THE JASKHADELPHYOIDEA, A NEW CLADE OF TINY-SIZED PALEOGENE METATHERIANS (MAMMALIA, METATHERIA): DIVERSITY, SYSTEMATICS, PALEOECOLOGY AND PALEOBIOGEOGRAPHY

The metatherians represented the most diverse group of mammals of the Paleogene of South America. Among them, the Jaskhadelphyidae was represented by very small metatherians, which claim the status of smallest mammals that have ever existed. This group was represented by _Jaskhadelphys_, from the lower Paleocene of the El Molino Formation, Tiupampa, Bolivia; _Minusculodelphis_, from the lower Eocene of the Itaboraí Basin, Brazil; and _Kiruwamaq_ from the upper Eocene of the Yuahuarango Formation, Peru; all of these with a weight of around 10 g and a strict insectivorous diet. However, these three genera represent just a small fraction of a more diverse group. Previous studies have proposed the inclusion of _Szalinia_, _Tiulordia_, _Gaylordia_, _Incadelphys_, and _Marmosopsis_ as sister taxa of the Jaskhadelphyidae. Herein, we present the Jaskhadelphyoidea, a new Paleogene South American metatherian group. The phylogenetic analysis was performed with a new technology search, as it comprehended 129 taxa and 515 characters. The analysis recovered four most parsimonious trees (Best Score: 2069; CI: 0.327; RI: 0.663). The Jaskhadelphyoidea is supported by the presence of the following synapomorphies: single and large lacrimal foramen, upper molars mesiodistally compressed, mesiodistally compressed hypoconid; entoconid reduced on lower molars, being virtually fused to the hypoconulid on m4; hypoconulid distolingual to the entoconid, and talonid much shorter than the trigonid. The affinities with the Pucadelphyida are supported based on the presence of a hypoconulid as the largest talonid cuspid on m4, and angled coronoid process of the dentary. _Gaylordia_+ _Tiulordia_ share the presence of robust third lower premolars, mesiodistally compressed trigonids and robust dentary. _Incadelphys_+ _Marmosopsis_ share a moderately developed stylar shelf and eccentric protocone (i.e. aligned with StB and paracone on same axis). The Jaskhadelphyoidea was the most diverse metatherian group during the earliest Paleocene of Bolivia, with four described taxa. Nevertheless, their apex of diversity was recorded on the Itaboraí Basin, with five species; probably linked to the events of the Paleocene-Eocene Thermal Maximum. The extinction of the group may be related to the global cooling that happened through the Eocene, as _Kiruwamaq_ is the last representative currently known for this group.