Systematic relationships of the Guianan brush-tailed rat (Isothrix sinnamariensis ) and its first occurrence in Guyana / Systématique du rat à queue en brosse de Guyane ( Isothrix sinnamariensis ) et première découverte en Guyane

Isothrix sinnamariensis was originally described and known only from French Guiana. We document the first occurence of this species from Guyana. A single sub-adult male was collected in lowland tropical rainforest near the headwaters of the Essequibo River. Although smaller in external and cranial size than the two adult females of the type description, our third known specimen represents the basal lineage to the type-series clade, with an average sequence difference of 1.72% based on cytochrome b mitochondrial DNA sequences. However, the degree of morphological and genetic divergence (2.67%) between I. sinnamariensis and its sister taxon I. pagurus is substantially less than the chromosomal differences originally used to distinguish them as two separate species. In contrast, distinctive karyotypes, morphological differences, and higher levels of mtDNA divergence (13.48%) between the other two species recognized, I. negrensis that was recently separated from I. bistriata , suggest an earlier and deeper diversification for these taxa.