Priusaulax wilsoni, Korth, 2017

Korth, William W., 2017, A new tribe of castoroidine beavers from the late Arikareean to Hemphillian (Oligocene-Miocene) of western North America, Acta Palaeontologica Polonica 62 (2), pp. 249-258 : 251-254

Priusaulax wilsoni sp. nov.

Figs. 1–3 ; Table 1.

1960 Anchitheriomys ? sp.; Wilson 1960: 66, figs. 68–70.

1980 Hystricops senrudi ( Wood, 1945) ; Cassilliano 1980: 36, fig. 14 (in part).

1998 Hystricops sp. ; Korth 1998b: 313, fig. 6.

2006 Priusaulax sp. ; Korth and Bailey 2006: 240.

Holotype: KU 10173 , partial cranium with right side of rostrum, maxilla and orbital wall containing right P4–M3.

Type locality: Quarry A, Logan County, Colorado, USA ( Wilson 1960).

Type horizon: Pawnee Creek Formation, Early Hemingfordian (early Miocene), He1of Janis et al. (2008).

Material.— KU 10175 , left m1 or m2 ; KU 10174 , right M1 or M2; from the type locality . UW 10010 , right m3 from Horse Creek Quarry (“…about 2.5 m above the contact with the Harrison Formation ”) Laramie County Wyoming, USA (Cassiliano 1980: 36) . UNSM 26599 , right dentary with dp4 (p 4 in crypt)–m3 ; UNSM 11984 , m3; from Runningwater Formation, Dawes and Box Butte counties, Nebraska, USA ( Korth and Bailey 2006: 240) .

Diagnosis.—Slightly larger than Priusaulax browni and smaller than P. senrudi ; no anterior re-entrant valley or accessory cuspule (anterostylid) on p4 as in P. browni .

Description.— Wilson (1960: 66) only briefly described the cranium of P. wilsoni (as Anchitheriomys ? sp.), KU 10173, but provided a table of measurements ( Wilson 1960: 67). The following description will focus on the details not noted by Wilson (1960). In dorsal view, the rostrum is long and parallel-sided ( Fig. 1A ). The nasals taper in width posteriorly, and end well posterior of the frontal-premaxillary suture in a point (approximately 9.2 mm). A small piece of the lacrimal bone is present at the anteromedial corner of the orbit. Although the jugal bone is missing, the jugal-maxillary suture is preserved on the dorsal surface of the zygomatic arch, and it is evident that the most-anterior extent of the jugal did contact the lacrimal bone. Well-developed temporal crests are present on the frontal bones. There is a slight swelling at the anterior end of the crests that converge posteriorly in a slight curve. However, the two crests do not meet at the posterior end of the frontals. The parietals are lacking on the cranium, so it cannot be determined whether the temporal crests are united more posteriorly.

In lateral view ( Fig. 1A ), the diastema is long (38.5 mm); approximately 1.6 times longer than the length of the tooth row. Its ventral surface is gently arched dorsally. The infraorbital foramen is just above the ventral surface of the diastema at its posterior end (below the anterodorsal end of the zygomatic arch). The foramen opens anteriorly and is laterally compressed. A flange of bone is present lateral to the foramen (= attachment of masseter lateralis). The orbital wall is badly broken so little can be determined. The nasolacrimal foramen is apparently large and in the anterodorsal corner of the orbital wall. The only other observable foramen is the sphenopalatine which is within the maxillary bone, dorsal to M1. All other sutures and foramina are unobservable.

Although the upper incisor is lacking, it is evident from the alveolus that the anterior enamel surface was slightly convex ( Fig. 1A ), and in cross-section, the tooth was slightly longer than wide (alveolus: length = 7.6 mm; width = 6.3 mm). The anterior and posterior ends of the incisive foramen are preserved so that it can be measured (8.1 mm). The ratio of incisive foramen length to the length of the diastema is 0.21. The premaxillary-maxillary suture crosses the incisive foramen at its posterior end. There is a distinct groove running anteroposteriorly along the maxilla medial to the tooth row that ends posteriorly at the level of M2, at the posterior palatine foramen. The palatine bone is not preserved, but the maxillary-palatine suture is evident and joins the posterior palatine foramen.

