Abstract:

Two new species and genera of extinct muroid rodents are described from fossil material dating to the early Pleistocene of Argentina (Sanandresian Substage/Subage). They are both tetralophodont forms belonging to the tribe Phyllotini (Sigmodontinae) and are assigned to Ichthyurodon ameghinoi, gen. et sp. nov., and Olympicomys vossi, gen. et sp. nov. Phylogenetic analyses involving extant phyllotines, the two new species, and two other undescribed extinct phyllotines result in a refined and expanded concept of the clade consisting of Loxodontomys and the Reithrodon generic group. The generic diversity of this clade is thus doubled from four to eight. Several of the fossil genera appear to represent transitional forms, although Ichthyurodon, gen. nov., possesses dental traits unique among the members of the subfamily Sigmodontinae. The significance of the new species and their faunal context for our understanding of the paleoecology of the southern cone of South America is discussed.

	Figure 2. Holotype of Ichthyurodon ameghinoi MLP 91-VI-25-166, right maxilla with M1-3. Scale bar = 1 mm.		Figure 3. Holotype of Olympicomys vossi MLP 91-VI-25-179, right mandible with m1-3. Scale bar = 1 mm.

Figure 4. Lower toothrow of A) Loxodontomys. Upper toothrows of B) Euneomys, C) Neotomys, and D) Reithrodon.

Figure 5. Phylogram of one of three most parsimonious trees from the phylogenetic analysis. The other two trees differ only in the relationships of Ichthyurodon and genus T (Ortiz et al, in press) relative to node "B." Branch lengths are proportional to the number of unambiguous character state changes. Node "A" defines the Reithrodon group sensu lato, node "B" defines the Reithrodon group sensu stricto. Numbers to the left of nodes are the bootstrap percentages: numbers above the lines found in 200 replicates with the complete data set (although topology of majority-rule bootstrap consensus tree may not be congruent); numbers below the lines are from bootstraps excluding Olympicomys.