Castoroidinae Allen, 1877

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 : 250-251

Subfamily Castoroidinae Allen, 1877

Tribe Hystricopini nov.

Type genus: Hystricops Leidy, 1958 .

Included genera: Hystricops Leidy, 1958 ; Priusaulax Korth and Bailey, 2006 .

Diagnosis.—Derived features shared with castorines, castoroidines, and palaeocastorines: P3 lacking; P4/p4 larger than molars; upper tooth rows slightly diverge posteriorly; upper diastema arched; posterior palatine foramina intersected by maxillary-palatine suture; palatal surface distinctly grooved; length of incisive foramen less than 30% length of upper diastema; loss of stapedial foramen in auditory bulla. Derived characters shared with castoroidines: larger size (compared to contemporaneous species); angle of dentary laterally expanded with flattened ventral surface, mandibular condyle medial to angle and ascending ramus (= zig-zag pattern). Retained primitive features of castorids: cheek teeth mesodont in crown-height; occlusal morphology of cheek teeth with single, persistent re-entrant (lingual hypoflexus on upper; buccal hypoflexid on lowers) and three persistent, transversely oriented fossettes (-ids); jugal bone contacts lacrimal at its anterior end; enamel surface of incisors smooth, and unornamented; nasal bones extend more posteriorly than premaxillaries on dorsal surface of cranium; rostrum elongated; upper incisors slightly procumbent.

Discussion.—The Hystricopini share nearly all of the derived cranial characters of the Castoroidinae ( Korth 2001b: 289, fig. 4). The only major difference is in the contact of the lacrimal bone with the jugal. In more derived castoroidines the jugal ends anteriorly well before joining the lacrimal. This feature in hystricopines is clearly primitive among castorids. In the original description of Priusaulax , it was suggested that the presence of most of the castoroidine features and presence of the lacrimal-jugal contact suggested a basal position of the genus within the castoroidines. In addition, the cheek teeth of both Hystricops and Priusaulax are similar to other early castoroidines (e.g., Monosaulax Stirton, 1935 ) being mesodont in crown-height and retaining a single re-entrant and three transversely elongated enamel fossettes after minimal to moderate wear.

Hystricopini differ from the nothodipoidine castoroidines in being larger and lacking the fossorial adaptions of the skull of the latter (smaller size; higher-crowned cheek teeth; low, wide cranium; more procumbent incisors; greater postorbital constriction; diastema more strongly arched; Korth 2007: 1536), as well as retaining a more primitive occlusal pattern of the cheek teeth (never attaining the S-pattern of nothodipoidines; Korth 2002: 21). The Hystriocopini also differ from the Castoroidini in being smaller in size, having lower-crowned cheek teeth, and lacking the specialized pattern of the cheek teeth (S-pattern) and ridged incisors of the most derived genera ( Korth 2001b: 289). As previously suggested for Priusaulax ( Korth and Bailey 2006) , the Hystricopini appear to be basal castoroidines.

Stratigraphic and geographic range.—Late Arikareean (latest Oligocene) to Hemphillian (late Miocene) of western North America.

Genus Priusaulax Korth and Bailey, 2006

Type species: P. browni Korth and Bailey, 2006 , UNSM locality Sf- 105E/N, Nebraska, USA, Upper Arikaree Group , latest Arikareean (early Miocene) .

Emended diagnosis.—Hystricopine; markedly smaller and less robust than species of Hystricops ; anterior enamel surface on i1 gently convex, cross-section longer than wide; cheek teeth slightly lower-crowned.

Stratigraphic and geographic range.—Late Arikareean (late Oligocene: Ar4) to early Barstovian (middle Miocene: Ba1) of western North America.