Napoleon Bonaparte and the fate of an Amazonian rat: new data on the taxonomy of Mesomys hispidus (Rodentia: Echimyidae).

The spiny rat Mesomys hispidus is one of many South American rodents that lack adequate taxonomic definition. The few sampled populations of this broadly distributed trans-Amazonian arboreal rat have come from widely separated regions and are typically highly divergent. The holotype was described in 1817 by A.-G. Desmarest, after Napoleon's army brought it to Paris following the plunder of Lisbon in 1808; however, the locality of origin has remained unknown. Here we examine the taxonomic status of this species by direct comparison of 50 extant individuals with the holotype at the morphometric and genetic levels, the latter based on 331 bp of the mitochondrial cytochrome b gene retrieved from a small skin fragment of the holotype with ancient DNA technology. Extensive sequence divergence is present among samples of M. hispidus collected from throughout its range, from French Guiana across Amazonia to Bolivia and Peru, with at least seven mitochondrial clades recognized (average divergence of 7.7% Kimura 2-parameter distance). Sequence from the holotype is, however, only weakly divergent from those of recent samples from French Guiana. Moreover, the holotype clusters with greater that 99% posterior probability with samples from this part of Amazonia in a discriminant analysis based on 22 cranial and dental measurements. Thus, we suggest that the holotype was originally obtained in eastern Amazonia north of the Amazon River, most likely in the Brazilian state of Amapá. Despite the high level of sequence diversity and marked morphological differences in size across the range of M. hispidus, we continue to regard this assemblage as a single species until additional samples and analyses suggest otherwise.

Morphological convergence in Hippidion and Equus (Amerhippus) South American equids elucidated by ancient DNA analysis.

Unusual equids named hippidions inhabited South America for more than 2 MY (million years). Like many other animals they succumbed to the worldwide climatic change that occurred 10 KY (thousand years) ago and completely disappeared during the great late Pleistocene megafaunal extinction. According to fossil records and numerous dental, cranial, and postcranial characters, Hippidion and Equus lineages are known to have diverged prior to 10 MY. Some equid bones from Rio Verde and Ultima Esperanza (Patagonia, Chile) dating back to the late Pleistocene period (8-13 KY) have been identified as Hippidion saldiasi, while a few teeth have been assigned to Equus. Six samples of those remains have been obtained from the Zoological Museum of Amsterdam for ancient DNA analysis to try to place Hippidion in the evolutive tree of Perissodactyla. Two samples of Hippidion and one sample of Equus yielded 241-394 bp of the mtDNA control region and 172-296 bp of the cytochrome b gene. Unexpectedly, all the sequences clustered deep inside the Equus genus, casting doubt on the initial identification of the bones. For paleontologists, one of the striking and classical diagnostic characters of Hippidion is their extremely short and massive metapodials, a probable locomotory adaptation to the Andine steep slopes. However, our DNA analysis reveals that a very Hippidion-like metapod might also have been possessed by another South American equid, i.e., Equus (Amerhippus), an interpretation supported by complementary anatomical observations. This adaptive convergence between members of the two South American equid genera may lead paleontologists to limb bone misidentification.