'' '' Features that characterize many omomyids include large orbits (eye sockets), shortened
rostra and dental arcades, loss of anterior premolars, cheek teeth adapted for insectivorous or
frugivorous diets, and relatively small body mass (i.e., less than 500 g). However, by the late middle Eocene (about 40 mya), some North American omomyids evolved body masses in excess of and frugivorous or
folivorous diets.
The largest omomyids were
Macrotarsius and
Ourayia, both at in weight. Large orbits in genera such as
Tetonius,
Shoshonius,
Necrolemur, and
Microchoerus indicate that these taxa were probably
nocturnal. At least one omomyid genus from the late Eocene of Texas (
Rooneyia) had small orbits and was probably
diurnal. Like
primates alive today, omomyids had grasping hands and feet with digits tipped by nails instead of claws, although they possessed
toilet claws like modern lemurs. Features of their skeletons strongly indicate that omomyids lived in trees. • stem
tarsiiformes [i.e., basal offshoots of the tarsier lineage]. • stem primates more closely related to adapids than to living primate taxa. Recent research suggests the Omomyiformes are stem haplorhines, making them likely a
paraphyletic grouping. Attempts to link omomyids to living groups have been complicated by their primitive (
plesiomorphic) skeletal anatomy. For example, omomyids lack the numerous skeletal specializations of living haplorhines. These haplorhine adaptations - absent in omomyids - include: • significant reduction of the canal for the
stapedial branch of the
internal carotid artery. • route of the canal to house the promontory branch of the internal carotid artery through the
auditory bulla of the temporal bone, i.e. "perbullar" (rather than across the
promontory of tympanic cavity, "transpromontorial") . • contact between the
alisphenoid and
zygomatic bones. • presence of an anterior accessory cavity confluent with the
tympanic cavity. Omomyids further demonstrate a gap between the upper central incisors, which presumably indicates the presence of a
rhinarium and
philtrum to channel fluids into the
vomeronasal organ. Omomyids as a group also lack most of the derived specializations of living tarsiers, such as extremely enlarged orbits (
Shoshonius is a possible exception), a large supra-
meatal foramen for an
anastomosis between the
posterior auricular and
middle meningeal circulation (again,
Shoshonius is a possible exception, but the contents of the foramen in this extinct taxon are unknown), and extreme postcranial adaptations for leaping. Among primates, omomyids have a uniquely derived characteristic. This is the presence of an aphaneric (not visible or readily distinguishable, due to its position), or "intrabullar" (within the bullae),
ectotympanic bone, connected to the lateral wall of the auditory bulla by an unbroken annular bridge. ==Classification==