Proischyromys, Samuels & Korth, 2017

Samuels, Joshua X. & Korth, William W., 2017, The first Eocene rodents from the Pacific Northwest, USA, Palaeontologia Electronica 53 (4), pp. 1-18 : 8-9

Subfamily ISCHYROMYINAE Alston, 1876

Included Genera. Ischyromys Leidy, 1856 (including Titanotheriomys Matthew, 1910 ); Spurimus Black, 1971 ; and Proischyromys n. gen.

Range. Late Uintan (middle Eocene; Ui3) to early Whitneyan (early Oligocene; Wh1) of western North America.

Definition. Small to intermediate sized ischyromyids with brachydont to mesodont and lophate cheek teeth; hypocones large on upper molars; lower molars with: 1) hypolophid complete, 2) mesoconid lacking, 3) metalophulid II complete enclosing trigonid basin posteriorly, 4) entoconid separated from posterior cingulid by deep valley.

Remarks. There has been a debate over several decades as to the validity of Titanotheriomys as a distinct genus or subgenus of Ischyromys ( Black, 1968; Wood, 1976, Heaton, 1996; also see discussion in Korth, 1994:49). This taxonomic question will not be addressed here, and any reference to Ischyromys will include all species previously referred to either the latter or Titanotheriomys for convenience.

Previously, Ischyromys (including Titanotheriomys ) has been the only genus referred to the subfamily Ischryomyinae ( Korth, 1994; McKenna and Bell, 1997; Anderson, 2008). Wood (1937, 1955, 1962, 1976, 1980) recognized it as the family Ischyromyidae rather than a subfamily, but also limited it to Ischyromys . This genus (and subfamily) was the latest occurring of the Ischyromyidae , originating in the Duchesnean and continuing through the Whitneyan ( Korth, 1994). Anderson (2008, figure 18.3) illustrated the range of Ischyromys extending back into the Washatchian, but this cannot be verified and seems to have been in error. Only a single occurrence prior to the Duchesnean is listed in Anderson (2008), Ischyromys cf. douglassi from the late Uintan (Ui3) Hough Draw Local Fauna in Montana that appeared as? Ischyromys sp. on a faunal list by Tabrum et al. (1996).

No earlier or contemporaneous genus has been identified as ancestral or closely related to Ischyromys in the past. However, it appears that Spurimus and a new genus described below occur earlier and are morphologically more primitive than Ischyromys and are here referred to the subfamily. Anderson (2008) suggested that Ischyromys was closely related to the microparamyines (see Anderson, 2008, figure 18.2). However, this is very unlikely due to the overall size, degree of lophodonty, and other characteristic features of Ischyromys . Ischyormys also lacks the diagnostic characters of a microparamyine ( Wood, 1962; Korth, 1994; Anderson, 2008).

Black (1971) first described the rodent Spurimus from the Badwater fauna of central Wyoming, recognizing two species S. selbyi and S. scottii (type species) from the Uintan and Duchesnean, respectively. Both species were represented by isolated cheek teeth only. He placed the genus in the subfamily Prosciurinae of the family Ischyromyidae . Additional specimens of an indeterminate species of Spurimus were reported later from the earliest Chadronian of Wyoming ( Sutton and Black, 1975). Shortly afterward, Rensberger (1975) placed the Prosciurinae (along with Spurimus ) in the family Aplodontiidae , an allocation that has been followed by most subsequent authors (e.g., Wood, 1980; Korth, 1994). McKenna and Bell included Spurimus in the family Allomyidae within the superfamily Aplodontoidea . Flynn and Jacobs (2008) included Spurimus in the Aplodontiidae along with several other early genera as “basal aplodontids.” In all these cases, Spurimus has been associated taxonomically with the aplodontiids.

However, in recent cladistic analyses of the Aplodontoidea , Spurimus has been purposely excluded from consideration ( Hopkins, 2008; Vianey-Liaud et al., 2013). Rensberger (1975) was the first to question the inclusion of Spurimus in the Prosciurinae based on a number of dental morphologies that differed from prosciurines (upper molars: protocone directly connected to protoconule, low anterior cingulum, large hypocone, metaloph connects metaconule and metacone at apices; lower molars: mesoconid minute or lacking, hypolophid high and distinct; also see Wood, 1980). The dental features cited by Rensberger (1975) are all characteristic of Ischyromys . Spurimus is clearly separable from Ischyromys by its much smaller size and brachydont cheek teeth that are markedly less lophate. Spurimus has lower molars (m1 and m2) that are longer than wide, while the m1 and m2 are typically equidimensional or wider than long in Ischyromys . However, the large hypocone and metaloph morphology of the upper molars and the well-developed hypolophid and lack of a mesoconid on the lower molars observed in Spurimus are more similar to the features of Ischyromys than prosciurine aplodontiids, which have a small to minute hypocone and a metaloph that connects the metacone and metaconule along their anterior edges on the upper molars, and a weak or incomplete hypolophid and distinct mesoconid on the lower molars. The new genus described below has these same shared characters of the lower molars seen in Spurimus and Ischyromys , and is therefore assignable to the Ischyromyinae , but clearly distinct from both of these latter genera.