The Origin and Evolution of Mammals - Moodle

234 THE ORIGIN AND EVOLUTION OF MAMMALS
superficially rodent-like, with enlarged incisors
and reduced canines. The molars are rectangular
and the cusps blunt, indicating an omnivorous, or
perhaps frugivorous diet. Most plesiadapiforms
were small, such as Carpolestes (Fig. 7.5(b)) whose
head and body length is about 15 cm (Bloch and
Boyer 2003a), and skeleton adapted for an arboreal
existence. Others, notably Plesiadapis, were relatively
large in body size, weighing up to 5 kg
(Fig. 7.5(c)). The skeleton of this particular genus
has been variously interpreted as either terrestrially
adapted on the basis of the length of the limbs, or
alternatively arboreally adapted as suggested by the
well-developed claws and the structure of the ankle
joint (Szalay and Delson 1979).
‘Condylarthra’
The concept of ‘Condylarthra’ as a group of basal
ungulate herbivores has been very important in the
development of ideas about Palaeocene placental
evolution. It contains a variety of small, primitive
members, but more importantly a number of lineages
of specialised and significantly larger herbivores, and
secondary carnivores are included. Furthermore, the
ancestral roots of all the later, specialised ungulate
orders of placentals have been assumed to lie within
the group. Thus the ‘condylarths’ are profoundly
paraphyletic, and the interrelationships of this mixture
of ancestral and derived condylarths are practically
impossible to disentangle. Most agree that,
while the term has long since outlived any formal
usefulness, it is still virtually impossible to do without
it: hence its retention but with quotation marks,
or alternatively use of the synonymous expression
‘archaic ungulates’. The characteristics of ‘condylarths’
are simply the ancestral features of ungulates
generally (Archibald 1998; Nessov et al. 1998). The
molar have evolved a crushing action in place of the
primitive shearing function. They are low-crowned,
with bunodont cusps. The uppers are rectangular in
occlusal view with a fourth cusp, the hypocone,
developed. The trigonid of the lower molar is shortened
from front to back and bears a prominent
metaconid cusp. The second feature of ‘condylarths’
concerns the terminal phalanges of the digits. They
are elongated and are not grooved or fissured, and
are therefore at least incipiently hoof-like rather
than claw-like.
The distinction between ‘condylarth’ molars and
those of the presumed ancestral grade seen in
zhelestids are not marked, and nor are there clear
differences between the most progressive ‘condylarths’
and stem members of some of the ungulate
placental orders. Nevertheless, accepting ‘condylarths’
as a grade of primitive ungulates, several
groups can be distinguished at least some of which
are probably monophyletic (Archibald 1988;
Prothero et al. 1998). The great majority are from the
Palaeocene of North America, where more than half
the Early and Middle Palaeocene mammal species
are ‘condylarths’. Many are herbivorous and
include species of large body size. Others have a
modified dentition suitable for an omnivorous, or a
carnivorous diet, and amongst the latter are the first
placental mammals to enter the medium to large
body-sized carnivorous habitat.
Protoungulatum. Protungulatum (Fig. 7.6(a)) is the
earliest and most primitive ‘condylarth’, a genus
that occurs in North America at the base of the Early
Palaeocene. Apart from a few cranial and postcranial
fragments, it is only represented by teeth and
jaws, which have the ancestral ‘condylarth’ form.
The dental formula is I3/3: C1/1: PM4/4: M3/3,
there is no diastema behind the incisors, and the
premolar teeth show only a slight tendency to
become more molariform. The cusps of the molars
are relatively tall, and the upper molars are transversely
wide. The hypocone is barely developed.
Arctocyonidae. Several families of ‘condylarths’
appear in the Early Palaeocene, distinguished
mostly by details of the dentition. The most primitive
are the Arctocynonidae, which occur in North
America and Europe. Protoungulatum is sometimes
included as the most basal member, although it shares
only ancestral characters with them. Arctocyonid
molar teeth are relatively low-crowned, and there is a
fully developed hypocone cusp on the uppers, indicating
that the crushing function had fully evolved.
A virtually complete skull of Arctocyon (Fig. 7.6(b))
from Europe shows lower canines, a deep sagittal
crest, and a low position of the mandibular articulation.
All these characters indicate an ability to take
significant live prey, even though there was no tendency
to develop any form of carnassial, or specialised
shearing teeth. Thus, arctocyonids seem to