Monosaulax skinneri 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 : 171-174

Monosaulax skinneri Stout, 1999

Figs. 2, 3 , 4A–D .

1935 Monosaulax pansus (Cope, 1874) ; Stirton 1935 (in part): 416.

1990 Monosaulax pansus (Cope, 1874) ; Voorhies 1990a (in part): tables 1, 2.

1990 “ Monosaulax ” sp.; Voorhies 1990b: A94–A95.

1999 Monosaulax skinneri sp. nov.; Stout 1999 in Evander 1999: 246, fig. 7.

Holotype: UNSM 67241, left mandible with p4–m3.

Type locality: Railway Quarry ( UNSM locality Cr 12), Cherry County,

Nebraska, USA.

Type horizon: Late Barstovian (middle Miocene) Crookston Bridge

Member, Valentine Formation.

Material.— UNSM 49027 and 49028, nearly complete crania; UNSM 49030 and 49031, mandibles with cheek teeth; FAM 64292, 64317, 64315, 64956, partial mandibles with incisor and cheek teeth; FAM 64322, isolated (unworn) p4.

Emended diagnosis.—Largest species of the genus; cheek teeth lower−crowned than other species; p4 three−rooted (two−rooted in other species); parafossettid curved and accessory fossettids common on p4.

Description.—The two crania of Monosaulax skinneri are the most complete ever described for the genus. UNSM 49027 is an undistorted skull lacking only the zygomatic arches and the basicranium, with complete dentition. UNSM 49028 has a complete dentition and lacks only the zygomatic arches. However, the latter cranium is laterally crushed, with only the palate between the tooth rows not distorted, but the basicranial area is intact, allowing for a description of the complete cranium. UNSM 49027 is slightly larger than UNSM 49028 ( Table 1).

In dorsal view ( Fig. 2A), the rostrum is parallel sided and longer relative to the length of the entire skull compared to that of Castor , but less so than in contemporaneous Eucastor ( Korth 2007a) . The nasals extend posteriorly to a point even with the center of the anterior root of the zygomatic arches, just slightly posterior to the end of the premaxillaries. The nasals widen anteriorly, making up the entire width of the rostrum at its anterior end. The lacrimal bones are missing on both specimens, but it is evident that the anterior extent of the jugal bone does not contact it on the zygomatic arch, a character diagnostic of castoroidines ( Xu 1995; Korth 2001). The frontal bones are dorsally flat and extend together onto the neurocranium in a posteriorly pointing V−shape. The postorbital constriction occurs at the posterior end of the orbits and is nearly as great as in Eucastor ( Table 1; Korth 2007a: tables 1, 3). The surface of the parietals is rugose, typical of castorids. The temporal crests are very low and originate dorsal to the orbits, converging gradually posteriorly, meeting at a sagittal crest that forms only on the interperital, just anterior to the nuchal crest. There are two temporal foramina along the squamosal−parietal suture; one just anterior to the nuchal crest, the other more anterior, dorsal to the posterior end of the zygomatic arch. This same feature of two temporal foramina is also present in Dipoides ( Wagner 1983) and on the referred skull of Monosaulax pansus from New Mexico ( Korth 2002b). The interparietal is triangular in outline, abutting the nuchal crest. The neurocranium is widest at the posterior base of the zygomatic arches.

In lateral view ( Fig. 2B), the skull is very similar in shape to that of Dipoides and Prodipoides ( Wagner 1983: figs. 2, 3; Korth 2007a: fig. 6). The upper diastema slopes slightly upward towards the incisors. The upper incisors are directed ventrally, showing no signs of procumbency. The skull is sciuromorphous with the infraorbital foramen opening anteriorly, just anterior to the zygomatic plate at mid−depth of the rostrum. The maxillary−premaxillary suture descends the side of the rostrum in a nearly viertical line anterior to the infraorbital foramen.

The orbital wall is well preserved and undistorted on UNSM 49027 ( Fig. 2B). The nasolacrimal foramen is large and in the anterodorsal corner of the orbital wall. A sphenopalatine foramen is high on the orbital wall, about at mid−depth of the skull, dorsal to M1. It is entirely within the maxilla, just below the maxillary−frontal suture. Directly posterior to the sphenopalatine foramen, dorsal to the posterior margin of M2, is the optic foramen. It is a small oval within the orbitosphenoid, opening anterodorsally. The ethmoid foramen is entirely within the frontal bone, positioned dorsal to the optic and sphenopalatine foramina, about midway between them. There are three identifiable interorbital foramina. The smallest is just anterior and slightly ventral to the optic foramen, a second is posterior and ventral to the optic, and a third, much larger, oval foramen is posterior and ventral to the second interorbital foramen. All are within the orbitosphenoid bone. The palatine contribution to the floor of the orbital wall, and the dorsal palatine foramen that it contains, is blocked laterally by the alisphenoid and cannot be seen on Fig. 2B. In the posteroventral corner of the alisphenoid are two small foramsphenopalatine vacuity posterior palatine foramen ina. The anterior of the two is interpreted as the masticatory and the posterior as the buccinator foramen. A large postglenoid foramen is present just posterior to the glenoid fossa.

