Mioperognathus willardi, 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 : 604-609

Mioperognathus willardi n. sp.

( Fig. 4 ; Table 1 )

Perognathus minutus – Korth, 1998 (in part).

HOLOTYPE. — FAM 129674 , nearly complete cranium with incisors and right M1 (partial)-M3.

OCCURRENCE. — Lower Pojoaque Bluffs, Pojoaque Member , Tesuque Formation , New Mexico.

AGE. — Late Barstovian (middle Miocene).

DIAGNOSIS. — Only species of the genus.

ETYMOLOGY. — Patronym for Willard Korth for his physical and moral support of my research.

DENTAL MEASUREMENTS. — M1, tr = 1.00 mm; M2, a-p = 0.69 mm, tr = 0.89 mm; M3, a-p = 0.43 mm, tr = 0.55 mm (from Korth 1998: table 1). Abbreviations as in Table 2 .

DESCRIPTION

The cranium and upper molars of Mioperognathus n. gen., FAM 129674, have been previously described in detail as those of Perognathus minutus ( Korth 1998: 133). Since there is no additional material available, nothing will be added to the original description.

DISCUSSION

Korth (1998) originally referred FAM 129674 to Perognathus minutus James, 1963 because of its similar size and time equivalent occurrence to the species from southern California ( Lindsay 1972). However , based on dental measurements, the described specimen is distinctly smaller than the Barstovian sample of “ P. minutus ” from California ( Lindsay 1972: table 9; Korth 1998: table 1). The only described Tertiary species of Perognathus that is similar in size is “ P. ” trojectioansrum from the Barstovian of the northern Great Plains .

In addition to overall size, there is another feature that separates FAM 129674 from specimens of P. minutus ; the relative sizes of the upper molars ( Fig. 4D ). In all described species of Perognathus , M1 and M2 are nearly identical in size, M2 being approximately 95% the width of M1. On the holotype of M. willardi n. sp., M2 is less than 90% the size of M1. Even more strikingly, M3 on FAM 129674 is markedly smaller than M2 having a transverse width of only 62% that of M2 (Korth 1998: table 1). On all species of Perognathus , M3 is usually smaller than M2 but has over 80% the transverse width of M2. Besides its smaller size, M3 of M. willardi n. sp. is circular in occlusal outline, not oval as in species of Perognathus and other perognathines. “ Perognathus ” trojectioansrum is similar in size, but it differs from M. willardi n. sp. in lacking the difference in proportional size of the upper molars. Although no M2s or M3s of “ P.” trojectioansrum were reported, measurements taken from the figured maxilla with alveoli for these teeth (UNSM 56313; Korth 1979: fig. 3A) demonstrate that the width of M2 is 98% that of M1, M3 has 80% the width of M2, and M3 is wider than long (oval in occlusal outline).

Mioperognathus n. gen. differs from Eochaetodipus n. gen. in several features of the skull that are shared with extant Perognathus : 1) loss of the accessory foramen ovale; 2) greater inflation of the bulla and mastoid; 3) shorter posterior extent of the temporalis muscle scar; 4) presence of an unossified area on the orbital wall; 5) squamosal reduced to a thin bar of bone posteriorly; and 6) sulcate upper incisors. The inflation of the mastoid, extent of the temporalis muscle scar, and width of the interparietal bone in Mioperognathus n. gen., are not as derived as in Recent Perognathus or Chaetodipus . Mioperognathus n. gen. also retains the temporal foramen as in Eochaetodipus n. gen. In one cranial feature, the earlier and generally more primitive Eochaetodipus n. gen. is more derived (closer to the morphology of Perognathus ) than Mioperognathus n. gen. The relative length of the incisive foramina compared to the length of the upper diastema in the latter is approximately 23%, whereas in Eochaetodipus n. gen.and extant perognathines, it is less than 15%. Mioperognathus willardi n. sp. is also much smaller in size than E. asulcatus n. sp. Eochaetodipus n. gen. also lacks the marked reduction in size of the upper molars present in Mioperognathus n. gen. DISCUSSION

