The Origin and Evolution of Mammals - Moodle

272 THE ORIGIN AND EVOLUTION OF MAMMALS
The other modern anthropoid group are the
Catarrhini, the Old World monkeys and the apes.
Several primitive members occur in the Late
Eocene levels of the Fayum deposits, such as
Catopithecus, for which there are several skulls preserved
(Simons and Rasmussen 1996). It combines
derived catarrhine characters of the dentition,
including a reduction from three to two premolars
and fused symphysis, with primitive features of the
skull such as the relatively small brain and elongated
snout. Aegyptopithecus is the most familiar
basal catarrhine, coming from the slightly younger
Early Oligocene level in the Fayum sequence. It
was the size of a typical monkey, well adapted for
arboreal life, and the rounded cusps of the molar
teeth indicate a diet of fruit. These basal catarrhines
are conveniently grouped as the Propliopithecoidea,
despite presumably being a paraphyletic group
defined only by primitive characters, and containing
the ancestors of the later anthropoid taxa
(Rasmussen 2002). As with the platyrrhines, there is
no agreement about precisely what the relationship
is between the known basal anthropoids and the
catarrhines.
There is a considerable gap in the fossil record
between the Early Oligocene basal catarrhines such
as Aegyptopithecus, and the first occurrences of the
derived lineages. The Old World monkeys,
Cercopithecoidea, are unknown until the Early
Miocene of Africa, when the Kenyan Victoriapithecus
is found. The first of the apes, Hominoidea, may be
slightly earlier if the tooth-bearing maxillary fragment
referred to as Kamoyapithecus (Leakey et al.
1995) is correctly identified and correctly dated as
Late Oligocene. Otherwise, hominoids too commence
their radiation in the Early Miocene of Africa
in the form of the well-known genus Proconsul
(Harrison 2002). By the Middle Miocene, radiations
of hominoids were also well under way in Asia and
Europe. The third derived catarrhine group are the
Pliopithecoidea, an exclusively Middle and Late
Miocene group restricted to Europe and Asia.
During their brief radiation, a range of size and
adaptations comparable to that of the Old World
monkeys in Africa evolved (Begun 2002).
The story of the origin and evolution of the
Hominidae, the australopithecines and hominines
of Africa and their subsequent worldwide dispersal
has been told so many times that it does not need
repeating here. As far as mammalian radiation is
concerned, the hominids were a very minor group
of trivial significance, prior to the ecological effect
of one species, Homo sapiens, since the Late
Pleistocene a few thousand years ago.
Dermoptera
The sole living genus of the order Dermoptera is
the colugo or flying lemur Cynocephalus, although
despite this paucity, there is fossil evidence for a
very long, if not very diverse history of the group.
The dentition is particularly distinctive. There is a
gap between the front of the upper tooth rows, and
the two pairs of lower incisors have a unique,
comb-like construction, consisting of many separate
‘tines’. The cheek teeth are wide and have a
curiously wrinkled enamel that creates a series of
sharp edges after a certain amount of wear. The
postcranial skeleton is also uniquely designed for
supporting a gliding membrane stretched between
all four limbs. On the basis of these characters, a
fully evolved fossil dermopteran, Dermotherium, is
recognisable in the Lower Eocene of Thailand
(Ducrocq et al. 1992). There are much earlier specimens
sometimes regarded as members of the order,
namely the plagiomenids that occurred from the
Early Palaeocene to Middle Eocene of North
America. The lower dentition of a form such as
Plagiomene certainly resembles that of Cynocephalus
(Rose and Simons 1977), although others doubt the
relationship (MacPhee et al. 1989).
Dermoptera have been linked by several authors to
the plesiadapiforms, the group of mainly Palaeocene
forms sometimes regarded as basal primates. Kay et
al. (1990) described similarities in the ear region, and
Szalay and Lucas (1993) detected homologies in the
postcranial skeleton of the two groups. Other authors
have, however, disputed the association of any, let
alone all the plesiadapiforms with the Dermoptera,
regarding the similarities as merely superficial,
or primitive for the orders (Wible 1993; Bloch and
Silcox 2001).
Scandentia
The tree shrews, at various times confidently classified
as Insectivora, and as basal, virtually ancestral
primates, are a distinct mammalian order, but one