Thylamys

Abstract: The holotype and only known specimen of Marmosa formosa Shamel, a nominal species currently synonymized with Gracilinanus agilis Burmeister, is strikingly unlike any other known didelphid marsupial. Phylogenetic analyses based on nonmolecular characters and IRBP sequences suggest that formosa is either the sister-taxon of Thylamys (including Lestodelphys) or Monodelphis. Because neither alternative is strongly supported by the data at hand, and because including formosa in Thylamys or in Monodelphis would compromise the diagnosability of those taxa, a new genus— Chacodelphys—is proposed to contain it. Currently known only from northern Argentina, Chacodelphys formosamay be widely distributed in the Chaco and other adjacent Neotropical biomes.

Abstract: The recent increase in mammal inventories and the widespread use of pitfall traps as a major capture method in Brazil have increased the number of non-volant small mammal specimens in scientific collections, providing new information on natural history, geographical range, and taxonomic status of many marsupials and rodents. To date, however, little is known about the Brazilian species of the marsupial genus Thylamys: the number of species in the country is disputed, as are their names and geographic distribution. Thus, the aim of this study was to define the Brazilian species of the genus, delimiting their taxonomic status and distributional range. We provide qualitative and quantitative data on external and skull morphology for specimens housed in scientific collections. A study of the material available allowed us to recognize the occurrence of three species in Brazil: Thylamys karimii (Petter, 1968), T. macrurus (Olfers, 1818) and T. velutinus (Wagner, 1842). For each of these we provide redescriptions, a discussion of local and geographic variations, geographic distribution, taxonomy, age class differentiation, sexual dimorphism, and natural history.