RODENTIA Bowdich, 1821

Samuels, Joshua X. & Korth, William W., 2017, The first Eocene rodents from the Pacific Northwest, USA, Palaeontologia Electronica 53 (4), pp. 1-18 : 5-6

Order RODENTIA Bowdich, 1821 Rodentia incertae sedis

( Figure 3 )

Referred specimens. JODA 16822, UOMNH F-28304, UOMNH F-28307, incisor fragments.

Occurrence. JDNM- 13 ( UC V75203, UO 2473), Hancock Mammal Quarry, Wheeler County, Oregon, Clarno Formation.

Age. Late middle Eocene (either latest Uintan or Duchesnean NALMA), bracketed between Stony Tuff dated 42.7 ± 0.3 Ma ( Bestland et al., 1999) and Member A Ignimbrite dated 39.22 ± 0.03 Ma ( Bestland et al., 1999).

Measurements. UOMNH F-28304: anteroposterior length = 1.73 mm, transverse width = 0.87 mm; blade (wear surface) length 2.67 mm; JODA 16822: transverse width = 0.91 mm, blade (wear surface) length = 0.69 mm.

Description. Both UOMNH F-28304 and F-28307 are partially imbedded in matrix, preventing observation of some details of their structure, while the posteromedial portion of the incisor is broken in JODA 16822. In UOMNH F-28304, the incisor is oval in cross-section and approximately twice as long anteroposteriorly as it is wide. In JODA 16822, the preserved portion does suggest a similar overall shape to UOMNH F-28304. The anterior and lateral surfaces of the incisors are rounded, while the anteromedial edge is somewhat flattened. The enamel in both UOMNH F-28304 and JODA 16822 is relatively thick and has a slightly rugose texture, though it lacks grooves or other ornamentation. In UOMNH F-28304, the anterior surface and about 1/3 of the lateral surface of the incisor is covered in enamel.

Remarks. These are the first described rodent specimens from the Clarno Formation. and the oldest rodents known from the Pacific Northwest. Though fragmentary, these specimens are also significant because they are by far the smallest vertebrates ever noted from the Hancock Mammal Quarry. The previous smallest vertebrate from the site was the equid Epihippus ( Hanson, 1996) . JODA 16822 was collected by screen-washing sediments, specifically the spoils-piles, from earlier excavations at the site. The presence of these very small rodents indicates that the apparent lack of rodents at the site is likely a result of collecting bias, rather than preservational bias.

The diminutive size of these incisors and their shape suggest that they could all belong to one species, likely an eomyid, dipodoid, or possibly Microparamys . Though no Eocene eomyids have been documented from the Northwest, there are five Oligocene (Arikareean) eomyid species known from the John Day Formation ( Korth and Samuels, 2015) and a number of eomyids known from the middle Eocene of California (Flynn, 2008). Metanoiamys agorus is known from the early Uintan (Ui1) age Friars Formation, and the middle Uintan (Ui2) age Murray Canyon and Mesa Drive Local Faunas ( Chiment and Korth, 1996; Flynn, 2008); while M. fantasma is known from the Uintan or Duchesnean age Hartman Ranch Local Fauna (Lindsay, 1968).? Simiacritomys whistleri and Paradjidaumo reynoldsi are known from the Duchesnean age Simi Valley Landfill Local Fauna ( Kelly, 1992). In addition to size, the rounded anterior face and relatively thick enamel of the Clarno incisors are similar to other eomyids ( Wood, 1974).

Several dipodoids are also known from the middle and late Eocene of California. Simimys simplex is known from several late Uintan (Ui3) and Duchesnean age sites in southern California ( Wilson, 1935; Lillegraven and Wilson, 1975; Flynn, 2008). A second species, S. landeri , is known from the Duchesnean age Simi Valley Landfill Local Fauna ( Kelly, 1992). Nonomys gutzleri is known from the Duchesnean or early Chadronian (Ch1) age Upper Pomerado Conglomerate ( Walsh, 2010). While the incisors or the Bridgerian dipodoid Elymys have rather thin enamel ( Emry, 2007), a lower incisor of the Chadronian N. simplicidens described by Emry and Dawson (1972) had thick enamel like that of the Clarno incisors, but it is substantially smaller. A single dipodid is known from the Oligocene of Oregon, Plesiosminthus fremdi from the late Arikareean (Ar3) age Kimberly Member of the John Day Formation ( Korth and Samuels, 2015).

The incisors from the Hancock Mammal Quarry are also similar in morphology to the smallest members of the Ischyromyidae , including some members of the subfamily Reithroparamyinae ( Wood, 1962, 1974; Anderson, 2008). The tiny reithroparamyine ischyromyids Microparamys, Lophiparamys , and Uriscus are known from sites in the western United States, including multiple Uintan age sites in California ( Anderson, 2008). Like the Clarno incisors, the incisors of some Microparamys, Lophiparamys , and Uriscus are 1) oval in cross-section, 2) have smooth, round anterior, and lateral enamel surfaces, and 3) a flattened medial surface ( Wood, 1962). However, the Clarno specimens are smaller than even the smallest specimens of Reithroparamyinae that have been described ( Wood, 1962). The Clarno incisors have thick enamel and no evidence of a groove, unlike Reithroparamys , which has thin enamel and sometimes grooved incisors ( Wood, 1962). Unlike the Clarno specimens, the incisors of Acritoparamys have enamel limited to their anterior surface and sometimes bear a shallow groove ( Wood, 1962; Korth, 1984). The Clarno incisors also differ substantially from those known of mid and late Eocene geomorph rodents (ex. Heliscomys, Floresomys, Meliakrouniomys ), which are strongly laterally compressed ( Korth et al., 1991; Flynn et al., 2008).

Figures

FIGURE 3. Rodentia incisor fragments from the Clarno Formation, Oregon. JODA 16822, 3.1. anterior view, 3.2. lateral view, 3.3. distal view. UOMNH F-28304, 3.4. anterior view, 3.5. lateral view, 3.6. distal view. Scale bar equals 1 mm.