Temperocastor valentinensis (Stout, 1999)

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 : 178-181

publication ID	https://doi.org/ 10.4202/app.2008.0201
persistent identifier	https://treatment.plazi.org/id/03DA87E5-FFC5-FF9A-FCE4-715DFF55F90D
treatment provided by	Felipe
scientific name	Temperocastor valentinensis (Stout, 1999)
status

Treatment

Figs. 7D–E View Fig , 8 View Fig .

1935 Monosaulax curtis [sic] (Matthew and Cook); Stirton 1935 (in part): 420.

1990 “ Monosaulax ” curtus (Matthew and Cook); Voorhies 1990a: tables 1, 2.

1990 Monosaulax ” cf. curtus (Matthew and Cook); Voorhies 1990b:

A93–A94. 1999 Eucastor valentinensis sp. nov.; Stout, 1999 in Evander 1999:

250–251, fig. 10. 1999 Monosaulax valentinensis (Stout) ; Korth 1999: 262. Holotype: UNSM 67201 About UNSM , right mandible with p4–m3. Type locality: Railway Quarry A, Cherry County, Nebraska. Type horizon: Late Barstovian (middle Miocene), Crookston Bridge Member , Valentine Formation.

Material.—UNSM 45392, partial skull with associated mandibles, and right scapula and radius; UNSM 45393, 49395, 49398, 49399, mandibles with cheek teeth. (Also see Evan− der [1999: 250] for list of topotypic specimens.)

Diagnosis.—Only species of the genus.

Description.—UNSM 45392 consists of nearly the entire right half of the skull and both associated mandibles ( Fig. 8 View Fig ). The occiput and basicanial area are almost entirely missing, and the nasal is lacking on the side that is preserved. The cranium is also laterally flattened. In overall outline, the skull strongly resembles that of Eucastor tortus ( Korth 2007a) . The dorsal diastema is long and strongly arched dorsally. The ratio of the length of the upper diastema to the alveolar length of the cheek teeth is 1.70, at the lower end of the range observed for Eucastor and Nothodipoides ( Korth 2007a: table 3). The upper incisors are procumbent with angles of greater than 100 °, greater than in Monosaulax and typical of nothodipoidines ( Korth 2007a: table 2). Dorsally, the premaxillary bone extends posteriorly even with the lacrimal, more posterior than the nasals. The lacrimal is very small and at the anteromedial corner of the orbit. The jugal extends anteriorly along the dorsal side of the zygomatic arch, but ends anteriorly about 2 mm short of contacting the lacrimal. The frontals are flat. The postorbital constriction is not as narrow as in Eucastor . The least width of the frontals relative to the width of the rostrum is 0.79, greater than in E. tortus , and similar to Prodipoides ( Korth 2007a: table 3). The temporal crest is extremely low. It arises dorsal to the orbits. On the parietals, the temporal crest runs parallel to the sagittal suture to the interparietal. Due to breakage, the posterior end of the temporal crest is not preserved. However, a triangular interparietal is present with a distinct sagittal crest that fuses posteriorly with a high nuchal crest. The surface of the parietals is rugose. There appears to be a small temporal foramen along the parietal−squamosal suture just anterior to the nuchal crest. A small, circular postglenoid foramen is just posterior to the glenoid fossa within the squamosal.

The skull is sciuromorphous. The infraorbital foramen is C−shaped and opens anteriorly. It is positioned just anterior to the zygomatic plate at about mid−depth of the rostrum. The premaxillary−maxillary suture runs almost directly ventrally just anterior to the zyogmatic plate and crosses the center of the palate at the posterior end of the incisive foramen. The incisive foramen is at the center of the diastema (slightly posterior to the center in Eucastor ) and its length is 25% that of the length of the upper diastema, typical of all castoroidines ( Korth 2007a: table 3). Due to breakage, very little can be determined about the palate. The tooth rows strongly diverge posteriorly, and there are recognizable anteroposterior grooves present, but all sutures and foramina are obscured by damage to the specimen. The only part of the acoustic bulla that remains is the outline of the external meatus which is a tube that extends posterodorsally as in most castoroidines. A small foramen for the posttemporal canal is present along the mastoid squamosal suture posterior to the external acoustic meatus as in Eucastor and other castorids. The only other identifiable foramen on the occiput is a mastoid foramen along the mastoid−occipital suture, medial to the posttemporal canal. The mastoid contribution to the occiput is very small.

The orbital wall is badly damaged, so the only observable foramen is the nasolacrimal foramen in the anterodorsal corner of the orbit. All other foramina and sutures are unrecognizable. Along the posterodorsal wall of the orbit is the short ridge that is also present in E. tortus and Nothodipoides ( Korth 2007a, b). The zygomatic arch is complete. It is relatively light in structure compared to Castor , being thickest at a point where the maxillary contribution and the jugal overlap, marking the posterior margin of the orbit.

