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Allow me to shed a little light on this mysterious bear.......... by Ghari Sher
As for those first two possibilities you bring up, the Kew Bridge ulna is much younger than the oldest know remains of polar bears, which is a mandible dating back ~130kya, and they are believed to have diverged from brown bears quite early on, about 400,000 years ago. So even if the ulna was a polar bear, it would not have and early example of this species by any means, nor an intermediate, at least not one on the line toward extant polar bears.
Of course, you mention, the scientists at the NHM are convinced that the ulna is actually a brown bear. This is based on personal communication with Andy Currant (now retired).
This isolated ulna fits into perspective once we take into account some of Currant's other writings - he has written extensively on the mammals of Pleistocene Britain, and the different assemblages which existed through the different climatic shifts, etc.
One detail which might raise eyebrows is when the Currant & Jacobi (2001) [1] describe one particular period of the British late Pleistocene - a period known as MIS 5a, which spanned from approximately 85-65 thousand years ago, known in the British biostratigraphy as the Banwell Bone Cave MAZ (mammal assemblage-zone), after the cave site where this period was first recognized. During this period, we witness a very low diversity cold-climate fauna inhabiting the British Isles at various sites across the country including the aforementioned locus typica. Only two large carnivore species - wolves and brown bear, are known to have existed in the carnivore guild.
The brown bear remains are consistently described as being extremely large in the literature, and what few images I can find of the remains from the locus typicus certainly suggest as much.
21 ( quote ) The strange assemblage from Banwell Bone Cave includes a giant bear, probably an even more formidable predator than the polar bear. The top of its humerus (above) is compared with the whole humerus of a brown bear (left).
When they describe the brown bear briefly in their 2001 paper, Currant & Jacobi interestingly cite Kurten (1964) to exemplify the huge brown bears at this time, suggesting that by this point they had already re-assigned the Kew Bridge ulna to Ursus arctos, long before the personal communication sent by Currant in 2008.
( quote ) Bison and reindeer are the dominant elements of this fauna, with wolf, wolverine, mountain hare and an extremely large variety of brown bear (e.g. Kurten, 1964) as their consistent companions.
Looking at Currant (2004)'s description of the brown bear of this period raises suspicions further:
( quote ) One of the most spectacular features of this collection are the remains of a huge form of brown bear which closely matches the living polar bear in many features. This appears to have been the dominant predator in this restricted, cold stage assemblage.
This extremely large form of brown bear which closely resembles the polar bear.............. where does that ring a bell?
Taking a closer look at the lithostratigraphy and biostratigraphy at the Kew bridge site, we get our suspense settled. The site dates to between MIS 6 and MIS 4.
The fauna found at the site are very characteristic of the period of MIS 5a in the British Isles.
( quote ) Based on the dominance of reindeer and the co-occurrence of very large brown bear, the assemblage from Kew Bridge was assigned to the Banwell Bone Cave MAZ and consequently attributed to MIS 5a (Currant and Jacobi, 1997, 2001, 2002; Gilmour et al., 2007).
This makes the Ursus "maritimus tyrannus" holotype another specimen representing the extremely large form of brown bear that inhabited the British Isles at this period. Interestingly, the fact that this brown bear ulna was misidentified as being a polar bear seems to largely fit in with what I have read of the descriptions of the MIS 5a brown bears of Britain from other sites(namely Banwell Bone cave) - these brown bears seem to be likened to the polar bear very often - they seemed to be very polar bear-like in their morphology.
( quote ) Predators likewise changed, as the extremely narrow carnivore guild enjoyed rich herbivore pickings. Fearsome brown bears became so large that in the past their remains were misidentified as polar bear and, like the polar bear, they were probably active and formidable hunters.
Various other sites yield this same pattern of fauna, including the brown bear remains, such as the nearby Isleworth site just an hour's walk away from Kew Bridge, which possibly has a similar depositional history:
These bears are consistently larger than the brown bears found in the British Isles during other periods, as show above. Seems that Kurtén misidentified this bear as a polar bear.
Brobear: The Pleistocene bear called "the tyrant sea bear" was even larger than the big modern polar bears of today. Could this be because he was once a hunter of musk ox and perhaps some now extinct tundra herbivores and a scavenger of mammoths who then gradually learned to hunt seals?
This is where we move to the paleoecological context behind this very large brown bear form that inhabited the British Isles during this narrow period of the late Pleistocene.
