Arboreal or Terrestrial Ancestry of Placental Mammals

Abstract

External form and internal structure allow most pentadactyl mammalian hands to be classified as convergent, in which the digits fall together when they are flexed on the palm; clasping, in which the contrahentes muscles to the marginal digits are more or less transversely arranged; hands with opposable pollices, in which the 2 digits oppose the other 3 and hands in which opposability has been lost. The hand of Herpestes ichneumon is described as an example of a strictly terrestrial convergent type, with some cursorial adaptations. The clasping and fully prehensile types appear to have been derived from the convergent type. Specializations of the hands, feet, and tail such as are characteristic of many arboreal animals, particularly of slow, deliberate climbers, cannot, so far as can be judged from the skeleton, be recognized in the earliest placental mammals. In spite of the suggestion frequently made that the placental mammals as well as the marsupials were primitively arboreal in habit, it seems more probable from the structure of the hand of Herpestes and a comparison between it and other types that the earliest known placentals had hands of convergent type, and like Herpestes were terrestrial in habit. The early Cretaceous forests appear to have offered little scope for arboreal life, but since their replacement by angiospermous forests of modern type the abundance of insects and fruits available as food has led to the adoption of arboreal habits by several distinct groups of mammals.