The mandible is similar in shape to that of contemporaneous castoroidines ( Fig. 8C View Fig ). It is deep for the size of the animal. The diastema is shallow and long, being only slightly shorter than the length of the tooth row. A single mental foramen is present below the anterior margin of the premolar near the center of the depth of the mandible. On the right side of UNSM 45392, there are two additional, minute foramina dorsal to the mental foramen. On two other specimens, UNSM 45395 and UNSM 45398, there is a single additional foramen. These accessory mental foramina are not present on any of the other specimens. The diagastric process, along the ventral border of the mandible just below the mental foramen, is very small. The masseteric scar extends anteriorly to below the center of p 4 in a V−shape. The ascending ramus arises even with m1, blocking m2 and m3 completely from lateral view. This is identical to the condition in Eucastor tortus and more anterior than in Monosaulax where m2 is visible. Although the complete angle of the mandible is not present on any specimen, the coronoid process, condyle and angle of the jaw appear to form the zig−zag pattern typical of castoroidines where the condyle is medial to the other processes. The only difference is that the coronoid is not displaced as far laterally as in Eucastor and other castoroidines. On the lateral side of the ascending ramus is a very deep pocket just anterior to the lateral bulge of bone that is formed by the base of the incisor. This appears relatively deeper than in specimens of Monosaulax , suggesting a larger prominence of bone, indicating a more rapidly growing incisor, typical of tooth−digging rodents (Korth and Rybczynski 2003).

Evander (1999) provided a complete description of the lower cheek teeth but had no maxillary teeth in his collection. The upper dentition has not been previously described, although Voorhies (1990b) listed some maxillary specimens. The partial skull, USNM 45392, retains the upper incisor and all of the cheek teeth on the right side ( Fig. 7D View Fig ). The anterior enamel surface of the upper incisor is nearly completely flattened with only a very slight convexity along the edges. There are no grooves or ridges present on the enamel surface. The enamel extends onto only about 1/3 the medial and lateral sides of the tooth. In cross−section, the tooth is slightly wider than long.

Typically, the premolar is the largest of the cheek teeth that decrease in size posteriorly. The teeth are mesodont and rooted. The occlusal surface is at a stage of moderate wear; there are no open buccal flexi, a fossette has formed in all cases. The hypoflexus remains open lingually on all the cheek teeth. The occlusal surface of P4 consists of three fossettes and the hypoflexus. The hypostria extends to the base of the crown, thus the hypoflexus will remain open lingually until very late stages of wear. The hypoflexus is oriented anterobuccally and extends across about half the occlusal surface of the tooth. The parafossette abuts the buccal end of the hypoflexus and is oriented slightly obliquely, ending just short of the buccal border of the tooth. The mesofossette is the longest of the fossettes, extending nearly the entire width of the tooth. It curves posteriorly on its lingual half, posterior to the hypoflexus, but paralleling it. The metafossette is very short and obliquely oriented, anterobuccal−posterolingually. It is in the posterobuccal corner of the tooth.

M1 and M2 are similar in size and morphology. M1 is slightly more worn and also shorter because of its slightly later stage of wear. On both molars the hypoflexus is angled anterobuccally and crosses about three−quarters of the occlusal surface, ending near the anterobuccal corner of the tooth just short of the margin of the tooth. A minute parafossette is at the buccal end of the hypoflexus. On M1 the mesofossette is straight, short, and slanted posteriorly, blocking off the posterobuccal corner of the tooth. There is no evidence of a metafossette. On M2 the mesofossette is slightly longer and gently curved, similar to the morphology of the premolar. There is a minute, circular metafossette in the posterobuccal corner of the occlusal surface. Evander (1999) mentioned in his description of the lower molars that the parafossettid was shallower than the metafossettid and would disappear earlier in wear. It appears that the same is true for the metafossette of the upper molars; it is shallower so it disappears before the parafossette.

M3 is markedly smaller than M1 or M2 and nearly equidimesional. It is roughly triangular shaped in occlusal outline. The hypoflexus is similar to that of the premolar, angled anteriorly and extending only about half the width of the tooth with the parafossette abutting the buccal end. The parafossette is short and is slanted slightly posterobuccally, the opposite of the orientation of the parafossette on the premolar. The mesofossette is small and C−shaped. Because the tooth is much narrower posteriorly, the mesofossette nearly reaches both the lingual and buccal borders of the tooth. The metafossette is a minute, circular fossette in the posterobuccal corner of the occlusal surface as in M2.

Discussion.— Eucastor valentinensis was first described by Evander (1999) based on several mandibles with cheek teeth. The generic allocation was based on higher crown−height of the cheek teeth than in Monosaulax and the beginning of the development of the Dipoides −like S−pattern on the occlusal surface of the molars on some specimens (= shallower parafossettid). Shortly afterwards, Korth (1999) transferred the species to Monosaulax because the original diagnosis of the genus and its distinction from Eucastor was based on the development of the lingual striids on the lower premolar ( Stirton 1935). The specimens referred to E. valentinensis had the morphology of Monosaulax , and the molars had not fully attained the occlusal S−pattern ( Evander 1999: fig. 10).

It is evident from the morphology of the skull that Temperocastor valentinensis had the fossorial adaptations of the skull present in Eucastor and Nothodipoides ( Korth 2007a, b), but maintained the primitive morphology of the dentition as in Monosaulax . This suggests that Temperocastor is a basal nothodipoidine. If this is the case, the Nothodipoidini must be defined by the cranial morphologies for tooth−digging and the development of the more advanced dentition occurred secondarily within the group.

Stratigraphic and geographic range.— Previously referred specimens from several other UNSM localities at the same horizon as the holotype and UNSM quarries from the early late Barstovian of Nebraska (see Voorhies 1990a) . Specimens described above from the late Barstovian UNSM locality Wt−15A, base of the Ogallala Group , Webster County, Nebraska .