A

B Mioheteromyinae

Eochaetodipus

Mioperognathus Perognathus

Heteromys

Eochaetodipus n. gen. is referred to the Perognathinae based on several derived characters of the skull and skeleton shared with extant perognathines. Eochaetodipus n. gen. has a reduced incisive foramen (length less than 15% that of upper diastema). In mioheteromyines and harrymyines, the ratio of the length of the incisive foramen to the total length of the upper diastema ranges from 24 to 46% ( Korth 1997: table 5). Primitively, in mioheteromyines and harrymyines, the buccinator and masticatory foramina are fused into a single foramen. In Eochaetodipus n. gen., these foramina are separated as in extant perognathines. The interparietal in harrymyines and mioheteromyines is triangular in shape, similar to extant heteromyines. In Eochaetodipus n. gen., the interparietal is oval in shape and very broad. This same broad, oval shape is also present in Perognathus and Chaetodipus . The bulla and mastoid of Eochaetodipus n. gen. show the beginnings of inflation but it is not as marked as in extant perognathines, but is clearly greater than that in heteromyines. One mioheteromyine shows similar inflation of the mastoid and bulla, Schizodontomys Rensberger 1973 (Korth et al. 1990). The harrymyine genus Harrymys also has an inflated bulla and mastoid (Wahlert 1991). It is suggested here that the bullar inflation occurred separately in Schizodontomys and Harrymys because of the distinctive nature of their cheek teeth (increased crown height, occlusalpattern) and lack of any of the other derived cranial characters of perognathines. Because other mioheteromyines, such as Balantiomys, lack this inflation ( Gazin 1932; Wood 1935; Korth 1997), it is not likely a primitive condition for that subfamily or for the entire Heteromyidae .

The final feature of Eochaetodipus n. gen. that is shared with extant perognathines is the slenderness of the limb bones. The bones of Eochaetodipus n. gen. are more delicate than any known for a mioheteromyine.

Eochaetodipus n. gen. cannot be referred to either of the extant genera of perognathines because it lacks a number of derived features: 1) grooved upper incisor; 2) loss of temporal foramen; 3) loss of accessory foramen ovale; 4) unossified area on orbital wall; 5) reduced (shortened) temporalis ridge; 6) greater inflation of mastoid and bulla; 7) angle of mandible deflected laterally; and 8) central union of lophs on p4. The character state of all these features in Eochaetodipus n. gen. is primitive, and similar to that of mioheteromyines and harrymyines.

The cranium of Mioperognathus n. gen. is markedly similar to the skulls of modern perognathines but retains a large temporal foramen, has less inflation of the mastoids, and has the primitive condition of the temporalis muscle scar (continuous for full length of parietals). However, this skull has several features of extant perognathines that are lacking in Eochaetodipus n. gen.: 1) greater inflation of mastoid; 2) grooved upper incisor; 3) loss of accessory foramen ovale; and 4) unossified area on orbital wall. Mioperognathus n. gen. is clearly intermediate in morphology between Eochaetodipus n. gen. and extant perognathines ( Fig. 5A ). PAUP analysis was run on the cranial and dental features of the perognathines discussed (Tables 3, 4). The resulting phylogeny ( Fig. 5B ) shows these same relationships. Mioperognathus n. gen. is also temporally intermediate between Eochaetodipus n. gen. and perognathines.

The morphocline for many cranial and dental characters in perognathines goes from Eochaetodipus n. gen., to Mioperognathus n. gen., to Chaetodipus , and finally to Perognathus . This sequence is also reflected in the fossil occurrence of these genera: Eochaetodipus n. gen. is late Arikareean; Mioperognathus n. gen. is Barstovian; and all fossil species previously referred to Perognathus are Barstovian and younger.

Extant heteromyines have been generally grouped with the perognathines because of several derivecharacters: flange above the orbit; reduced size of incisive foramen; loss of the temporal foramen; more gracile limb bones; deeper parapterygoid fossa; and loss of the accessory foramen ovale.With the recognition of Eochaetodipus n. gen. and Mioperognathus n. gen., it appears that several of these features were arrived at independently from the perognathines.Extant heteromyines retain an uninflated bulla and mastoid, and fusion of the buccinator and masticatory foramina. Both these features are derived in both Eochaetodipus n. gen. and Mioperognathus n. gen., and shared with perognathines. The loss of the accessory foramen ovale in heteromyines is also present in Mioperognathus n. gen., suggesting a separate development of this feature. The only feature not present in either Eochaetodipus n. gen. or Mioperognathus n. gen. is the bony flange above the orbits. Extant heteromyines also have lost the stapedial foramen, a derived feature not present in any other heteromyids, extant or fossil (unknown in Eochaetodipus n. gen.).

