Were there giant polar bears in the Pleistocene?
Skye McDavid, May 4, 2022
Fossils are often found in remote, inaccessible places: the Gobi Desert and the Badlands of Alberta have both yielded numerous amazing dinosaur specimens. But sometimes, the remains of extinct animals are found in the centers of bustling cities. One particularly mysterious specimen was found in the sediments of the Thames, near Kew Bridge in London.
NHMUK PV OR 24361 is the left ulna of a bear, mostly complete but missing the distal end, dating to the Late Pleistocene. Bears are, of course, nothing out of the ordinary, but this ulna still raises some questions. First, it's huge. The incomplete ulna is 440mm long, and was estimated to be 485mm long when complete. This makes this specimen the largest known specimen of the genus Ursus, comparable in size to the giant Tremarctines (often misleadingly called "short-faced bears") of the Americas.
Based on its enormous size, NHMUK PV OR 24361 was made the holotype of a new subspecies, Ursus maritimus tyrannus by Björn Kurtén in 1964. He identified it as a polar bear, Ursus maritimus, and believed the specimen to represent a subadult, meaning that the adults would have been even larger.
The presence of Polar Bears in Southern England may be surprising to some, but this specimen is from the Pleistocene, when various animals that are restricted to the Arctic today had much wider distributions. Walruses (Odobenus rosmarus), for example, are found as far South as New Jersey. Several other polar bear fossils have been found south of their current range, including as far South as Jutland. In other words, it's entirely possible that there were Polar Bears in Southern England during the Pleistocene.
However, the assignment of this ulna to the species Ursus maritimus is under question. It may represent Ursus arctos, a brown bear. Ingólfsson and Wiig (2009) described a subfossil mandible of a modern polar bear (Ursus maritimus maritimus) from Svalbard, and briefly mentioned Ursus maritimus tyrannus. Citing personal communication with Andy Currant, they say that unnamed NHMUK researchers reanalyzed the specimen and are "confident" that it represents Ursus arctos. The NHMUK's online database also identifies it as Ursus arctos.
The analysis that found NHMUK PV OR 24361 to be a brown bear was never formally published, though I can only speculate as to why. One possibility is that the paper was rejected during peer review because the reviewers agreed with Kurtén's original identification of Ursus maritimus. Another, possibly more likely explanation is that the project was simply abandoned due to lack of time or because one or more of the researchers moved to a different institution. I know of several examples of potentially important projects being abandoned for these reasons.
Although I am not a mammal paleontologist (I work primarily with Mesozoic Archosaurs), I consider the subspecies Ursus maritimus tyrannus to be questionable because its diagnosis is inadequate. Kurtén's diagnosis is simply "a Polar Bear greatly exceeding the living form in size." The holotype could simply be an exceptionally large individual. At this point, I think it would be safest to consider NHMUK PV OR 24361 a particularly large individual of Ursus sp.
If NHMUK PV OR 24361 is a polar bear, it would be somewhat larger than modern ones. If it is a brown bear, it is much larger than modern ones. It is clear that there was at least one enormous bear that walked what would eventually become London during the Pleistocene. But it is not clear whether it was a Polar Bear or Brown Bear, and since only a single individual is known, at this stage it's impossible to know whether it represents an exceptional individual or a larger population or subspecies.
Kurtén, Björn (1964) The evolution of the Polar Bear, Ursus maritimus Phipps. Acta Zoologica Fennica 108 http://hdl.handle.net/10138/37762
Natural History Museum London Data Portal NHMUK PV OR 24361 (retrieved May 4, 2022) https://rb.gy/3s2by5
Ingólfsson, Ólafur, & Wiig, Øystein (2009). Late Pleistocene fossil find in Svalbard: the oldest remains of a polar bear (Ursus maritimus Phipps, 1744) ever discovered. Polar Research, 28(3), 455-462. https://doi.org/10.3402/polar.v28i3.6131