FOSSIL BATS OF THE AMERICAS
Icaronycteris index [see also - UCMP Chiroptera: Fossil Record]
Type specimen: Yale Peabody Museum/Princeton University 18150; skeleton lacking right fibula and several toe bones.
Type Locality and Horizon: 8 km W Kemmerer, Lincoln County, Wyoming, USA; early Eocene (late Wasatchian); Green River Formation.
Diagnosis (from Jepsen 1966): From head to tail and from limb girdles to limb ends I. index has the following distinctive combination of qualities, with the few that are especially characteristic of Megachiroptera indicated by M: long narrow cranium; premaxillaries not united at midline (M); dental formula, (188.8.131.52.)/(184.108.40.206.) = 38; diastema between upper incisors; one root on P2 and p2, two on p3 and p4, three on P3 and P4; W-shaped labial wall of upper molars; metaconid and long, deep talonid basin (postfossid) on p4; long nasal bones (M); shallow eye orbits; no postorbital processes on frontals or jugals; zygomatic arch slender, long, and complete; very small sagittal and occipital ridges; palate projected rearward beyond posterior molars; stylohyals long, slender, and articulating with bullae; dentary body long, low, and slender; mental foramina below i3 and p2; ascending process of dentary broad anteroposteriorly, with high, rounded superior border (M); condyle of dentary well above line of tops of molar cusps; angle of dentary hook-shaped and pointed; vertebral formula 7-12-7-3-13 or 14; no vertebral fusion except in sacrum; no coalesced ribs; segments of sternum not fused; mesosternum not keeled; pubic bones loosely united at symphysis; pubic spine short and robust; long and free tail; 5th to 7th caudal vertebrae larger than others; ail tapers abruptly near tip; large supraglenoid tuberosity on scapula; coracoid process of scapula long and slendernot bifid; clavicle heavy and not expanded at ends; trochiter of humerus large and articulates with scapula; high flange-like deltoid crest on straight, slender humerus; relatively short radius; no trace of sesamoid at end of ulna; large scapholunar; very flexible metacarpophalangeal joint; claw on thumb not hooded; claw on index finger (M); digital formula 2-3-3-3-3 (wing) and 2-3-3-3-3 (foot); all claws of wings and feet compressed laterally; decreasing order of finger length 3-4-5-2-1; femur comparatively robust, very short neck between head and shaft; femur head and neck at angle to shaft; fibula slender and well developed; fibula slightly longer than tibia; tibia shorter than femur; metatarsal I shorter and heavier than others; first toe shorter than other toes; no calcar; decreasing order of toe length, 4-3-2-5-1.
Primary references: Jepsen 1966; Jepsen 1970; Simmons and Geisler 1998