Proischyromys perditus, 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 : 10-12

Proischyromys perditus n. gen., n. sp.

( Figure 5 ; Table 2)

zoobank.org/ 260754CE-F428-4741-AB2B-15409A4812EE

Type and Only Specimen. UCMP 170698 , right dentary with m1 to m3 .

Occurrence. UC V92002, Lost Creek, Crook County, Oregon, Big Basin Member, John Day Formation.

Age. Chadronian (late Eocene, Ch2 or later), above White Ash of Post dated 36.21 ± 0.26 Ma ( Manchester and McIntosh, 2007).

Diagnosis. Only species of the genus.

Etymology. Latin, perditus , adjective meaning lost; referring both to the name of the site where it was found and the long missing Eocene fauna to which this rodent belongs.

Description. The dentary is typical of ischyromyids in its primitive morphology. The masseteric scar extends anteriorly in a U-shape to a point below m2. The ascending ramus originates lateral to the anterior margin of m3, blocking only the posterior half of m3 from lateral view. The dorsal margin of the diastema is shallow. The incisor is much longer than wide in cross-section and rounded anteriorly.

Only the alveoli for p4 are preserved. The tooth is the smallest of the cheek teeth, two-rooted, and narrower anteriorly than posteriorly. The m1 is broken along the lingual edge of the tooth, so the metaconid and entoconid are lacking. The m1 trigonid is a small basin anterolingual to the protoconid. It is enclosed both anteriorly and posteriorly by the anterior cingulid and metalophulid II, respectively. A low, broad lophid runs distobuccally along the posterior margin of the metalophulid II and is separated from the ectolophid by a gap. The ectolophid extends posterolingually from the protoconid, and then turns posterobuccally at the junction with the hypolophid. The re-entrant valley between the protoconid and hypoconid is narrow, oriented posterolingually, and extends approximately 1/3 the width of the tooth. The hypoconid is round and slightly anteroposteriorly compressed. The posterior cingluid extends lingually from its posterolingual corner. A small but distinct hypoconulid is present on the posterior cingulid just lingual to its junction with the hypoconid. The hypolophid extends directly lingual from the ectolophid. The valley between the posterolophid and the hylolophid is narrow and deep. The valley that separates the hypolophid from the metalophid II is broader, and there is a broad sloping surface extending buccally from the base of the metaconid.

The m2 is similar to m 1 in morphology but is complete, with only a small chip of enamel missing off of the lingual slope of the metaconid. The metaconid is large and slightly anteriorly placed relative to the protoconid and is slightly obliquely compressed. There is a minute swelling along the posterolingual slope of the metaconid that may indicate the presence of a metastylid, but this cannot be verified due to breakage. The trigonid basin is a narrow valley between the anterior cingulid and the metalophulid II, both of which are complete from the metaconid to the protoconid. The hypoconid and ectolophid are as in m1, but the ectolophid is slightly more lingually placed, making the re-entrant valley slightly deeper than in m1. The hypoconid is round in occlusal shape, and as in m1 the posterior cingulid extends lingually from its posterolingual corner. The posterolophid is broad, and due to wear it is impossible to determine whether a distinct hypoconulid was present. The hypolophid extends to a prominent entoconid and is separated from the posterior cingulid by a deep, narrow valley. Anterior to the hypolophid is a shallower and broader valley. There is a low, broad lophid extending distobuccally from the metaconid along the posterior margin of the metalophulid II. The loph is separated from the ectolophid by a small gap and is slightly more distinct than in m1.

The m3 is the smallest of the molars. Unlike the other molars, the trigonid is not closed posteriorly, as the posterior arm of the protoconid extends less than half the distance to the metaconid. The anterior cingulid is complete. The protoconid and ectolophid are similar to those of the anterior molars. The entoconid on m3 is isolated from the hypolophid. The hypolophid originates at the ectolophid as in the anterior molars, but extends in a more posterolingual direction, joining the lingual end of the posterior cingulid, posterior and buccal to the entoconid. The posterior cingulid and the hypolophid enclose a narrow slit-like basin. On the posterolingual slope of the metaconid are two minute cuspules (metastylids). There is no indication of an accessory loph extending lingually from the metaconid as in m1 or m2.

