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.