In earlier interstadial deposits in Britain, we have evidence of warm-climate fauna in MIS 5c and 5e, e.g. Hippopotamus & Paleoloxodon, as well as predators such as cave lion (Panthera spelaea) and hyena (Crocuta crocuta spelaea), but after the cold period of MIS 5b, it appears that these fauna were forced to leave/were extirpated. By the milder, though still somewhat cold period of MIS 5a, the geological evidence suggests that the now warmer climate raised sea levels just high enough to isolate the British Isles from the European mainland by a presumable-narrow English channel, though the extent of the risen sea levels is unknown. Certainly, glaciers still existed in the north of the country. The ability of species to swim, or even, perhaps, their ability to make crossings over sea-ice, may have acted as a natural biological filter. Whatever animals were able to survive the cold glacial conditions of MIS 5b, were now isolated from the rest of Europe on a somewhat-milder Island.[5] The environment was likely still cold however, with a mean winter temperature estimated at -20±10℃ .[6]
Among the large herbivores, only steppe bison and reindeer are known to have been present at this time, while the European mainland housed the "standard" herbivore guild of the mammoth steppe (woolly mammoth, rhino, horse, and giant deer in addition to the aforementioned two).
The carnivore guild was also strikingly different. Only wolves, and this aforementioned gigantic brown bear are recorded in the British Isles. Meanwhile the mainland guild contains the additional cave lion and cave hyena. Another predator that once frequented the British Isles is missing at this period - man. Neanderthals are known to have existed in Britain before MIS 5a, but during this period there is no trace of them. This strongly suggests that Neanderthals were absent in the British Isles at this period, returing only in MIS 4, about 60,000 years ago, when the glaciers expanded and a land-bridge arose once more (the same is true for most of the other large fauna), before being replaced by their more slender African cousins about 20,000 years afterwards.[5]
In their ecological liberation - the wolves and brown bears had this island home all to themselves. Mass estimates suggest that both the wolves and brown bear were the largest ever to have lived in the British Isles. Analysis of the teeth of the wolves suggest some heightened level of durophagy, suggesting they had expanded into the niche of the absent hyena.[7]
Looking at the postcrania of the bear remains, the informaton that I can derive is that they were adapted to hunting the large ungulates that lived on the island, but past that I can't gather very much. I suppose this raptorial hunting style led to some convergence with the polar bear, but you be the judge of that.
The bear remains are often huge, suggesting a powerfully built runner with small teeth, and enormous claws, an active hunter like a polar bear.
Analysis of the wear on the teeth of brown bears from Banwell Bone Cave suggests a highly hypercarnivorous diet, sharing many similarities to the polar bear in the nature of the tooth wear.
The microwear results as seen in the bivariate plots from the Banwell Bone Cave bears reveal many similarities with high-latitude species such as U. maritimus, U. americanus and U. arctos from Russia. In addition, the PCA revealed that the dietary ecospace of these bears had commonalities with those of U. maritimus and U. thibetanus at the present day. Some of the main microwear features noted on these large extinct brown bears are the highest score for scratches width after U. maritimus, an absence of puncture pits and a moderate percentage of coarse scratches (Fig. 4.19.E). The absence of puncture pits indicates that fruits with seeds did not play a part in the diet of U. arctos from Banwell Bone Cave (e.g. Semprebon et al ., 2004). As well as hunting, flesh may also have been scavenged from (frozen) carcasses, as seen in modern polar bear utilisation of marine mammals (Bentzen et al., 2007).
In regards to them hunting muskoxen or scavenging mammoth - this wasn't possible since neither coexisted with these isolated island bears. They main prey was almost certainly the steppe bison and reindeer they coexisted with
Will have more to say in good time.
Sources:
[1] doc.rero.ch/record/209912/files/PAL_E4236.pdf
[2] books.google.co.uk/books?id=1lcd...wQ6AEIXDAJ
[3]
www.bleadon.org.uk/media/other/24400/BleadonCavern_HES_150_Years_Chapter_7.pdf[4] pure.royalholloway.ac.uk/portal/files/4059178/2011JubyCPhD.pdf#page=336
[5] books.google.co.uk/books?id=Xeye...0Q6AEIKjAA
[6] onlinelibrary.wiley.com/doi/abs/...0.CO%3B2-N
[7] s3.amazonaws.com/academia.edu.do...variab.pdf
[8] pure.royalholloway.ac.uk/portal/...hd.pdf.pdf
