Ann. Zool. Fennici 61: 357–376 ISSN 0003-455X • eISSN 1797-2450 Helsinki 19 November 2024 © Finnish Zoological and Botanical Publishing Board Dance of the Cave Bear: Honouring the Scientific Legacy of Björn Kurtén Bear feet with Björn: tarsal evolution in the origin of polar bears P. David Polly Earth & Atmospheric Sciences, Indiana University, 1001 E 10th Street, Bloomington, IN 47401, USA (pdpolly@indiana.edu) Received 14 Mar. 2024, final version received 26 May 2024, accepted 31 May 2024 Polly, P. D. 2024: Bear feet with Björn: tarsal evolution in the origin of polar bears. — Ann. Zool. Fennici 61: 357–376. Polar bears (Ursus maritimus) diverged from brown bears (U. arctos) in the last one million years. Polar bears have a strikingly different external appearance because of their white pelts and large size, but osteological differences are thought to be limited to flatter crania and altered dentition, with little difference in the postcrania despite swimming frequently and living almost entirely on icy substrates. This paper shows that polar bears have substantial differences in their tarsal form and function from other bears, including their close relatives, the brown bears. Ankle gear ratio records the major functional transition from semi-cursorial hemicyonines to the more plan- tigrade locomotion of crown ursines. Analysis of tarsal morphology among seven extant ursines show that the arboreal species Helarctos malayanus, Melursus ursinus, Tremarctos ornatus, U. thibetanus, and U. americanus have morphologies that permit greater movement at the transverse tarsal and lower ankle joints, especially broader and more gently curved astragalocalcaneal and sustentacular facets on the calcaneum and their articular equivalents on the astragalus, as well as broader and more gently curved navicular facets on the astragalar head. U. arctos and U. maritimus, which are strongly terrestrial, have smaller sustentacular facets and pronounced interlocking between astragalus and calcaneum at the sustentaculum. Polar bears, however, differ from brown bears in that this interlocking is less tight and thus permits more move- ment at the lower ankle joint. The phylogenetic comparative analysis of shape shows that the divergence in ankle morphology of the polar bear from the brown is one the most rapid bursts of tarsal evolution in ursines. Introduction Just remember that’s a bear there in the bunch with you, And they just don’t come no better than a bear. (Steven Fromholz 1975) As a recently, strikingly, and perhaps incom- pletely diverged species, the origin and fate of the polar bear, Ursus maritimus, is of interest from both evolutionary and ecological perspectives. Closely related to the brown bear Ursus arctos, polar bears are members of the tribe Ursini that includes all extant bears except the spectacled bear Tremarctos ornatus and the giant panda Ailu- ropoda melanoleuca (Fig. 1). Polar bears are the Edited by Oscar E. Wilson, University of Helsinki