Eucastor tortus ( Leidy, 1858 )

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 : 177-178

Eucastor tortus ( Leidy, 1858)

Figs. 6B , 7A–C .

1858 Castor (Eucastor) tortus sp. nov.; Leidy 1858: 23.

1877 Eucastor tortus ( Leidy, 1858) ; Allen 1877: 449.

Holotype: USNM 11020 , partial palate and rostrum with incisors and right and left P4–M3.

Type locality: “ Niobrara River , Loup Fork beds. Exact locality unknown.” ( Stirton 1935), Nebraska .

Type horizon: Barstovian (middle Miocene), Valentine Formation.

Material.— FAM 64966, nearly complete left and right mandibles with all cheek teeth; FAM 65262, right partial mandible with incisor and p4–m2; FAM 65252, fragmentary cranium with both incisors, left P4–M2 and right M1; CM 18876, partial left mandible with p4–m2; CM 18877, fragmentary left mandible with m2–m3; CM 18878, fragmentary right mandible with p4–m1; and CM 19672, partial maxilla with right M1–M2.

Description.—The partial cranium, FAM 65257, is badly broken and partially covered with matrix. The observable features are the same as those of the previously described skulls from lower in the Valentine Formation. The ratio of the length of the rostrum to that of the upper cheek tooth row is 1.70, at the lower end of the range for this species ( Korth 2007a: table 3). The upper diastema is strongly dorsally arched, again as in other skulls of this species. The procumbency indices are: A= 100 ° and B= 110 °. The only other diagnostic feature that can be determined is the relative size of M3. Although the tooth is lacking on the specimen, the alveolus is present on the left side and is much smaller than that of M2. All of these features indicate that FAM 65257 is referable to Eucastor tortus .

The morphology of the cheek teeth does not differ from that described previously ( Stirton 1935; Voorhies 1990b, Evander 1999; Korth 2005 [as “ Monosaulax ” valentinensis ]). The mesostriid on all observable p4s reaches more than half the height of the tooth. All of the specimens are at a level of wear that the parastriid is eliminated, forming the parafossettid ( Fig. 7A–C ). This indicates that the parastriid was very short. The only difference in the morphology of the premolar is that the parafossettid is gently concave anteriorly. Usually in E. tortus this fossettid is straight.

Voorhies (1990b) noted that the S−pattern of the occlusal surface of the molars was not fully attained in specimens of E. tortus from the earlier Cornell Dam Member. Evander (1999) described the molars of E. tortus from the Crookston Bridge Member as having eliminated the para− and metafossettids on the lower molars after the formation of the mesofossettid. Similarly, Korth (2005: fig. 2) figured specimens of E. tortus (misidentified as Monosaulax valentinensis [see Korth 2006]) from the same horizon as Evander’s sample where only a minute metafossettid was present shortly after the formation of the mesofossettid. All of the specimens of E. tortus from the Devil’s Gulch Member have the para− and metafossettids absent by the time of the formation of the mesofossettid. This appears to be a progressive change through the stratigraphic section where the molars become more Dipoides −like in the development of the occlusal pattern of the molars.

The nearly complete mandibles of E. tortus, FAM 64966, preserve the entire posterior end of the mandible, lacking only the top of the bony knob that covers the base of the incisor. It is evident from the size of the knob and the deep pocket on the ascending ramus lateral to it, that it was very large ( Fig. 6B ). This is a characteristic of tooth−digging rodents where rapid growth of the incisor is necessary (Korth and Rybczynski 2003) and compliments the tooth−digging adaptations of the cranium ( Korth 2007a).

Discussion.— Evander (1999) described the rodent fauna from the Railway Quarries that were from the same age and geographical area as the Crookston Bridge Quarry. Evander identified three species of castoroidine beavers including a sample of E. tortus that had a very wide range of size of dental measurements, particularly p4–m3 ( Evander 1999: table 4; also repeated in Korth 2006: table 1); these measurements are much wider than the size range of this species that had been reported previously ( Voorhies 1990b: A97). One of the sets of measurements presented by Evander date had a coefficient of variation value exceeding 10, usually an indication of the presence of more than one species. It appears that the Railway Quarry sample of E. tortus included a specimen of Monosaulax skinneri, FAM 64292 ( Evander 1999: fig. 8A) and one specimen of Prodipoides burgensis (see above discussion) that expanded the range of size of the sample.

FAM 64292, originally assigned to E. tortus by Evander (1999), contains a p4 that is not completely erupted so the crown−height index can be determined. The crown−height index for this specimen is 1.20, far below the range for E. tortus (average = 1.50), and similar to the lower−crowned Monosaulax skinneri (see above and Korth 1999: table 2). The morphology of the parafossettid on FAM 65292 is also that of M. skinneri ; the parafossettid is curved with an accessory fossettid anterior to it. In E. tortus the parafossettid is usually straight and there are no accessory fossettids. Clearly, FAM 64292 is a specimen of M. skinneri .

All of the specimens referred here to Eucastor tortus resemble those from populations of this species described elsewhere ( Stirton 1935; Voorhies 1990b). This is the first recognition of this species in the Devil’s Gulch Member of the Valentine Formation. All specimens referred previously to this species were from the subjacent Cornell Dam and Crookston Bridge Members of the Valentine Formation.

Stratigraphic and geographic range.— Previously cited specimens are from localities in the Cornell Dam and Crookston Bridge Members of the Valentine Formation, Nebraska ( Voorhies 1990a). All referred specimens listed above are from Devil’s Gulch Member, Valentine Formation, Nebraska. FAM 64966 and FAM 65262 from Rattlesnake Gulch Quarry, Brown County; FAM 65257 from Devil’s Gulch Horse Quarry, Cherry County; and all CM specimens from Verdigre Quarry, Knox County (see Voorhies 1969, for locality data).

Figures

Fig. 6. The beavers from the middle Miocene Valentine Formation of Nebraska, USA. A. Prodipoides burgensis (Korth, 2002). B. Eucastor tortus (Leidy, 1858). Left mandibles in lateral view. A. FAM 64857, from Crookston Bridge Member. B. FAM 64966, from the Devil’s Gulch Member.

Fig. 7. The beavers. A, B. Eucastor tortus (Leidy, 1858) from th latest Barstovian Devil’s Gulch Member of the Valentine Formation. C, D. Temperocastor valentinensis (Stout, 1999) from the late Barstovian Crookston Bridge Member equivalent of the Valentine Formation. Cheek teeth:A. FAM 65262, right p4–m2 in occlusal (A1) and medial (A2) views. B. CM 18876, left p4–m2. C. UNSM 46392, right P4–M3. D. UNSM 45385, right p4–m3. A, B, anterior to left; C, D, anterior to right.