temporal foramen 10 mm optic foramen ethmoid foramen nasolacrimal foramen postglenoid foramen infraorbital foramen posterior maxillary foramen 10 mm foramen ovale incisive foramen foramen magnum

Ventrally, the premaxillary−maxillary suture crosses the center of the rostrum near the center of the incisive foramina, which are almost exactly in the center of the upper diastema ( Fig. 2C). The length of the incisive foramen is 24% that of the total length of the upper diastema ( Table 1) which is in the low end of the range for castoroidines ( Korth 2007a: table 3). On the uncrushed skull, UNSM 49027, there is a fairly large oval opening just anterior to the tooth row on both sides of the skull. These are not foramina, but mark the base of the upper incisor where the bone is very thin and is often broken away. In life, these openings would not have been present.

carotid canal jugular foramen hypoglossal foramen foramen magnum 10 mm

The tooth rows diverge strongly posteriorly and P4 is the largest of the cheek teeth. There are two distinct grooves on the palate, one on either side of the midline, running from the posterior margin of the incisive foramen for the entire length of the palate, widening at the posterior margin of the palate where the tooth rows are the farthest apart. The maxillary−palatine suture starts in the center of the palate at a point even with the lingual roots of M2 and runs just medial to the alveolar margin of the tooth rows. A posterior palatine foramen is at the anterior margin of this suture within the grooves of the palate medial to M2. Posterior to M3, there is a small posterior maxillary foramen within the maxillary−palatine suture. The pterygoid flanges are broken away on both available specimens. The large posterior opening for the sphenopalatine vacuity opens posteriorly at the back end of the palate, even with M3. The foramen ovale is just posterior to the tooth row and anterior to the bulla.

The basicranial area is only preserved on the crushed skull, UNSM 49028, and is slightly distorted ( Fig. 3A ). The bulla is inflated as in Dipoides and other early castoroidines. The external acoustic meatus is a short tube that runs posterodorsally. Just posterior to the opening for the external acoustic meatus is a small foramen for the posttemporal canal, along the nuchal crest, within the mastoid−squamosal suture. Two foramina are recognizable along the medial border of the bulla: the jugular foramen at the posteromedial corner of the bulla and the opening for the carotid canal near the anteromedial corner of the bulla. In Fig. 3A , the carotid canal appears more posterior because the right side of the skull was pushed backward when the cranium was crushed. The hypoglossal foramen is on the ventral part of the occipital, just anterior to the occipital condyle.

On the occiput, the identifiable foramina are the foramen magnum, the posttemporal canal and a small mastoid foramen ( Fig. 3B ). The mastoid foramen is along the occipital−mastoid suture above the level of the top of the foramen magnum and lateral to it. The mastoid contribution to the occiput is minimal, extending less than one−fourth the width of the occiput from the side. The occiput is nearly vertical in orientation and there is no indication of any well−defined ridges.

In Evander’s (1999) original description of this species, he described the lower molars but did not describe the upper dentition of Monosaulax skinneri because he had no upper teeth in the hypodigm. The two crania of M. skinneri have complete dentitions, allowing for a complete description of the upper teeth.

The cheek teeth are mesodont and rooted. The cheek teeth of both specimens are well worn. P4 is the largest of the upexternal acoustic posttemporal canal meatus per cheek teeth. On the lingual side the hypostriia extends nearly to the base of the crown, so the hypoflexus remains open lingually. It is oriented anterobuccally, and extends about half the width of the occlusal surface ( Fig. 4D ). On the buccal half of the occlusal surface there are three fossettes formed. The anterior parafossette is transversely oriented. The lingual end of the parafossette abuts the buccal end of the hypoflexus on UNSM 49027. The mesofossette is the longest of the lingual fossettes, extending nearly the entire width of the tooth. It curves posteriorly, ending lingually posterior to the hypoflexus. The metafossette is the shortest of the fossettes, reaching less than half way across the occlusal surface of the tooth.

On both specimens, M1 is the most worn tooth. Due to the tapering of the tooth, it is shortened anteroposteriorly. Only the hypoflexus and mesofossette are preserved. The hypoflexus is oriented as in P4, and reaches about three−fourths the way across the surface of the tooth. The mesofossette is short, not reaching the center of the tooth, and curved slightly posteriorly. The second upper molar is nearly identical to the first but less worn, and the parafossette and metafossette are preserved as minute circles along the buccal edge of the occlusal surface, anterior and posterior to the mesofossette, respectively.

