The Origin and Evolution of Mammals - Moodle

etween Primates, Dermoptera (colugos), and
Scandentia (tree shrews), which formed part of the
old supraordinal taxon Archonta. However, the
anatomy of neither fossil nor living forms has ever
offered support for either excluding the bats, or
including the rodents and lagomorphs with them,
as is now indicated by molecular evidence.
Primates
The fossil record of primates (Fig. 7.23) is notoriously
very poor because they are largely arboreal,
forest-dwelling animals, and the early members at
least were all small in size (Taveré et al. 2002). On
the other hand, the degree of effort devoted to finding
them and the level of attention given to their
interpretation tends to mitigate their sparseness.
The Plesiadapiformes (page 232) used to be
regarded as the most primitive primates, indicating
a very early, immediately post-Cretaceous origin
for the order, followed by a radiation through the
Palaeocene resulting in at least five plesiadapiform
families, distributed in North America and Europe.
However, the lack of such fundamental primate
characters as a postorbital bar, shortened snout, or
opposable hallux and pollex eventually led most
authors to exclude Plesiadapiformes from the
order, interpreting them instead as basal archontans,
with the possible relationship to Dermoptera
mentioned later (p272). The issue has re-emerged
recently with the description by Block and Boyer
(2003a,b) of a more or less complete skeleton of the
carpolestid plesiadapiform Carpolestes (Fig. 7.5(b)),
showing it does in fact possess opposable, nailbearing
first digits. Their phylogenetic analysis has
carpolestids as the sister-group of what they term
the Euprimates, and they conclude that plesiadapiforms
ought to be included in the order Primates
after all. Kirk et al. (2003) are unconvinced and
believe that the postcranial similarities are convergences
in two separate lineages of arboreal euarchontan
mammals.
Living primates fall into two informal categories.
The ‘prosimians’ are the primitive primates and
include lemurs, galagos, lorises, and tarsiers. They
retain a series of ancestral characters, including a
small brain, a long snout, and a postorbital bar that
is not expanded to enclose the back of the orbit. The
incisors are rounded rather than spatulate and
LIVING AND FOSSIL PLACENTALS 269
the symphysis of the mandible is unfused. The
anthropoids are advanced primates and consist of
the monkeys and apes. ‘Prosimii’ is a paraphyletic
group because it is based entirely on ancestral characters;
using derived characters demonstrates that
some prosimians are basal to the anthropoids. Two
monophyletic groups are now recognised. The
lemurs, galagos, and lorises are the Strepsirhini,
characterised by the presence of a grooming toothcomb
formed from the compressed lower incisors
and canines. The tarsiers and anthropoids are the
Haplorhini, characterised, amongst other features,
by loss of the moist rhinarium on the nose and
cleft upper lip, loss of the tapetum lucidum, and
presence of a haemochorial placenta.
‘Prosimian’ grade primates first appear in the
earliest Eocene, and there followed immediately
an Eocene radiation throughout the northern
continents, that produced several distinct lineages.
The relationships of the Eocene to the modern
‘prosimian’ groups are far from clear, but most
authors accept that both the strepsirhines and the
haplorhines were represented, and that the latter
had already divided into the tarsier and the anthropoid
lineages (Covert 2002). All were relatively
small, and arboreal, and variously had insectivorous
and frugivorous diets.
Adapiformes are first represented by dentitions
and partial skulls of Cantius (Fig. 7.23b), from the
earliest Eocene of Europe and North America. They
had unspecialised incisors and non-shearing
molars, suggesting a primarily frugivorous diet.
Later forms were generally quite large for Eocene
primates. Notharctus, for example, had an estimated
weight of 7 kg (Gebo 2002) and proportions resembling
a lemur. A number of characters suggest that
adapiforms are strepsirhines (Kay et al. 1997; Ross
et al. 1998). These include the lemuriform-like structure
of the ear region, with a ring-shaped ectotympanic
bone within the tympanic cavity, and
similarities in the bones of the hand and the foot.
The Omomyoidea are the second group of primates
to occur in the Early Eocene of Europe and
North America. Teilhardina was only about 70 g in
estimated body weight, and no omomyoid was
larger than a squirrel. Their unspecialised dentition
suggests that insects formed an important part of
the diet, and at least some members of the family