Toothcomb - Evolution in Lemuriforms

Evolution in Lemuriforms

The origins of the lemuriform toothcomb and the clade it characterizes have been the center of considerable debate for more than a century. In 1920, British palaeoanthropologist Wilfrid Le Gros Clark proposed that the toothcomb found in treeshrews (which he believed were primates) was an early version of the dental structure found in lemuriforms. Because he viewed the fossil lorisoids from the Miocene as not having fully developed the modern lemuriform toothcomb, he implied that lemurs and lorisoids had evolved the trait independently. This view was later overturned, and the monophyletic relationship between lemurs and lorisoids is now accepted.

The ancestral condition of the anterior dentition on the lower jaw, based on Eocene primate fossils, suggests that earliest primates had lacked a differentiated toothcomb. Most fossil strepsirrhines lacked the stereotypic lemuriform toothcomb. Collectively, early strepsirrhine primates are known as adapiforms. Adapiforms are considered to be a paraphyletic group (containing many but not all of the descendants of the last common ancestor of the group's members) because the lemuriforms are assumed to have evolved from one of several groups of adapiforms. In terms of ecology, the evolution of the toothcomb is assumed to have required a folivorous (leaf-eating) diet among the ancestral adapiform population, since that would select for reduced incisors, which would serve as an exaptation (a trait with adaptive value for something other than what it was originally selected for), which could then be used for personal or social grooming. However, the inclusion of the canines into the toothcomb must have required exceptional conditions, since large lemuriforms have secondarily modified caniniform premolars to substitute for the loss.

A popular hypothesis about the origins of the lemuriform clade is that they evolved from European adapiforms known as adapids. In some adapids, the crests of the lower incisors and canines align to form functional cropping unit, and the American paleontologist Philip D. Gingerich has suggested this foreshadowed the development of the lemuriform toothcomb. However, no lemuriform toothcomb has been found in the fossil record of the Eocene, and the European adapid lower jaws from that time did not resemble the derived state seen in lemuriforms.

Lemuriforms are currently thought to have evolved in Africa, and the earliest known prosimian primates from Africa are azibiids from the early Eocene, which likely descended from a very early colonization of the Afro-Arabian land mass in the Paleocene (65 to 55 mya). Stem lemuriforms, including Djebelemur and 'Anchomomys' milleri, have been found in Africa and date from 50 to 48 mya and were very distinct from European adapiforms. However, they lack a toothcomb. These stem lemuriforms suggest an early common ancestry with cercamoniines from outside of Europe. Based on large, procumbent lower teeth, Plesiopithecus, a fossil primate found in late Eocene deposits at the Fayum Depression in Egypt, is thought to be most closely related to lemuriforms. Together, Djebelemur, ‘Anchomomys’ milleri, and Plesiopithecus are considered to be sister taxa (the closest relatives) of lemuriform primates.