RODENTIA

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 70-71

CROWN RODENTIA (77)

Node Calibrated. The common ancestor of the three major, extant rodent clades: Muroidea (mouse-related.), Sciuromorpha (squirrel-related.), and Ctenohystrica (guinea-pig related.), following Churakov et al. (2010).

Fossil Taxon and Specimen. Paramys atavus (type specimen YPM-PU 14200; Yale Peabody Museum Princeton Collection; Jepsen, 1937) from the Fort Union Formation of the Eagle Mine, Carbon County, Montana .

Phylogenetic Justification. Phylogenetic analyses place Paramys in Sciuromorpha, the squirrel-related clade, based on characters of the ear region ( Korth, 1984; McKenna and Bell, 1997; Marivaux et al., 2004). Paramys is then nested well within crown Rodentia .

Minimum Age. 56 Ma

Soft Maximum Age. 66 Ma

Age Justification. An index taxon for the Clarkforkian (Rose, 1981; Anemone and Dirks, 2009), Paramys is known from late Paleocene localities in North America and as such correlates with the Thanetian stage, the minimum bound of which is 56 Ma ± 0.0 Myr = 56 Ma ( Gradstein et al., 2012).

Taxa such as Heomys and Mimotona are Glires, but do not belong within crown Rodentia or Lagomorpha (Meng et al., 2003; Asher et al., 2005). Therefore, a reasonable soft maximum constraint on the base of crown Rodentia could be set by these stem rodents at the base of the Paleocene, at 66.04 Ma ± 0.4 Myr = 66 Ma.

Discussion. The relationships of major rodent groups to each other enjoy a moderate level of consensus, although several questions remain. Recent studies generally recognize three groups: mouse-related (muroids plus castoromorphs), squirrel-related (sciuroids plus glirids), and guinea pig-related (Ctenohystrica). Atkins et al. (2003) and Blanga-Kanfi et al. (2009) support a topology in which squirrel-like rodents are basal to a mouselike-ctenohystricid clade. Churakov et al. (2010) supported this arrangement but noted the likely influence of incomplete lineage sorting and very short branches near the root.