P4 is the largest of the cheek teeth which decrease in size toward M3. The tooth row diverges slightly posteriorly ( Fig. 1 , 2A ; Wilson 1960: fig. 70c). The width of palate can be estimated (at P4 approximately 8 mm; at M3 approximately 15 mm). The occlusal morphology of the cheek teeth have been well described by Wilson (1960). On P4, M2, and M3, there is a smaller accessory enamel lake posterior to the mesofossette or mesoflexus. These are smaller than the other fossettes and will likely be eliminated with additional wear, thus producing the basic three-fossette pattern. There is also an additional enamel fossette on the anterior half of M3 that appears to be a split of the parafossette.

The dentary referred to Priusaulax sp. by Korth and Bailey (2006), UNSM 26599, is slightly larger than those of P. browni , but very similar in morphology ( Fig. 3 ; also see Korth and Bailey 2006: 241–243 for complete description). This specimen is comparable in size to the type of P. wilsoni , and is here referred to the latter. The age of occurrence of UNSM 26599 is early Hemingfordian, also compatible with that of the type of P. wilsoni . The greatest difference in the morphology of the cheek teeth of UNSM 26599 with those of P. browni is that of p4. In P. browni , there is a distinct anterostylid (or anteroconid) and a narrow valley along the anterior margin of the tooth. In the unerupted p4 of UNSM 26599, there is no distinct anterostylid and the anterior lophid is continuous for its entire width ( Fig. 2B ).

Discussion.—Korth (1998A: 317) suggested that the cheek teeth of KU 10173, originally referred to Anchitheriomys ? sp. by Wilson (1960), were more similar to those of H. venustus than any other castorid (large size, mesodonty, hypoflexus with three fossettes, P4 larger than M1). The cheek teeth differ from contemporaneous and similarly sized castoroidines (e.g., Monosaulax ) in being lower crowned, even though the occlusal pattern is similar, and having the tooth rows not diverge quite as strongly (palatal width of M3 much greater in castoroidines; see Stirton 1935: fig. 92; Korth 2008: fig. 2C). Several other cranial features separate KU 10173 from castoroidines: jugal bone contacts the lacrimal in KU 10173 (no contact in castoroidines), and the infraorbital foramen is slightly lower on the rostrum in KU 10173, just above the surface of the diastema, whereas it is higher in castoroidines, approximately one-third to one-half the height of the rostrum.

However, all of these features of KU 10173 are similar to those of the reported cranium of Priusaulax browni ( Fig. 1B ) from the late Arikareean of western Nebraska ( Korth and Bailey 2006). P. wilsoni differs from P. browni in being slightly larger ( Table 1). The best comparison is in the length of the upper and lower tooth rows. For the type of P. wilsoni the length of P4–M2 is 18.1 mm and that of P. browni is 16.5–16.7 mm (mistakenly listed as P4–M 3 in Korth and Bailey 2006: table 1). Similarly, the length of m1–m 3 in the referred specimen of P. wilsoni is 17.8 mm, and that of P. browni ranges from 15.5–16.0 mm.

A single isolated m3 from Wyoming originally referred to H. senrudi by Casilliano (1980) is transfered to H. wilsoni based on the occlusal morphology and age of occurrence (see below discussion of H. senrudi ).

Stratigraphic and geographic range.—The Hemingfordian (early Miocene) of Colorado, Nebraska, and Wyoming ( USA) .

Priusaulax browni Korth and Bailey, 2006

Fig. 1B .

Material.— UNSM 119711 , dentary with p4–m3 (holotype) ; UNSM 119707 , nearly complete skull lacking teeth ; UNSM 119708 dentary with incisor and p4–m3 ; UNSM 119709 , dentary fragment with i1 and p4 ; UNSM 119710 fragment of dentary with m2–m3; late Arikareean (earliest Miocene) Stage Hill I local fauna, Scott’s Bluff County, Nebraska, USA ( Korth and Bailey 2006) .

Description.—No new material of this species has been recovered since its original description ( Korth and Bailey 2006). The described cranium, UNSM 119707, is partially distorted (dorsoventral compaction of rostrum and anterior displacement of basicranium; Korth and Bailey 2006: fig. 1), but is nearly identical to the partial cranium of P. wilsoni in all observable features ( Fig. 1B ).

Discussion.— P. browni differs from P. wilsoni in being slightly smaller and in the morphology of p4 (see above discussion of P. wilsoni ).

Stratigraphic and geographic range.—The late Arikareean (latest Oligocene) of western Nebraska, USA.

Priusaulax senrudi ( Wood, 1945)

Figs. 4 , 5 , Table 1.

1945 Monosaulax senrudi ; Wood 1945: 2, fig. 1.

1917 Hystricops senrudi (Wood) ; Stout and Stone 1971: 281.

1980 non Hystricops senrudi (Wood) ; Cassilliano 1980: 36.

1994 Amblycastor sp. nov.; Korth 1994: 133.

1994? Monosaulax senrudi Wood ; Korth 1994: 148.

1998? Oligotheriomys senrudi (Wood) ; Korth 1998a: 127.

2001 Anchitheriomys senrudi (Wood) ; Korth 2001a: 54.

2010 “ Anchitheriomys ” senrudi (Wood) ; Mörs and Hulbert 2010: 1901.

Holotype: AMNH 39415 , left dentary with incisor and p4–m3.

Type locality: “On top of Fighting Butte, at the northwest tip of Chalk Buttes, section 30, Township 1 South, Range 57 East” ( Wood 1945: 2), Carter County, Montana, USA.

Type horizon: Barstovian (medial Miocene), Ba1 of Janis et al. (2008).

Material.— Holotype only.

Diagnosis.—Slightly larger than P. wilsoni ; additional enamel fossette posterior to hypoflexid variably present on lower cheek teeth.

Description.—The holotype and only specimen of this species has been fully described and figured by Wood (1945). No additional description is necessary.

Discussion.— Monosaulax senrudi Wood, 1945 , has been included in a number of castorid genera (see above list of synonymies). Most recently, it has been referred to Anchitheriomys Roger, 1898 by Korth (2001a) and Flynn and Jacobs (2008). However, this allocation was based on the erroneous recognition of a ridged lower incisor. The incisor of the holotype was originally described as having “… fine, slightly divergent striations on the convex enamel face of the incisor...” ( Wood 1945: 2). However, no such striations are visible on the incisor ( Fig. 5B ) clearly not the deeper, very distinct parallel ridges and grooves of the known species of Anchitheriomys ( Fig. 5C ; also see Korth 2001a; Mörs and Hulbert 2010). The overall morphology of the type specimen is relatively simple, and similar to that of Priusaulax . It differs from the latter only in the presence of an additional enamel fossettid posterior to the mesofossettid on p4 and m1 ( Fig. 4B ), not known in any specimens of H. venustus , but similar to the accessory fossettes on the upper cheek teeth of P. wilsoni ( Fig. 2A ). The isolated lower cheek teeth referred to P. wilsoni do not have the accessory fossettid as in P. senrudi ( Wilson 1960: fig. 68). The angle of the dentary is not preserved on the type specimen, so it cannot be determined whether there is a lateral extension of the bone or a flattening of the ventral margin at the anterior margin of the angle.

