This essay explores the reasons that Tarsiers are so hard to classify and why different Primatologists Classify them differently.
Tarsiers are notoriously hard to classify in any given Taxonomy. In fact as one peruses differing classification schemes you will find that different scientists create differing classification schemes based solely on the traits that they find most relevant. The question boils down to whether you think they most closely resemble Prosimians or Anthropoids. In general we consider Prosimians as being primitive primates like Lemurs and Marmosets and Anthropoids are advanced primates like Monkeys, apes and humans. The uses of the terms primitive and advanced are misnomers in of themselves. Fleagle who is a leader in the field of primatology uses these terms for lack of terms that would be more appropriate. Those terms imply connotations that are unintended. Primitive only refers to derived characteristic that more closely resemble a non-primate ancestral common ancestor. Conversely advanced characteristics refer to traits that more closely resemble rapid evolutionary processes that resemble more human characteristics. Frankly these terms are anthropomorphic and misleading but they will suffice for the discussion that follows.
There are generally accepted at this time 5 different species of Tarsiers classified under the Genus of Tarsius though from author to author these tend to vary as well.
Philippine tarsier Tarsius syrichta
Bornean tarsier Tarsius bancanus
Spectral tarsier Tarsius spectrum
Diane’s tarsier Tarsius dianae
Pygmy tarsier Tarsius pumilus
Primate classification as whole is extremely difficult especially at the species level. You can find the same picture of a primate in any given book and the species names will differ from source to source. This speaks to the subjective nature of the application of any given taxonomic system. Tarsiers on the other hand present problems as far up the taxonomic ladder as family and even order. The following is a breakdown of the confounding traits exhibited by tarsiers that make them very difficult to classify.
Confounding features of Tarsiers:
Fig #1 from Faulk and primate Diversity.
Prosimian like features
(Similarities to prosimians are considered primitive features, Fleagle 1999)
1. Unfused mandibular symphysis.
2. Molar teeth with high cusps
3. Grooming claws on 2nd and 3rd toes.
4. Multiple nipples
5. Bicornate uterus.
6. Diet – Faunivorous (Insects, arachnids, small vertebrates like snakes and lizards)
7. Locomotion, vertical clinger and leaper. Intermembral index’s of 52 (Tarsius bancanus) and 58 (Tarsius syrichta).
Anthropoid like features
(Derived specializations indicative of a phyletic relationship, Fleagle 1999)
1. Bony eye socket, partly closed, resembles higher primates but not conclusive.
2. Lack of attached upper lip with a median fold.
3. Internal to the nose, greatly reduced turbinates and absence of a sphenoid recess
4. Major blood supply to brain from the promontory branch of internal carotid.
5. Teeth resemble Anthropoids in overall proportions, Lg. upper central incisors and canines, and small lower incisors.
6. Inner ear structure of Anthropoid.
7. Hemochorial placenta instead of an epitheliochorial type found in lemurs.
8. Large offspring (30% of female’s body weight) with monthly menstruation cycles
9. No dental comb.
Unique Tarsier traits
1. Size of eyes, each larger than the brain and stomach.
2. Nocturnal primate yet lacks a Tapeta lucidum. Resembles diurnal primates having a retinal fovea.
3. Dental formula 18.104.22.168./22.214.171.124. Unique
4. Social groups of all kinds from monogamy to multi-male / multi-female types.
As you can see, these differing traits would make it hard for anyone to classify the Tarsier as either a Prosimian or an Anthropoid. For accuracies sake if not simplicities sake, tarsiers need to be classified under their own heading somewhere in between Prosimians and Anthropoids. Fleagle classifies Tarsiers under infraorder Tarsiforms which falls under Suborder Prosimii. Given the size and habitat of tarsiers this makes a lot of sense from a cladistic standpoint. Falk does the same thing only sets them in a different clade. When the smoke clears I go with Fleagel’s interpretation as Fleagle is hard for anyone to argue with as he is one of the foremost thinkers in his field of evolutionary adaptation.