The Origin and Evolution of Mammals - Moodle

264 THE ORIGIN AND EVOLUTION OF MAMMALS
amongst the modern mammalian fauna (Janis et al.
1997(c)). It has always been a relatively conservative
order in terms of size and body form, not producing
any really bizarre forms such as are found
in the perissodactyls. On the other hand, artiodactyls
were eventually to prove far and away the
most diverse of all the ungulate orders, creating
extraordinary difficulties for phylogenetic analysis.
Homoplasy of morphological characters is rife,
and disagreements at the lower taxonomic levels
widespread.
The more basal forms are included in the
Suiformes, or Bunodontia, in which the molar teeth
are bunodont, with low, rounded cusps appropriate
for browsing and omnivory. The anthracotheriids
(Fig. 7.20(d)) were the earliest family to radiate, in
the Middle Eocene of Europe and Asia. By the
Late Eocene, North American representatives are
found (Kron and Manning 1997), and by the
Oligocene they had arrived in Africa (Black 1978).
Anthracotheriids varied in size from that of a small
pig to that of a large hippopotamus, and being relatively
heavily built and short-limbed, it is usually
assumed that they had a semiaquatic, hippo-like
existence. Indeed their decline in importance during
the Miocene and onwards has sometimes been
attributed to the Miocene origin of a second
suiform family, the Hippopotamidae. However, this
family never entered North America, even though
anthracotheriids disappeared there too. The third
suiform family, Suidae, are first found in the
Oligocene of Eurasia, and like the hippos went on
to radiate from the Miocene onwards exclusively in
the Old World. In contrast, the fourth group,
Tayassuidae, or peccaries, were exclusively New
World in distribution (Wright 1998). These generally
pig-like forms appeared and radiated widely
during the Miocene, and entered South America
during the Plio-Pleistocene.
All other artiodactyls are included in a taxon
Selenodontia, on the basis of the evolution of selenodont
cheek teeth, whose crescentic-shaped crests
contributed to a structure capable of dealing with
tougher vegetation. The least modified of the selenodonts
are the Tylopoda, a primarily North
American group. It consists of the Middle Eocene
and Miocene Oreodontidae (Fig. 7.20(e)), which
were pig-like forms (Lander 1998), and the
Camelidae. The latter were a very important part
of the large herbivore fauna during the Miocene,
and only finally disappeared from that continent in
the Late Pleistocene (Honey et al. 1998). The familiar
Eurasian and African camels of today are the
descendants of Late Miocene immigrants (Agustí
and Antón 2002).
The Ruminantia constitute by far the majority
of artiodactyls, most of which are Old World in
distribution. Fossil ruminants are recognisable by
the loss of the upper incisors, and fusion of the
cuboid and navicular bones of the forefoot. The earliest,
and most primitive members are represented
by families in North America and Eurasia by the
Upper Eocene, and in Africa by the Early Miocene.
A complex evolutionary radiation and dispersal
pattern of the ruminants followed in all three areas
(Gentry and Hooker 1988; Janis and Scott 1988;
Janis et al. 1998c), culminating in the dominant position
amongst ungulate mammal faunas that they
hold today. The more progressive members, the
pecorans were always predominantly Old World
forms, and include the Giraffidae, and particularly
the Bovidae that are the most diverse artiodactyls
of Africa. The Cervidae were always more cosmopolitan,
and are the dominant Eurasian ruminants.
Only one or two rare fossils of either cervids or
bovids have been discovered in North America
before the Late Pleistocene. In fact, only one pecoran
family at all, Antilocapridae, was ever very significant
in North America. They were quite
abundant during the Miocene, but disappeared
during the Plio-Pleistocene with the sole exception
of Antilocapra, the living pronghorn antelope (Janis
and Manning 1998).
The very strong molecular evidence for a sistergroup
relationship between whales and hippos was
discussed earlier in the chapter. Until recently, the
fossil evidence did not support this, and on the
basis of dental and braincase characters in particular,
cetaceans were regarded as related to, indeed
descended from mesonychid ‘condylarths’. The
fossil record of cetaceans is actually remarkably
good, owing to their marine habitat, and a diversity
of primitive, Eocene whales have been described
(Williams 1998; Thewissen and Williams 2002).
They are mainly from the Indian subcontinent and
Egypt, both of which bordered the Tethys Sea at the