Competing Classification Systems

          Historically, there have been two competing taxonomic systems used to classify the suborders of primates. Tarsiers were initially categorized alongside lemurs and lorises, and apart from humans, apes, and monkeys. This original taxonomic system, known as the gradistic division, held that the two suborders of primates were Prosimii and Anthropoidea. Prosimians, which means “before apes”,  were comprised of lemurs, lorises, and tarsiers, due to the perception that they represented grades of evolution. They were seen to possess many of the same traits, such as similar teeth, skull, and limb anatomy to early, now extinct, primates. These “primitive” features being shared amongst the three extant groups were believed to be evidence of close relation, and were thought to set them apart from the “more evolved” characteristics of anthropoids. The more recent categorization, known as the phyletic division, posits that tarsiers should actually be grouped alongside monkeys, apes, and humans, and apart from lemurs and lorises. In this taxonomic system, the two suborders of primates are instead Strepsirhini and Haplorhini. The reasoning behind grouping tarsiers with the formerly named anthropoids as a new group called haplorhines is that humans, apes, monkeys, and tarsiers all have shared derived features that indicate closer relation amongst them than with the lemurs and lorises which comprise strepsirrhines. Strepsirrhines are defined by features such as wet rhinarium, the presences of a tooth comb, a laterally flaring talus, and a grooming claw on the second digit of the foot. Tarsiers are distinct from the strepsirrhines in that they have a dry rhinarium, lack a tooth comb, as well as having certain skeletal and physiological traits that are more similar to the other haplorrhines.  Furthermore, the extant haplorhines share a number of derived cranial features, including postorbital closure, a retinal fovea in their eyes, a reduced number of nasal conchae, a short, vertical nasolacrimal duct and the lack of a moist rhinarium, giving them the dry nose and continuous upper lip from which the name haplorhine derives. They also all have a hemochorial placenta and an inability to synthesize vitamin D. The tarsiers’ similarities to other prosimians are primitive features, like an unfused mandibular symphysis, molar teeth with high cusps, grooming claws on their second toes, multiple nipples, and a bicornuate uterus. In contrast, their similarities to anthropoid primates seem to be derived specializations indicative of a more recent common ancestor, a hypothesis that has been supported by genomic analysis. The taxonomic system with greater evidentiary support is, therefore, the phyletic division of haplorhines and strepsirrhines.