The Origin and Evolution of Mammals - Moodle

260 THE ORIGIN AND EVOLUTION OF MAMMALS
were a predominantly North American family
(Munthe 1998a), where the small-dog sized
Hesperocyon (Fig. 7.18(a)) of the Middle Eocene was
the earliest, and the family was not represented in
Europe until well into the Miocene, nor Asia and
Africa until the Pliocene. The Ursidae (bears) also
first occur in North America (Hunt 1998), in the
Late Eocene, as the very small, gracile Parictis that
had a skull only 7 cm long. Like the canids, this
family too does not appear in the Eurasian or
African record until the Miocene. In contrast, the
other caniform families Amphicyonidae (the extinct
beardogs), Procyonidae (raccoons), and Mustelidae
(weasels, otters etc.) all occur by the Late Eocene
or Early Oligocene in both the Old World and
the New.
The Pinnipedia are a monophyletic group related
to the caniforms. Both the morphological evidence
(Wyss and Flynn 1993; Janis et al. 1998b) and molecular
sequence data (Corneli 2003) indicate that they
are most closely related to the ursids. They occur
from the Oligocene. Enaliarctos (Fig. 7.18(e)) is a
Late Oligocene Californian genus known from
more or less complete skeletons. It was relatively
small, about 1 m in presacral length, and both fore
and hind feet had evolved into flippers, although
they are sufficiently well developed to imply a reasonable
degree of manoeuvrability on land (Berta
and Ray 1990). The molar teeth are relatively
unmodified carnassials.
Fossil members of the three constituent families,
which are identified by various cranial characters,
are all represented in the fossil record by the Early
to Middle Miocene, Otariidae (eared-seals) and
Obodenidae (walruses) on the North Pacific coasts,
Phocidae (true seals) on the North Atlantic coasts
(Berta and Sumich 1999; Berta 2002).
Perissodactyla
The earliest perissodactyls (Fig. 7.19(a)) date
from the beginning of the Eocene of North
America and Europe, and within the Early Eocene
representatives of all the three main lineages
are already present (Prothero and Schoch 1989). The
most primitive perissodactyls include famously
Hyracotherium (‘Eohippus’), although the taxonomic
situation is confusing because specimens attributable
respectively to true horses, Equoidea, and to
the primitive European group, Palaeotheriidae,
have been placed in this genus (Hooker 1994;
Froehlich 2002). Basal members of both these
groups consisted of animals no larger than a small
dog, with a primitive version of the perissodactyl
molar tooth structure consisting of two transverse
lophs connected externally by an ectoloph. Early
Eocene representatives of the other two major lineages,
recognisable by details of their dentitions,
were the titanothere Eotitanops and the moromorph
Homogalax.
For many years the ‘condylarth’ grade
phenacodontids (Fig. 7.7(e) to (g)) have been
accepted as the origin of the perissodactyls, on the
basis of similarities in the bilophodont tooth structure
(Prothero and Schoch 1989). However,
McKenna et al. (1989) described the skull and dentition
of Radinskya (Fig. 7.19), and demonstrated that
the molar teeth are similar to those of primitive
perissodactyls like Hyracotherium and Homogalax.
However, Radinskya also resembles the group of
Chinese ‘condylarth’ grade phenacolophids, usually
regarded as basal embrithopods. This proposed
relationship of phenacolophids and
perissodactyls excludes the phenacodontids from
consideration, and also implies that the perissodactyls
arose in Asia and dispersed to Europe
and North America around the end of the
Palaeocene (Beard 1998). The interrelationships
between this order, other ungulate orders, and
‘condylarths’ continue to be argued over without
agreement (e.g. Thewissen and Domning 1992;
Fischer and Tassy 1993; Holbrook 2001).
The subsequent Eocene radiation of perissodactyls
was spectacular, and the mere six surviving
genera today represent a small fraction of their erstwhile
diversity (Prothero and Schoch 1989). Several
lineages of brontotheres (Titanotheriomorpha)
evolved into very large, rhinoceros-like mammals
(Fig. 7.19(b)), with the skull adorned by a pair of
bony bosses or protuberances at the front of the
head, supposedly used more for intraspecific combat
than for protection (Mader 1997). The molar
teeth remained low-crowned and suitable for
browsing on relatively soft vegetation. Restricted
almost exclusively to North America and Asia,
brontotheres soon disappeared from the fossil
record, at the start of the Oligocene.