Phylogenic Evidence of Tarsiers as Anthropoids Essay

Phylogenic Evidence of Tarsiers as Anthropoids - Essay Example However a number of shared and less similar characters make definitive decision about direct ancestors of present day tarsiers almost impossible. There have been considerable debate and research over the tarsiers' relation to prosimians and anthropoids. A number of anatomic and molecular biological studies in recent times have shown a number of derived traits in tarsiers similar to anthropoids. Pocock in 1918 was the first to suggest an anthropoid connection to tarsiers based on its numerous shared derived traits or morphological synapomorphies and allocating it to the sub-order Haplorhini. Based on the data from Gursky (1999) tarsiers share conspicuous haplorhine traits such as a dry rhinarium, haemochorial placentation, reduced olfactory bulbs, promontory artery, fovea centralis and a flexible upper lip. However there are inconsistent views on these traits being purely anthropoidean as it is also argued that the traits have been attained due to homoplasy because of its nocturnal tropical niche over long time periods (Barrett and Dunbar, 2000). The visual system of tarsiers show high ganglion cell densities, high orbital convergence, retinal fovea, postorbital septum and lack tapetum lucedum which shows a stem lineage leading to a tarsier-anthropoid clade (Ross, 2000). Studies have implicated tarsiers to be derived from diurnal ancestors as in haplorhine primates, primarily due to the fact that they have lost the reflecting layer in the eye tapetum lucidum that maximizes its light gathering capacity. Comparison of the average measurement of eye and cornea diameter and ratio of these measurements in relation to the average body weight among primate specimens were considered by Jason Cowan in his attempt to solve the phylogenetic enigma. The eye and cornea diameter averages were closest to the anthropoidean relative Aotus. However the average eye diameter with respect to the average body length showed a prosimian connection. However it could be adaptations for nocturnal habitation and specific dietary reliance that require high visual acuity (Cowan, 2006). A diet reconstruction through character optimization shows a fruit and insect diet for the haplorhine stem lineage and visual predation on insects an adaptation of this (Ross, 1996). Another plausible scenario is that of a nocturnal tarsier like ancestry to anthropoids which occurred when a lineage diverged to adopt diurnality (Cartmill, 1980). Auditory System Tarsiers and anthropoids share an unusual feature in the petrous temporal bone which is the anterior accessory cavity. The cavity arises from the bony auditory tube as a pneumatic diverticulum ontogenetically and invades the developing petrosal plate it into a medial and lateral lamella that forms wall of the auditory bulla and tympanic cavity respectively (Cartmill&Kay, 1978, MacPhee & Cartmill 1986). The two chambers in the middle ear augment sensitivity to low and high frequency sound (Lomabard and Hetherington, 1993) that aid small nocturnal primates in visual predation. Brain components Analysis of brain components of tarsiers by comparing it with both anthropoid primates and prosimians showed a clear anthropoid connection in the work done by Joffe and Dunbar (1998). An increase in size of the visual brain components as a result of encephelization