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 : 9

Genus PROISCHYROMYS n. gen.

zoobank.org/ B2A8C0C3-6A38-4A8E-A916-D2063D7C0EED

Type Species. Proischyromys perditus n. gen., n. sp.

Range. Late Eocene (Chadronian) of Oregon.

Diagnosis. Cheek teeth brachydont; m1-m2 wider than long; trigonid basin on m1-m2 enclosed anteriorly by metalophulid I (=anterior cingulid) and posteriorly by metalophulid II; low, broad loph extending into center of the lower molars distobuccally from metaconid on m1-m2.

Etymology. Greek, pro-, prefix meaning before; intended to indicate the more primitive morphology of the genus relative to Ischyromys .

Remarks. Proischyromys n. gen. is referred to the Ischyromyinae because of the lophodonty and morphology of the lower molars. It shares with Ischyromys : 1) a complete hypolophid; 2) lack of a mesoconid; 3) short ectolophid situated well lingual to the buccal margin of the tooth; 4) entoconid separated from posterolophid by deep notch; and 5) metalophulid II complete, enclosing trigonid basin posteriorly on m1-m2. Proischyromys n. gen. differs from Ischyromys in: 1) smaller size (equal to only the smallest specimens of Ischyromys [see Table 2; Heaton, 1996]); 2) brachydont cheek teeth (mesodont in Ischyromys ); trigonid closed anteriorly by anterior cingulid (open at base of metaconid in Ischyromys ); and 3) low, broad lophid extending distobuccally from the metaconid on m1-m2. Proischyromys n. gen. differs from Reithroparamyinae in: 1) complete hypolophid on m1-2; 2) low, broad lophid extending distobuccally from the metaconid on m1-m2; and 3) buccal aspect of the posterolophid does not extend posterior to the entoconid.

Proischyromys n. gen. also shares the complete hypolophid, lack of a mesoconid, and separation of the entoconid from the posterolophid with Spurimus . The molars of Proischyromys n. gen. differ from those of Spurimus in: 1) being larger with higher and better developed lophs; 2) having a complete metalophulid II on m1-m2; and 3) m1-m2 wider than long (longer than wide in Spurimus ). Proischyromys n. gen. also lacks the distinct notch and cuspule anterior to the protoconid present on the lower molars of S. scottii (not present on S. selbyi ).