Hystricops venustus Leidy, 1858

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 : 254-257

Hystricops venustus Leidy, 1858

Figs. 6–8 ; Tables 1, 2.

1858 Hystrix (Hystricops) venustus ; Leidy 1858: 22.

1902 Erethizon venustus (Leidy) ; Hay 1902: 733.

1935 Hystricops venustus Leidy ; Stirton 1935: 414, figs. 60, 61.

1963? Hystricops sp. ; Shotwell and Russell 1963: 47, figs. 50, 51, table 7.

Holotype: USNM 1180 , right m1 or m2 and left p4.

Type locality: Uncertain locality in “Niobrara River, Loup Fork beds, horizon F of Hayden and Leidy’s survey” ( Stirton 1935: 414).

Type horizon: Late Barstovian and late Clarendonian (middle to early– late Miocene), B2–Cl3 of Janis et al. (2008).

Material.—F: AM 64048–64050 from Pratt Quarry, the Merritt Dam Member, Ash Hollow Formation (late Clarendonian), Brown County, Nebraska, USA. F: AM 65026, left maxillary fragment with P4–M1; from Gallop Quarry, the Merritt Dam Member, Ash Hollow Formation (late Clarendonian), Cherry County, Nebraska, USA ( Skinner and Johnson 1984; Korth 1997, 1998b). F: AM 64041, left maxilla with P4–M3 from East Kat Quarry, Xmas-Kat Channels (late Clarendonian), Cherry County, Nebraska, USA ( Skinner and Johnson 1984); three dentaries with p4–m 2 in the F: AM collections (Korth 1997:101). UNSM 75995, left dentary with p4–m1 from Cedar Creek Quarry, Valentine Formation (late Barstovian), Webster County, Nebraska, USA ( UNSM locality Wt-102; horizon equivalent to that of Myers Farm: Holman 1977). Ten isolated cheek teeth ( UNSM 101660, p4; UNSM 101661, 101662, 101775, m1 or m2; UNSM 101663, 101664, 101667, m3; UNSM 101655, 101666, P4; UNSM 101668, 101663, M1 or M2) and UNSM 101747, an incisor fragment ( Korth 1998b: 313), all from Pratt Quarry, the Merritt Dam Member, Ash Hollow Formation (late Clarendonian), Brown

A

B

County, Nebraska, USA. UOMNH F-2038 and UOMNH F-5802, left P4s, from Black Butte local fauna, Juntura Formation (Clarendonian), Malheur County, Oregon ( Bowen et al. 1963; Shotwell and Russell 1963).

Emended diagnosis.—Slightly smaller than H. browni ; striae not as deep on cheek teeth as in H. browni (fossettes [-ids] form at earlier stages of wear).

Description.—The upper dentition of H. venustus has been known previously from isolated cheek teeth ( Korth 1998b: fig. 6F–H). The two specimens of partial maxillae: F:AM 65026 and F:AM 64041, retain P4–M1 and P4–M3, respectively ( Fig. 6 ). Based on the stage of wear, F:AM 64041 is a much younger individual than F:AM 65024 because in the former, the mesoflexus, metaflexus, and paraflexus remain open on P4 and M3. On M1 and M2 of this specimen ( Fig. 6A ) the only buccal flexus that remains open is the mesoflexus. On M1 and M2, the parafossette and metafossette have been formed. All of the upper cheek teeth of F:AM 64041 have very a long mesoflexus that nearly reaches the lingual edge of the tooth and curves slightly posteriorly at its lingual end. The remaining parafossette and metafossette are transversely elongated and less than half the width of the tooth in length. M3 has a greatly reduced posterolingual corner of the tooth, resulting in a nearly circular occlusal outline of the tooth. In the heavily worn specimen, F:AM 65026, the P4 appears more elongated than in the little-worn specimen ( Fig. 6B ). The only flexus that remains open is the hypoflexus. All others have been closed buccally and formed fossettes, the mesofossette being the longest. The dimensions of the tooth are greater than those of the little-worn specimen. On the M1 of F:AM 65026, no flexi remain open. Only two fossettes remain; the mesofossette and the hypofossette. The tooth is much shorter (anteroposteriorly) than the unworn M1. The difference in size and proportions of the cheek teeth of these two maxillary specimens is clearly due to the stage of wear, which demonstrates the change in proportions of the teeth as they wear (molars become shorter, P4 increases in all dimensions).

No cranial material has previously been described for Hystricops . F:AM 64041 preserves the base of the zygomatic arch and much of the palate ( Fig. 7A ). The zygomatic arch is sciuromorphous and originates lateral to the P4 and M1, typical of castorids. The palate has two gentle grooves that run anteroposteriorly on either side of the mid-line of the palate. The maxillary-palatine suture extends anterior to the level of the anterior edge of M1. The posterior palatine foramina are along the suture. Little else can be determined from the specimens available.

Discussion.—The cheek teeth of Hystricops venustus have been described and figured in detail elsewhere (Leidy 1856; Stirton 1935: figs. 60, 61; Korth 1997: fig. 3; Korth 1998b: fig.6). However, an additional specimen of a dentary, UNSM 75995 ( Fig. 8 ) preserves enough of the posterior end to show that there is a flattened ventral surface of the angle indicating the zig-zag pattern of mandibular processes characteristic of the Castoroidinae (see Korth 2001a: fig. 3B). The species is distinct from the earlier occurring Priusaulax in its much larger size ( Table 1; Korth 1997: table 3; Korth 1998b: table 6) and flattened anterior incisor. H. venustus differs from the later occurring H. browni (Hemphillian) in having buccal striae on the upper cheek teeth that do not extend as far toward the base of the crown as the latter, making the flexi less persistent ( Fig. 7A ; Shotwell 1963: figs. 104, 105).

Shotwell and Russell (1963) described two P4s from the Clarendonian of Oregon referred to “? Hystricops sp. ” Both specimens do not differ from the general occlusal morphology of P4s from Nebraska ( Shotwell and Russell 1963: figs. 50, 51; Korth 1998b: figs. 6F, G). In size, the Oregon specimens are slightly larger but not appreciably so ( Table 2; Shotwell and Russell 1963: table 7; Korth 1998b: table 6). The Oregon specimens are best referred to Hystricops sp. , cf. H. venustus . The remainder of the specimens referred to H. venustus are from the late Clarendonian (Cl2) of northcentral Nebraska with one exception, UNSM 75995 ( Fig. 8 ). This specimen is from the Valentine Formation of southeastern Nebraska, approximately 300 km south of the more northern occurrences. The fauna associated with this specimen is typical of that of the late Barstovian (Ba2) Valentine Formation in the northern part of the state (equivalent to Myers Farm Quarry of Voorhies 1990: table 2). This is the earliest occurrence of Hystricops . This specimen is well within the size range of the specimens of H. venustus from the later horizons, and there are no morphologic differences in the dentary, lower cheek teeth or incisor between UNSM 75995 and the type and referred specimens from the later horizons ( Fig. 8 ).

Stratigraphic and geographic range.— Clarendonian (late Miocene) of Nebraska and Oregon, USA .

Hystricops browni Shotwell, 1963

Table 1 (figured in Shotwell 1963: figs. 104, 105).

Holotype: UOMNH F-15696, associated left and right P4–M1.

Type locality: UOMNH locality 2239, Drewsey Formation , Harney County, Oregon, USA (Shotwell 1963) .

Type horizon: Hemphillian (late Miocene), Hh2 of Janis et al. (2008).

Description.—The type and only specimen of H. browni was descriped by Shotwell (1963: 73–75). No additional specimens have been found, so no additional description is necessary.

Diagnosis.—Largest species; striae on upper cheek teeth deeper than in other species, more nearly reaching the base of the crown.

Discussion.—Shotwell (1963) fully described the type specimen of H. browni , and no additional specimens have been subsequently reported. It is similar in size to specimens of H. venustus , but the striae on the teeth more nearly reach the base of the crown (Shotwell 1963: figs. 104, 105), making the flexi more persistent than in H. venustus . It is the latest occurring species of the genus.

Stratigraphic and geographic range.—Hemphillian (latest Miocene) of Oregon, USA.

Figures

Fig. 6. Occlusal views of upper cheek teeth of castoroidine beaver Hystricops venustus Leidy, 1858, from Clarendonian (late Miocene) Ash Hollow Formation, Nebraska, USA.A. F:AM 64041, left P4–M3. B. F:AM 65026, left P4–M1.

Fig. 7. Maxilla and dentary of castoroidine beaver Hystricops venustus Leidy, 1858, from Clarendonian (late Miocene) Ash Hollow Formation, Nebraska, USA. A. F:AM 64041, left maxilla with P4–M3 in occlusal (A1) and lateral (A2) views. B. F:AM 64049, left dentary with i1, p4–m3 in occlusal (B1) and lateral (B2) views.

Fig. 8. Dentary of castoroidine beaver Hystricops venustus Leidy, 1858, UNSM 75995 (cast) from Barstovian (middle Miocene) Valentine Formation, southeastern Nebraska, USA, in dorsal (A), lateral (B), medial (C), and posterior (D) views.