Rodent systematics in an age of discovery: recent advances and prospects

With almost 2,600 species, Rodentia is the most diverse order of mammals. Here, we provide an overview of changes in our understanding of the systematics of living rodents, including species recognition and delimitation, phylogenetics, and classification, with emphasis on the last three decades. Roughly, this corresponds to the DNA sequencing era of rodent systematics, but the field is undergoing a transition into the genomic era. At least 248 species were newly described in the period 2000-2017, including novelties such as the first living member of Diatomyidae and a murid species without molars (Paucidentomys vermidax), thus highlighting the fact that our understanding of rodent diversity is going through an age of discovery. Mito-nuclear discordance (including that resulting from mitochondrial introgression) has been detected in some of the few taxonomic studies that have assessed variation of two or more unlinked loci. As studies incorporate more loci, incomplete lineage sorting and introgression are likely to gain recognition as widespread phenomena in the near future. Molecular phylogenetics has had a major impact in rodent phylogeny and allowed the identification of three major rodent clades, here recognized as suborders: 1) the Hystricomorpha (sometimes referred as the Ctenohystrica) and including two infraorders, Hystricognathi and Ctenodactylomorphi; 2) the Sciuromorpha; and 3) the Supramyomorpha, a new suborder that comprises the infraorders Castorimorphi, Anomalurimorphi, and Myomorphi. In spite of the greater understanding and ensuing stability of rodent phylogeny gained during the last three decades, several major areas of the rodent tree remain unresolved or poorly supported. We expect that the analysis of genomic-scale data will help resolve those areas of the radiation of Rodentia that still remain poorly understood.