The lower incisor is broken, but an oval crosssectional shape is clearly visible. The anterior and lateral surfaces of the incisor are round and the medial edge is flat. The enamel surface is smooth, and limited to the anterior face and about 1/3 of the lateral face of the incisor. The tooth is much deeper anteroposteriorly than it is transversely wide.

Discussion. The morphology and size of the lower molars of Proischyromys perditus n. gen., n. sp. are clearly intermediate between those of Spurimus and Ischyromys . In crown-height the molars of P. perditus n. gen., n. sp. are higher than those of Spurimus and lower than all species of Ischyromys . Similarly, the degree of lophodonty (height and completeness of lophs) in the molars of Proischyromys n. gen. is greater than in Spurimus and less than in Ischyromys .

Since the occurrence of Proischyromys n. gen. is from above an ash dated 36.21 Ma, it is late early Chadronian (Ch2) in age or younger. Presence of both Megacerops and Mesohippus at the Lost Creek site confirm that the site is late Eocene (Chadronian) in age, rather than early Oligocene. This means that P. perditus n. gen., n. sp. is likely later than any records of Spurimus , which is known from the late Uintan to earliest Chadronian ( Black, 1971; Flynn and Jacobs, 2008). The latest described record of Spurimus is from the Pilgrim Creek Local Fauna of Wyoming, which is earliest Chadronian in age ( Sutton and Black, 1975). Proischyromys n. gen. also predates most records of Ischyromys , which is known from the Duchesnean to the Whitneyan ( Korth, 1994; Flynn and Jacobs, 2008).

Records of Ischyromys that predate the appearance of Proischyromys perditus n. gen., n. sp. include several occurrences from the Duchesnean and earliest Chadronian. Duchesnean occurrences include: 1)? Ischyromys sp. from the Badwater fauna, Wyoming ( Black, 1971, figures 51-73), 2) Ischyromys sp. from the Lac Pelletier Lower Fauna in Saskatchewan ( Storer, 1988; Heaton, 1996), and 3) I. blacki from the Vieja Group in Texas ( Wood, 1974, figure 10; Flynn and Jacobs, 2008). Earliest Chadronian occurrences include Ischyromys junctus from the Calf Creek Local Fauna of Saskatchewan ( Russell, 1972) and an unnamed species from West Canyon Creek in Wyoming ( Heaton, 1996). Ischyromys sp. has been noted from the late Duchesnean or early Chadronian (Ch1) Upper Conglomerate Member of the Pomerado Conglomerate in California ( Walsh and Gutzler, 1999)

All of the earliest Ischyromys species have cheek teeth that are higher crowned than in Proischyromys perditus n. gen., n. sp. The m3 of P. perditus n. gen., n. sp. is shorter, relative to the m2, than in any other ischyromyid studied ( Table 2). In particular, both I. blacki and I. parvidens have a much longer m3 relative to m2 than in P. perditus n. gen., n. sp., as well as other species of Ischyromys ( Table 2; Wood, 1974; Heaton, 1996). The unnamed species of Ischyromys from Wyoming ( Heaton, 1996) also has accessory cuspules on the molars that are not present in P. perditus n. gen., n. sp. The only described species of Ischyromys as small as P.perditus n. gen., n. sp. is I. parvidens from the Orellan of South Dakota and Nebraska ( Table 2; Wood, 1937).

Figures

FIGURE 5. Proischyromys perditus n. gen., n. sp. right dentary with m1 – m3, UCMP 170698, from the Big Basin Member of the John Day Formation, Oregon. 5.1. occlusal view, 5.2. enlarged occlusal view, 5.3. lateral view, 5.4. anterior view. Scale bars equal 2 mm.