The Perognathinae could have easily arisen from a primitive mioheteromyine ancestor during the Arikareean. The earliest perognathine is Eochaetodipus n. gen., which shares a number of the primitive cranial and dental features of the mioheteromyines. Extant heteromyines represent a different clade that likely arose from a mioheteromyine ancestor as well, but separately from the perognathines.

Acknowledgements

The UNSM specimen was collected by B.E.Bailey and field crew as part of the Highway Salvage Paleontology Program, sponsored by UNSM and the Nebraska State Department of Roads.The FAM specimen was graciously loaned by Dr J. Flynn of the American Museum of Natural History.Permission to study the Recent mammal collections was granted by Drs J. Wible (CM) and R. McPhee (American Museum of Natural History, New York). Dr C. E. Mitchell of the Geology Department SUNY at Buffalo provided the camera-lucida for the illustrations and guided the processing of the PAUP analysis. Dr W. Hallahan of the Biology Department, Nazareth College of Rochester provided equipment and assistance for the photographs in Figure 4 . Dr C. Carrasco of the Foreign Language Department, Nazareth College of Rochester assisted in the translation of the title and abstract. Photographs for Figures 1 and 3 were taken by G. Brown of UNSM in the Biodiversity Synthesis Laboratory which was funded in part by National Science Foundation grant NSF-DBI 050076. This manuscript was critically read by Drs J. H. Wahlert and T. S. Kelly.

REFERENCES

GAZIN C. L. 1932. — A Miocene mammalian fauna from southeastern Oregon. Carnegie Institute of Washington Publications 418: 37-86.

HAFNER J. C. & HAFNER M. S. 1983. — Evolutionary relationships of heteromyid rodents. Great Basin Naturalist Memoirs 7: 3-29.

JAMES G. T. 1963. — Paleontology and nonmarine stratigraphy of the Cuyama Valley badlands, California. Part 1, Geology, faunal interpretations, and systematic descriptions of Chiroptera, Insectivora, and Rodentia. University of California Publications in Geological Sciences 45: 1-154.

KORTH W. W. 1979. — Geomyoid rodents from the Valentine Formation of Knox County, Nebraska. Annals of Carnegie Museum 48: 287-310.

KORTH W. W. 1993. — Review of the Oligocene (Orellan and Arikareean) genus Tenudomys Rensberger (Rodentia: Geomyoidea). Journal of Vertebrate Paleontology 13: 335-341.

KORTH W. W. 1997. — A new subfamily of primitive pocket mice (Heteromyidae, Rodentia) from the middle Tertiary. Paludicola 1: 33-66.

KORTH W. W. 1998. — Cranial morphology of two Tertiary pocket mice, Perognathus and Cupidinimus ( Rodentia , Heteromyidae ). Paludicola 1: 132-142.

KORTH W. W., BAILEY B. E. & HUNT R. M. 1990. — Geomyoid rodents from the early Hemingfordian (Miocene) of Nebraska. Annals of Carnegie Museum 59: 25-47.

LINDSAY E. H. 1972. — Small mammal fossils from the Barstow Formation, California. University of California Publications in Geological Sciences 93: 1-104.

REEDER W. G. 1960. — A new rodent genus (family Heteromyidae) from the Tick Canyon Formation of California. Bulletin of the Southern California Academy of Sciences 50: 121-132.

RENSBERGER J. M. 1973. — Pleurolicine rodents (Geomyoidea) of the John Day Formation, Oregon. University of California Publications in Geological Sciences 102: 1-95.

SUTTON J. F. & KORTH W. W. 1995. — Rodents (Mammalia) from the Barstovian (Miocene) Anceney local fauna, Montana. Annals of Carnegie Museum 64: 267-314.

WAHLERT J. H. 1974. — The cranial foramina of protrogomorphous rodents; an anatomical and phylogenetic study. Bulletin of the Museum of Comparative Zoology, Harvard 146: 363-410.

WAHLERT J. H. 1983. — Relationships of the Florentiamyidae (Rodentia, Geomyoidea) based on cranial and dental morphology. American Museum Novitates 2769: 1-23.

WAHLERT J. H. 1985. — Skull morphology and relationships of geomyoid rodents. American Museum Novitates 2819: 1-20.

WAHLERT J. H. 1991. — The Harrymyinae, a new heteromyid subfamily (Rodentia, Geomorpha) based on cranial and dental morphology of Harrymys Munthe, 1988. American Museum Novitates 3013: 1-23.

WAHLERT J. H. & SOUSA R. A. 1988. — Skull morphology of Gregorymys and relationships of the Entoptychinae (Rodentia, Geomyidae). American Museum Novitates 2922: 1-13.

