Morphometric analysis of metacarpal and metatarsal bones of cave bears (Carnivora, Ursidae)

Abstract

Abstract For the first time, morphometric variation has been studied in metacarpal and metatarsal bones of all known taxa of cave bears, which belong to different molecular genetic groups (deningeri, kudarensis, spelaeus, and ingressus haplotypes). The examined material involves nearly three thousand specimens from 28 localities of Europe, the Urals, Caucasus, and Central Asia. For comparison we used samples of fossil and recent Ursus arctos, as well as U. etruscus, regarded as a common ancestor of brown bears and cave bears. Methods of univariate and multivariate statistical analyses of metapodial bones were employed, providing an opportunity to ascertain the degree of sexual dimorphism in different taxa, the degree of morphological difference between taxa, and to define “size” and “shape” morphospaces for concise description of morphological diversity and classification of cave bears. Our study reveals that, on average, sexual dimorphism is more pronounced in U. arctos and U. kudarensis praekudarensis than in cave bears. Sexual dimorphism of bear metapodia is greater than sexual dimorphism of the skull (Baryshnikov and Puzachenko 2011). The contribution of sexual dimorphism to size of the metapodials is close to a contribution associated with morphological disparity between the bears belonging to different taxa. By the example of two chronosubspecies of Kudaro cave bear: U. kudarensis praekudarensis from Middle Pleistocene and U. k. kudarensis from Late Pleistocene, we succeeded in detecting a decrease of sexual dimorphism over time, which suggests that earlier cave bears inherited a pronounced sexual dimorphism from ancestral taxa. Metacarpal and metatarsal bones of cave bears are easily distinguished from those of U. etruscus and U. arctos, simultaneously demonstrating similarity between cave bears from different genetic groups, involving the species U. kudarensis (the basal taxon for all cave bears, including U. deningeri); some peculiarities of these bones are revealed only in the smaller U. rossicus. The examples have shown the presence (U. k. kudarensis) as well as absence (U. deningeri, U. kanivetz ingressus) of evident spatial (geographical) and temporal patterns in metapodial variability. It is determined that taxa can be better differentiated by metacarpals rather than by metatarsals, because the latter proved to be more “conservative” and less variable over time. It is hypothesized that very rapid modification of metapodial bones occurred at an early stage of evolution of this group, which was presumably a result of occupation of a special ecological niche by cave bears. This study reveals that the size and shape of metacarpal and metatarsal bones did not have an observable link with the taxonomic or evolutionary position of cave bears. At the same time, the clear morphological differences between brown bears and cave bears reflect an early evolutionary divergence between “arctoid” and “spelaeoid” lineages.