Mioperognathus, Korth, 2008

Korth, William W., 2008, Two new pocket mice (Mammalia, Rodentia, Heteromyidae) from the Miocene of Nebraska and New Mexico and the early evolution of the subfamily Perognathinae, Geodiversitas 30 (3), pp. 593-609 : 603-604

Genus Mioperognathus n. gen.

TYPE SPECIES. — Mioperognathus willardi n. sp.

RANGE. — Middle Barstovian (middle Miocene) of New Mexico.

DIAGNOSIS. — Derived perognathine features shared with Perognathus and Chaetodipus : mastoid and auditory bulla inflated; upper incisor grooved; accessory foramen ovale absent; unossified area present on medial orbital wall; premaxillary-maxillary suture crosses palate at posterior end of incisive foramina; buccinator and masticatory foramina separate; optic foramen greater than 1 mm in diameter. Primitive perognathine features: cheek teeth brachydont and bilophate; stapedial foramen present in bulla. Retained primitive heteromyid features lost in Perognathus and Chaetodipus : length of incisive foramina more than 15% of that of upper diastema; temporal foramen present on skull; scar for temporalis muscle extends onto parietal bones; mastoid and bulla inflated, but less than in Chaetodipus ; no bony flange above orbits. Autapomorphic features: molars greatly decrease in size from M1 to M3; M3 markedly smaller than M2 and circular in occlusal outline.

ETYMOLOGY. — The name is intended to reflect the age of occurrence (Miocene) and the close systematic relationship to Perognathus .

DISCUSSION

Mioperognathus n. gen. is clearly referable to the Perognathinae based on the cranial characters used by Wahlert (1985: 14) to distinguish the three extant subfamilies of the Heteromyidae (large optic foramen, non-ossification on orbital wall including ethmoid foramen, inflated auditory bullae with anteroventral processes approaching each other and touching basisphenoid, squamosal bone extending as thin process posteriorly above acoustic meatus, upper incisors grooved). In addition, the brachydont cheek teeth of Mioperognathus n. gen. are similar to that of perognathines, and do not attain the mesodonty or hypsodonty of hetermyines or dipodomyines.

Mioperognathus n. gen. differs from Perognathus and Chaetodipus in lacking a number of derived character states (shorter incisive foramina; loss of temporal foramen; scar for temporalis limited to frontal bone; marked mastoid and bulla inflation; bony flange above orbits). In Mioperognathus n. gen., these character states are expressed as: a large, distinct temporal foramen; the length of the incisive foramen is over 20% that of the upper diastema; the scar for the temporalis muscle extends posteriorly for nearly the entire length of the parietal bones; there is some bullar and mastoid inflation, but much less than in Chaetodipus (the extant perognathine with the least amount of inflation of this area); and there is no flange of bone above the orbits. The skull of Mioperognathus n. gen. is believed to be more primitive because these features are similar to those of mioheteromyines and other geomyoids.