WILLIAMS D. F., GENOWAYS H. H. & BRAUN J. K. 1993.— Taxonomy and systematics. Special Publication of the American Society of Mammalogists 10: 38-196.

WILSON D. E. AND REEDER D. M. 1993. — Mammal Species of the World: A Taxonomic and Geographic Reference. Smithsonian Institution Press, Washington, D.C., 1206 p.

WOOD A. E. 1935. — Evolution and relationships of the heteromyid rodents with new forms from the Tertiary of western North America. Annals of Carnegie Museum 24: 73-262.

WOOD A. E. 1936. — A new subfamily of heteromyid rodents from the Miocene of western United States. American Journal of Science 31: 41-49.

Submitted on 9 July 2007; accepted on 26 October 2007.

Figures

FIG. 4.— Cranium and upper molars of Mioperognathus willari n. sp. (FAM 129674) (holotype):A-C, photographs (left) and line diagrams of the cranium; A, dorsal view; B, ventral view; C, right lateral view (zygomatic arch removed); D, right M1(broken)-M3 (stereo pair; anterior toward top). Abbreviations:bu, buccinator foramen;fm, foramen magnum; fo, foramen ovale; in, incisive foramen;ju, jugular foramen; mbf, fissure medial to bulla; msc, masticatory foramen; op, optic foramen; pom, posterior maxillary foramen;ppl, posterior palatine foramen; spl, sphenopalatine foramen;spt, spheopterygoid canal;spv, sphenofrontal vacuity;rp, rostral perforation (including infraorbital foramen); st, stapedial foramen;sty, stylomastoid foramen;t, temporal foramen; uaf, unossified area between alisphenoid and frontal bones; uml, unossified area between maxillary and lacrimal bones; vf, venous foramen. Scale bars: A-C, 1 cm, D, 1 mm.

FIG. 5. — Phylogenies of selected heteromyid taxa: A, Cladogram of relationships of heteromyids. Explanation of nodes: 1, Heteromyidae; buccinator and masticatory foramen (fused) separated from accessory foramen ovale; rostral perforation associated with infraorbital foramen.2, More gracile skeleton; incisive foramen less than 15% length of upper diastema. 3, Heteromyinae; bony flange above orbit; loss of stapedial foramen; mesodonty; central basin on lower premolar; deeper parapterygoid fossa; loss of posterior border of accessory foramen ovale. 4, beginnings of bullar and mastoid inflation; buccinator and masticatory foramina separated; interparietal oval. 5, presence of unossified area on medial orbital wall;loss of accessory foramen ovale; premaxillary-maxillary suture on palate crosses incisive foramina at their posterior end;greater inflation of bulla and mastoid.6, central union of lophs on p4; deeper parapterygoid fossa; loss of temporal foramen; bony flange above orbit; even greater inflation of mastoid and bulla.7, Dipodomyinae; bulla enormously inflated; limb proportions for ricochetal locomotion; incisive foramina greatly enlarged; cheek teeth mesodont to hypsodont. B, Single most parsimonious tree based on PAUP analysis. Total length = 27 steps. Consistency index = 0.8889. Homoplasy index = 0.1111. Retention index = 0.7500. Multiple derived states for characters 2, 14, and 18 are ordered. See Table 3 for list of character states.

FIG. 1. — Photographs (left) and line diagrams (right) of the skull of Eochaetodipus asulcatus n. sp. based on the holotype UNSM 130000: A, dorsal view; B, lateral view (zygomatic arch removed); C, ventral view. Dashed lines indicate outlines of broken or covered areas; patterned area indicates missing bones. Abbreviations: bu, buccinator foramen; eth, ethmoid foramen; fo, foramen ovale; foa, accessory foramen ovale; fr, nutritive foramen in frontal; in, incisive foramen;mbf, fissure medial to bulla;msc, masticatory foramen; spv, sphenofrontal vacuity; pom, posterior maxillary foramen; ppl, posterior palatine foramen; t, temporal foramen; rp, rostral perforation (including infraorbital foramen); vf, venous foramen. Scale bar: 1 cm.

FIG. 3. — Postcranial elements and mandible of Eochaetodipus asulcatus n. sp. (UNSM 130000): A, ulna (distal end missing); B, tibiofibula (proximal end and fibula broken); C, humerus (proximal end missing); D, calcanium; E, lateral view of right mandible. Scale bar: 5 mm.