Prodipoides dividerus ( Stirton, 1935 )

Korth, William W., 2008, Cranial morphology, systematics and succession of beavers from the middle Miocene Valentine Formation of Nebraska, USA, Acta Palaeontologica Polonica 53 (2), pp. 169-182 : 175

Prodipoides dividerus ( Stirton, 1935)

Fig. 4E–H .

1935 Eucastor dividerus sp. nov.; Stirton 1935: 431, figs. 101–105. 2007 Prodipoides dividerus ( Stirton, 1935) ; Korth 2007a: 125, figs.

4B, C, 5, 6.

Holotype: UCMP 29691 , mandible with p4–m3.

Type locality: Fish Lake Valley beds, Esmeralda County, Nevada .

Type horizon: Clarendonian (Miocene) Esmeralda Formation.

Material.— FAM 64314, 64319, 64316, and 64318, mandibles with cheek teeth; and FAM 64324, partial cranium with incisors and right and left P4–M2.

Discussion.—There is no difference between the specimens cited here from the late Barstovian Devil’s Gulch Member and those from higher in the Nebraska section. A single specimen from the subjacent Crookston Bridge Member is present in the FAM collections, FAM 64314 from the Nenzel Quarry in Cherry County ( Fig. 4G ). This specimen is clearly referable to this species and not Monosaulax skinneri (the most common beaver from this fossil quarry in the Frick Collections). The lingual striids on p4, especially the parastriid, is much deeper on FAM 64314 than in any specimen of Monosaulax . The mesofossettids have not yet formed on the molars but the metafossettid is already eliminated on m1 and there are two minute fossettids anterior to the hypoflexid. These will likely disappear before the closure of the mesoflexid because the mesostriid extends another 1.22 mm below the current occlusal surface. Although Prodipoides burgensis is also present at this horizon (see below), FAM 64314 is clearly too large to belong to this species ( Table 2). This is the only specimen that can definitely be referred to P. dividerus from any horizon below the Devil’s Gulch Member.

This is the first recognition of P. dividerus from before the early Clarendonian ( Korth 2002a). As noted above, specimens of Monosaulax skinneri are also present in these lower horizons and are of similar size to specimens of P. dividerus . These two species are distinguishable based on the morphology of the lower premolar and molars (see above discussion). However, in very late stages of wear, when the para− and metafossettids are worn away on the molars in specimens of M. skinneri they cannot be separated from specimens of P. dividerus . For example, FAM 65344, a mandible with only an extremely worn p4 from the Devil’s Gulch Member, cannot be assigned here to P. dividerus or M. skinneri due to the lack of distinguishing characters.

Stratigraphic and geographic range.— Previously cited specimens from quarries in the early Clarendonian Burge Member of the Valentine Formation and late Clarendonian Merritt Dam Members of the Ash Hollow Formation of Nebraska ( Korth 2002a). Material described above: FAM 64318 is from Devil’s Gulch Horse Quarry; FAM 64316 is from Fairfield Creek Falls Quarry; FAM 64319 and 65344 are from Meisner Quarry (the latter three quarries are in Cherry County); and FAM 64324 is from Elliot Quarry, Brown County, all from the Devil’s Gulch Member, Valentine Formation, Nebraska. FAM 64314 is from late Barstovian Nenzel Quarry, Crookston Bridge Member, Valentine Formation, Cherry County, Nebraska.

Figures

Fig. 4. The beavers from the middle Miocene Valentine Formation of Nebraska, USA. A–C. Monosaulax skinneri Stout, 1999. D–F. Prodipoides dividerus (Stirton, 1935). Cheek teeth: A. FAM 64315, right p4–m2 from Devil’s Gulch Member in occlusal (A1) and medial (A2) views. B. FAM 64317, right p4–m2 from Devil’s Gulch Member. C. UNSM 49027, left P4–M3 from Crookston Bridge Member. D. FAM 64319, left p4–m3 from Devil’s Gulch Member; in occlusal (D1) and medial (D2) views. E. FAM 64316, right p4–m2 from Devil’s Gulch Member. F. FAM 64314, right p4–m2 (partial) from Crookston Bridge Member. Anterior to left for specimens of M. skinneri, anterior to right for specimens of P. dividerus.