Short-faced bear convergence and the diversity of American extinct bears

A neat new paper by Kieren Mitchell and colleagues in Biology Letters has an mtDNA phylogeny for some extinct bears of the Americas. The main conclusion is that the giant short-faced bears of North America and South America evolved convergently from smaller ancestors; earlier systematists had generally considered them to be sister taxa.

The first paragraph gives a great review of the past diversity of this group of bears:

The spectacled bear (Tremarctos ornatus) is the only living member of Tremarctinae, a previously diverse group of bears endemic to the Americas. The now-extinct Pleistocene diversity of Tremarctinae comprised the Florida spectacled bear (Tremarctos floridanus), South American short-faced bears (Arctotherium—five species; [1]) and North American short-faced bears (Arctodus—two species; [2]). These species ranged in size from the relatively small Arctotherium wingei (approx. 150 kg; [3]) to the giant short-faced bears Arctodus simus and Arctotherium angustidens, which may have attained body masses exceeding 1000 kg [4,5]. In addition, tremarctine bears displayed a diversity of foraging strategies, ranging from carnivorous/omnivorous (e.g. Arctodus simus, Arctotherium angustidens) to largely herbivorous (e.g. Arctotherium wingei, T. ornatus) [6–9]. The evolution and biogeography of this diverse group of bears is enigmatic, and currently lacks a robust phylogenetic framework.

I had no idea of the past phylogenetic diversity of bears of the Americas, it seems like a really cool story.

Here’s the phylogeny from the paper by Mitchell and colleagues, with all the species included:

American bear phylogeny
Figure 1 from Mitchell et al. 2016. Original caption: Relationships among tremarctine genera resulting from phylogenetic analysis of our mitochondrial genome dataset. Nodes reflect mean age estimates, whereas grey bars reflect 95% highest posterior densities (HPDs). Branch support values (BEAST posterior probability/RAxML bootstrap %) are given for each clade. The approximate temporal range of taxa of interest (see main text) is plotted based on the fossil record, and coloured according to distribution (North America, blue; South America, red). The extant taxon is marked with an asterisk.

The common ancestors of the Arctodus and Arctotherium clades lived in the Early Pliocene. This means that their Late Pleistocene representatives were separated by around 8–10 million years of evolutionary time. Looking at this as a paleoanthropologist, I’m tempted to compare to other similar instances of dietary convergence. For example, the robust australopiths have been argued to represent dietary convergence upon a large-molar and molarized premolar morphology, with small canine and incisor teeth, and large jaw muscles. They last existed during the later part of the Early Pleistocene, and if they were convergent in their anatomical configuration, their common ancestors may have lived in the Middle Pliocene. In this case their divergence may represent some 4 million years of evolutionary time.

Of course, that’s assuming that they are not truly sister taxa. Some evidence points in the direction of convergence, and with such clear demonstrations of convergence in other large mammal omnivore taxa, I don’t think we should rule it out.

The discussion of Mitchell and colleagues’ paper suggests an evolutionary scenario in which Arctotherium reached South America at around the same time as other members of the Pleistocene carnivore guild, including the sabretooths Smilodon and Homotherium and the modern puma and jaguar. The large-bodied Arctotherium first occurs with these predators, with no small-bodied precursor known from the fossil record of South America. They suggest that Arctodus in North America may have evolved large body size with a similar turnover of the carnivore community, in this case after the extinction of large scavengers like Agriotherium and Borophagus and the first introduction of bison into North America.

Obviously these are hypotheses to test further with better fossil samples, but I find them provocative because they do not suggest convergence after vicariance and slow adaptation to changing ecological conditions, the scenario usually described for hominin diversity in the Late Pliocene of Africa. The scenario sketched here for the bears is dynamic. A smaller-bodied Arctodus shifted to larger body size and greater scavenging when the herbivore and carnivore community of North America turned over in the Early Pleistocene. Meanwhile, Arctotherium rapidly entered an empty large scavenger niche as it dispersed along with the large cats into South America. The appearance of Arctotherium is then a parapatric event, and the establishment of the two large-bodied bear lineages in North and South America is due to the chance founding and rapid selection within one species with huge dispersal potential. This is not the isolation of two lineages in relative cul de sacs; it is the effect of rapid adaptation and a relatively small contact zone between the two genera. The weakness of any geographic and ecological barrier is further reinforced by the later dispersal of the extant South American spectacled bear, the relatives of which remained in North America from the Late Pliocene through most of the Pleistocene.

Reference

Mitchell KJ, Bray SC, Bover P, Soibelzon L, Schubert BW, Prevosti F, Prieto A, Martin F, Austin JJ, Cooper A. 2016. Ancient mitochondrial DNA reveals convergent evolution of giant short-faced bears (Tremarctinae) in North and South America. Biology Letters doi:10.1098/rsbl.2016.0062