, the North Sea has been a rich source of fossils in recent decades as bottom-trawling has become more prevalent.

The practice has yielded tens of thousands of pieces of the fossil record - many of which defy classification.

What is clear from the singular bone found by Mr Hoekman is that the animal from which it came fits neatly in the family of marine mammals known as Delphinids - the ocean-going dolphins that actually includes both killer and pilot whales.

More specific classification within this family is somewhat speculative.

The bone shows an unusually large tip region containing six teeth known as the premaxilla. This feature suggests the broad, blunt nature of the creature's snout.

Based on analyses of similar fossils and modern relatives within the family, the researchers are convinced they have found a new species whose closest living relative is the pilot whale.