The Origin and Evolution of Mammals - Moodle

236 THE ORIGIN AND EVOLUTION OF MAMMALS
have been generalist omnivores. In body size, they
ranged up to that of a small bear, and probably had
a comparably cosmopolitan diet. Chriacus (Fig. 7.6(c))
was a very lightly built, agile form.
Carnivorous ‘condylarths’. Arctocyonids were common
in the Early Palaeocene and structurally represent
the base of the ‘condylarth’ radiation, from
which various more specialised groups diverged.
Some became more adapted for carnivory, with a
tendency to evolve molar teeth with higher, sharper
cusps, and shearing edges, in place of the bunodont
cusps of the arctocyonid-grade (Szalay 1969). These
constituted the very first radiation of relatively
large, predaceous mammals. Three groups of
‘condylarths’ exhibiting an increasing expression of
this trend are usually combined into a taxon Cete
(Archibald 1998; Prothero et al. 1988). The triisodontids
are the most primitive members of this group,
and include one of the very earliest ‘condylarths’.
Isolated 5 mm long molars of Ragnarok occur at
the base of the Palaeocene 65 Ma. Carnivorous
characteristics of the dentition are only incipiently
developed. The later group Hapalodectidae are
more progressive in this respect, having higher,
sharper cusps. Hapalodectes was a small, rat-sized
animal, not found until the Eocene in North
America, but already present in the Late Palaeocene
of Asia. The Mesonychidae express to the greatest
extent the tendency towards specialised carnivory.
The molar teeth are laterally compressed, and shearing
edges have developed on the front of the lower
teeth, which acted against edges on the back of the
uppers. This condition is described as prevallidpostvallum
shearing, a system that is only analogous
to the postvallid-prevallum shearing system of
the carnivorous teeth that evolved in the later specialist
carnivore groups Creodonta and Carnivora. Some
Mesonyx such as Mesonyx itself (Fig. 7.6(d) and (e))
were the size of a wolf and had lightly built,
digitigrade limbs. Others, such as Harpagolestes
(Fig. 7.6(f)) were even larger, heavily built with powerful
jaws and teeth and were probably scavengers
rather than hunters. The mesonychids are also known
from Asia, where they occur in the Early Palaeocene
Shanghuan and the Late Palaeocene Nonshanian faunas
(Wang et al. 1998; Lucas 2001). One of the last
forms, Andrewsarchus (Fig. 7.6(g)), is from the Late
Eocene of Mongolia, and is often claimed to be the
largest terrestrial carnivorous mammal ever discovered.
Its skull was over 80 cm in length.
Herbivorous ‘condylarths’. Other lineages of ‘condylarths’
evolved increasingly specialised herbivorous
adaptations from an arctocyonid-like ancestry. To
varying degrees the premolar teeth enlarged and
became more molar like, and the molars evolved
broader, six-cusped occlusal surfaces. The limbs
tended to become digitigrade, with well developed
hooves on each of the digits. Four main families are
recognised (Archibald 1998) and all of them first
occur in the Early Palaeocene of China as well as
North America (Wang et al. 1998; Lucas 2001).
The hyopsodontids (Fig. 7.7(a)) were small,
rabbit-sized animals with short limbs. The canines
were very small, and the last premolar is slightly
enlarged. The paraconule and metaconule cusps of
the upper molars had enlarged to give the sixcusped
condition. Hyopsodus extended into the
Eocene and is therefore one of the last surviving
‘condylarths’.
The family Mioclaenidae (Fig. 7.7(b)) were also
relatively small, incipiently specialist herbivores. The
premolars were more molarised, and the occlusal
surfaces of the molar teeth were simplified by having
less distinct cusps, suggesting a more continuous
grinding action of tougher food. Mioclaenids, or at
least close relatives of this family occur in the Early
Palaeocene Tiupampan fauna of the Santa Lucia
Formation of Bolivia, where they have been implicated
in the origin of the indigenous South American
ungulate orders. Gheerbrant et al. (2001) have described
‘condylarth’ teeth of Early Eocene age from
Morocco in North Africa, one of which, Abdounodus
(Fig. 7.7(b)), is comparable to mioclaenids.
Periptychids varied from small, squirrel-sized
mammals to Ectoconus (Fig. 7.7(c)), which was the
size of a sheep. In this family, the premolars were
more or less fully molarised, and crescentic crests
had developed from the cusps of these and the
molars (Fig. 7.7(d)). Archibald (1998) interpreted the
tooth structure as an adaptation for dealing with
tough, fibrous vegetation that was first shredded by
the swollen premolars and then pulverised by the
molars. The skeleton of periptychids is rather heavily
built and the limbs short. The feet too are short,