6 Leptictida gregg f. gunnell, 1 thomas m. bown, 2 and jonathan i. bloch 3 1 Museum of Paleontology, University of Michigan, Ann Arbor, MI, USA 2 Erathem-Vanir Geological, Pocatello, ID, USA 3 Florida Museum of Natural History, University of Florida, Gainesville, FL, USA INTRODUCTION Leptictida is an enigmatic group of insectivorous mammals that has been viewed as ancestral or related to a variety of mam- malian groups including Lipotyphla, Erinaceomorpha, Tupaiidae, Primates, Macroscelididae, Apatemyidae, Microsyopoidea, Pan- tolestidae, and Rodentia among others (Gregory, 1910; Matthew, 1918, 1937; Simpson, 1945; Butler, 1956, 1972; McDowell, 1958; Van Valen, 1965, 1967; Lillegraven, 1969; McKenna, 1969, 1975; Szalay, 1969, 1977; Novacek, 1986; McKenna and Bell, 1997). Leptictidans first appeared in the Late Cretaceous of North Amer- ica, with their record continuing from the early Paleocene through the early Arikareean (late Oligocene). Leptictids are also known from the late Paleocene of Europe and the early Oligocene of Asia (McKenna and Bell, 1997). DEFINING FEATURES OF THE ORDER LEPTICTIDA CRANIAL Skull with elongate snout and nasal bones, lacrimal foramen small, and infraorbital canal short; jugal present and postglenoid process well developed; subsquamosal foramen present; maxilla with large orbital wing; auditory bullae formed by entotympanic and not com- pletely covering the tympanic cavities;flange of periotic at pos- terior wall of tympanic chamber absent and fenestra rotunda fully exposed; stylomastoid foramen not isolated from tympanic chamber by tympanohyal; small, triangular exposure of parietal on posterior occiput; deep antorbital fossa for snout muscles; inferior ramus of stapedial artery exits tympanic cavity near forward apex of facial canal; alisphenoid broadly exposed in orbital wall; short alisphenoid canal with anterior exit well behind sphenorbital fissure; subarcuate fossa very excavated (Novacek, 1977, 1986). Known leptictidan cra- nia include those of Eocene leptictid Palaeictops (Matthew, 1918; Novacek, 1986) and Oligocene leptictid Leptictis (Butler, 1956; Novacek, 1986) (Figure 6.1). DENTAL Dental formula I2/3-2, C1/1, P5-4/5-4, M3/3 (tooth nomenclature for leptictids follows Novacek, 1986); canines and dP1/dp1 single- rooted; p4 double-rooted and trenchant without basined talonid; p5 semi-molariform lacking paraconid or molariform with paraconid; p5 talonid basin fully formed or nearly trenchant; lower molar trigo- nids anteroposteriorly compressed and tall, talonid basins relatively large and shallow with entoconulid often present; m3 with mod- erately expanded hypoconulid; P4 with paracone, metacone, pro- tocone (secondarily lost in some taxa), anterior accessory cuspule, and posterior cingulum; molariform P5 with posterior and ante- rior cingula; upper molars with labially positioned paracone and metacone, narrow ectocingulum, paraconule and metaconule, pre- and postcingula, and weak-to-present hypocone (Clemens, 1973; Novacek, 1977, 1986). POSTCRANIAL Forelimb much shorter than hindlimb; robust humerus with promi- nent deltopectoral crest; radius and ulna relatively slender with relatively short olecranon process; stout metacarpals with exten- sor tuberosities on proximodorsal surface of metacarpals II and III; strongly keeled manubrium sterni; pelvis with prominent tuberosity for muscularis rectus femoris; slender femur, with narrow, elevated patellar trochlea and posteriorly directed lesser trochanter; variable degree of distal tibiofibular synostosis; deep astragalar trochlea; moderately elongate astragalar neck and metatarsals; pronounced peroneal process on metacarpal I; ungual phalanges with large exten- sor and flexor processes (Rose, 1999). Known leptictidan postcrania Evolution of Tertiary Mammals of North America, Vol. 2. ed. C. M. Janis, G. F. Gunnell, and M. D. Uhen. Published by Cambridge University Press. C  Cambridge University Press 2007. 82