The only specimen other than the holotype referred to H. senrudi was an isolated lower molar ( UW 10010) from the Hemingfordian of Wyoming that is essentially unworn and does not have an accessory fossettid ( Cassilliano 1980). In size, the UW specimen is closer to that of H. wilsoni ( Wilson 1960:67; Casilliano 1980: table 1). The age of the UW specimen is Hemingfordian, as is H. wilsoni , whereas the type of H. senrudi has been referred to the early Barstovian. This specimen is referred above to H. wilsoni .

Stratigraphic and geographic range.—Barstovian (middle Miocene) of Monanta, USA.

Genus Hystricops Leidy, 1858

Type species: Hystricops venustus Leidy, 1858 , “Niobrara River, Loup Fork beds of Hayden and Leidy’s survey. Exact locality unkown.”

( Stirton 1935). Believed to be Clarendonian (late Miocene), Ash Hollow Formation, Nebraska, USA.

Diagnosis.—Hystricopine; markedly larger with much more robust dentary than Priusaulax ; cheek teeth slightly higher-crowned; lower incisor with flattened anterior enamel surface, nearly as wide as high.

Discussion.— Hystricops is allied here with Priusaulax based on shared morphologies of the cheek teeth (large size; mesodont cheek teeth with simple occlusal morphology [one re-entrant and three transversely elongated fossettes], and dentary (the castoroid-like morphology of the mandibular processes [zig-zag pattern]) and derived castorid characters of the palate (posterior palatine foramen within the maxillary-palatine suture, grooved palate). Hystricops differs from Priusaulax in its much larger size and more robust dentary, higher-crowned cheek teeth, and having the lower incisor flattened anteriorly, and nearly as wide as high (narrower in Priusaulax ).

Stratigraphic and geographic range.—Late Barstovian (middle Miocene; Ba2) to Clarendonian (late Miocene; Cl2) of Nebraska and Clarendonian (late Miocene; Cl1) to Hemphillian (latest Miocene; Hh2) of Oregon.

Figures

Fig. 1. Crania of castoroidine beavers Priusaulax spp. A. Priusaulax wilsoni sp. nov., KU 10173 (holotype) from the early Hemingfordian (early Miocene) of Colorado, USA, in dorsal (A1), ventral (A2), and right lateral (A3) views. B. Priusaulax browni Korth and Bailey, 2006, UNSM 119707 from the late Arikareean (earliest Miocene) of Nebraska, USA, in right lateral view.

Fig.2.Cheek teeth of castoroidinebeaver Priusaulax wilsoni sp. nov.A. KU 10173 (holotype) from early Hemingfordian (early Miocene) Pawnee Creek Formation, Colorado, USA, right P4–M3 in occlusal view.B. UNSM 26599 from Hemingfordian (early Miocene) Upper Arikaree Group, Nebraska, USA, p4 (in crypt)–m3 (B1) and dp4 (B2) in occlusal views.

Fig. 3. Dentary of Priusaulax wilsoni sp. nov., UNSM 26599 from Hemingfordian (early Miocene) Upper Arikaree Group, Nebraska, USA, in right lateral (A) and posterior (B) views.

Fig. 4. Holotype of castoroidine beaver Priusaulax senrudi (Wood, 1945), AMNH 39415 (cast) from Barstovian (middle Miocene) Chalky Buttes, Montana, USA. Dentary in lateral view (A) and dorsal view with p4–m3 in occlusal view (B).

Fig. 5. Lower incisors of castoroidine beaver Priusaulax and Anchitheriomys in anterior views. A. Priusaulax wilsoni sp. nov., UNSM 119711 (holotype) from early Hemingfordian (early Miocene) Pawnee Creek Formation, Colorado, USA. B. Priusaulax senrudi (Wood, 1945), AMNH 39415 (holotype) from Barstovian (middle Miocene) Chalky Buttes, Montana, USA. C. Anchitheriomys stouti Korth, 2001a, AMNH 64017 (holotype) from Hemingfordian (early Miocene) Running Water Formation, Nebraska, USA.