In three separate incidents in the 1990s, fossil vertebrae (back bones) were dredged up from the North Sea, in the waters between Britain and the Netherlands. These vertebrae weren’t from any sea creature. Scientists identified them as being from musk oxen - animals that most definitely do not swim. So how did they end up in the North Sea?

The first bone was a thoracic vertebra (midback bone) trawled up from the bottom of the North Sea by the vessel SL27 in 1995. Next, the vessel ARM20 trawled up an axis (neck bone) in 1997 followed by another cervical vertebra (neck bone) in 1998.

In 1999, the three vertebrae were studied, described and published in a paper by a group of Dutch scientists, led by lead author Dick Mol of the Natural History Museum Rotterdam.

So if musk oxen don’t swim, how did the bones end up in the North Sea? During mulitple occasions during the Pleistocene Epoch, water levels in the North Sea were lower and a land-bridge connected Britain to continental Europe. This land existed in an on-again-off-again fashion throughout the Pleistocene as Earth underwent a series of ice ages.

Many species of animals lived in the land that connected Britain to the Continent during the Pleistocene, resulting in many fossil finds in the North Sea in the present era. As Mol explained:

The bottom of the North Sea between England and The Netherlands is the source of large quantities of mammal remains dating from the entire Pleistocene. The remains are trawled and brought ashore by fishermen, and they then find their way to collectors and museums.

Given that the fossil vertebrae were not found in situ in a specific geologic context (they were trawled up, after all), it isn’t possible to date them to a precise time period. However, in examining the fossils, Mol noted that the “bone is strongly mineralised and some iron concretions are adhering to it.” Mol explained that “these features characterise fossil land mammals from the late Early and Middle Pleistocene.”

By making detailed measurements on the centrum and neural canal of each vertebrae, Mol identified the fossils as belonging to the extinct musk ox species Praeovibos priscus. Bones of the extinct Praeovibos priscus are often quite similar to those of living musk oxen from the species Ovibos moschatus, but distinguished on the basis of their larger size. Mol measured the fossils and compared them to those of modern musk ox bones from Alaska in his museum’s collection, and determined that “all three North Sea fossils are larger than this recent muskox.” His measurements are included in the paper.

Considering that Praeovibos fossils are also known from England, Germany and France’s Arago Cave, it is not surprising that Praeovibos would have also lived in the land that is today the North Sea. However, these fossil finds help “fill in the gaps” of where Praeovibos priscus roamed during the Pleistocene.

Examining evidence from the known fossil sites in Western Europe, Mol concluded that:

Praeovibos did not live in the cold tundras in which we find the present-day muskoxen…All evidence therefore suggests that Praeovibos must be seen as a representative of a fauna from more temperate climatic stages in the Early and Middle Pleistocene.