M3 is approximately the same width as the first two molars, but is much longer. In UNSM 49027, it is slightly expanded at the posterobuccal corner. The morphology of the hypoflexus and parafossette is similar to that of P4. The mesofossette is short and strongly curved posteriorly, forming a C−shape and isolating the posterobuccal corner of the tooth. The metafossette is in the posterobuccal corner of the occlusal surface. It is a small oval that is obliquely oriented (anterobuccal−posterolingual). On the left M3 of UNSM 49027 there is a minute, circular accessory fossette anterior to the buccal end of the mesofossette.

The lower cheek teeth are similar to those of Monosaulax described elsewhere ( Stirton 1935; Evander 1999). The occlusal surface of the molars is primitive ( Fig. 4 ), retaining the parafossettid and metafossettid, well after the formation of the mesofossettid. FAM 64322 is an isolated, unworn p4. The crown height index (ratio of total crown−height to anteroposterior length) for the tooth is 1.24, much lower than species of Prodipoides or Eucastor that range from 1.50 to 1.70 (see Korth 1999: table 2; Korth 2002a: 17). The lower premolars are similarly primitive with a very short or absent parastriid, and a mesostriid that extends approximately half the total height of the crown. The morphology of the cheek teeth of the specimens from the Devil’s Gulch Member is identical to that from the earlier horizons.

Discussion.—Previously, the only castoroidine reported from the Crookston Bridge Quarry was Eucastor tortus (Voohries 1990a) . However, the measurements from the sample of beavers from this quarry exceed those of E. tortus reported elsewhere, and are closer in size to Monosaulax skinneri (see Korth 2006: table 1, and references therein). The primitive occlusal morphology of the molars (retention of the para− and metafossettes [−ids]), lower crown−height of the cheek teeth, and the shallow to absent lingual striids on p4 distinguish this material from Eucastor , and are the condition seen in specimens referred to M. skinneri .

The occurrence of M. skinneri from the Devil’s Gulch Member of the Valentine Formation is the latest occurrence of the species as well as the genus. Specimens of Prodipoides dividerus are reported from this horizon as well (see below) and are of similar size. These two species can be easily separated on the basis of the molar morphology: M. skinneri has the primitive condition of retaining the para− and metafossettids after the formation of the mesofossettid, and the specimens of P. dividerus have attained the S−pattern of the occlusal surface with the elimination of the para− and metafossettids. The crown−height of the premolars is also different, being greater in P. dividerus (>1.60). The depth of the lingual striids on p4, especially the parastriid ( Fig. 4B, F ), is also greater on the specimens of P. dividerus than those of M. skinneri .

The cranial morphology of Monosaulax skinneri is generally primitive for castoroidine beavers (see Korth 2007a). However, there are two features of the skull of M. skinneri that are shared with Hemphillian Dipoides that are not present in Clarendonian species is Prodipoides : the double temporal foramen along the parietal−squamosal suture, and multiple interorbital foramina posterior to the optic foramen, both otherwise unique to Dipoides ( Wagner 1983; Korth 2007a). In all other cranial features, the skull of M. skinneri is similar to that of other species of Monosaulax and Prodipoides ( Korth 2007a) . It is not likely that Monosaulax is closer to Dipoides than Prodipoides based on these features alone. The more derived dentition in Prodipoides , and the position of some of the cranial foramina of P. dividerus strongly suggest that M. skinneri is not directly ancestral to Dipoides . The precursor of Dipoides came from within Prodipoides with a more derived dental morphology.

Stratigraphic and geographic range.— Previously reported specimens are from UNSM quarries of the same horizon as the holotype in Nebraska , as well as early–late Barstovian UNSM quarries in Cornell Dam Member, Valentine Formation , Nebraska ( Voorhies 1990a) . Material discussed above: UNSM 49027 , 49028 , 49030 , and 49031, are from Crookston Bridge Quarry, and FAM 64292 from the Railway Quarry, both late Barstovian Crookston Bridge Member , Valentine Formation , Cherry County, Nebraska . FAM 64956 and FAM 64322 from latest Barstovian Devil’s Gulch Horse Quarry and FAM 64315 and FAM 64317 from Fairfield Creek Falls Quarry, both quarries in the Devil’s Gulch Member , Valentine Formation , Cherry County, Nebraska .

Figures

Fig. 3. The beaver Monosaulax skinneri Stout, 1999, specimen UNSM 49028 from the middle Miocene Valentine Formation of Nebraska, USA. Posterior fragment of skull. A. Cranial basis in ventral view (anterior to the top). B. Occipital plate in posterior view.

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.