Neonatal Ectotympanic Bone Variation among Lorisiforms and Tarsiers

Ectotympanic bone morphology is a classic characteristic used to differentiate primates and identify fossil affinities, often depicted as either a ring or tube falling along strict phylogenetic lines. Though ectotympanic bone morphology has been frequently cited in academic literature, the way in which this bone lengthens within and across taxa is still not well understood. The lorisiform ectotympanic bone is often characterized as ring‐like and fused to a slightly expanded petrosal bulla, however some authors have suggested that the degree to which the ectotympanic bone contributes to the external auditory meatus may vary within lorisiforms. The ectotympanic of tarsiers is often depicted as slightly elongate, though not as long as catarrhine primates. It is possible, and likely, that the lorisiform ear canal is longer than the lemuriforms though not as long as tarsiers. However, the degree to which the ectotympanic bone contributes to the ear canal has been a subject of some debate. Thus, can ontogeny resolve the questions surrounding the lorisiform and tarsier ectotympanic bones? Across lorisiforms, is the external auditory meatus derived from the petrosal bone, ectotympanic bone, or some combination of the two at birth? To address these questions, three genera of neonatal lorisiforms ( Galago senegalensis, Nycticebus pygmaeus , and Loris tardigradus ) and one species of tarsier ( Tarsius syrichta ) were qualitatively analyzed. It was found that in all three lorisiform species, the ectotympanic ring is formed by birth. The elongation of the ectotympanic bone in lorisiforms occurs almost immediately postnatally. The lorisiform and tarsier ectotympanic bones extend laterally as simple tubes, without forming the tubercles that are observed at intermediate stages in catarrhine species. The precise bony makeup of the lorisiform external auditory meatus varies among the genera studied. At birth among Loris and Nycticebus , the ear canal is already elongated but likely incorporates contributions from structures derived from the squamosal and petrous namely the post‐glenoid tubercle and the auditory bulla. The tarsier ectotympanic bone closely resembles the Galago at birth, and we suggest that the tarsier cranial proportions may be an underlying factor in the ultimate lengthening of the bone in this taxon. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .