The Origin and Evolution of Mammals - Moodle

240 THE ORIGIN AND EVOLUTION OF MAMMALS
was the size of a large dog, with a skull about 20 cm
in length. In North America, the group makes its
appearance in the middle of the Palaeocene, and it
survived into the Middle Eocene as represented by
Coryphodon. Teeth of pantodonts also occur in the
Palaeocene Tiupampan fauna of South America
(Muizon and Marshall, 1987, 1992) and the Eocene of
Antarctica (Reguero et al. 2002).
The dentition of pantodonts is complete, with
modest canines and no diastema. The most characteristic
feature is the development of lophs on
the postcanine teeth. In the more primitive
bemalambdids of Asia, the lophs are V-shaped, but
in all other forms, collectively classified as
Eupantodonta, the molar tooth lophs have evolved
a W-shape (Lucas 1993). The postcranial skeleton
varies from that of the lightly built, possibly
arboreal genus Archaeolambda, which is known
from a complete Late Palaeocene skeleton from
China, to the massive, graviportal Barylambda that
is often likened to a giant ground sloth, complete
with a massive pelvis and heavy tail that suggest a
habit of browsing on high vegetation.
Despite their large body size and herbivorous
habit, there are no dental or postcranial features
suggesting that pantodonts are related to the ‘condylarths’
as was once believed. McKenna (1975) followed
more recently by Lucas (1993, 1998) proposed
that, like the taeniodonts, pantodonts are possibly
related to the palaeoryctid group of small Cretaceous
insectivorous mammals, on the basis of features of
the dentition shared by the primitive pantodonts
and a form such as Didelphodus.
Tillodonta
The tillodonts are another group of mainly
Palaeocene herbivores, whose most primitive members
such as Lofochaius occur in the Early Palaeocene
of China, where they survived until the Late Eocene
(Lucas 2001). They make their appearance in North
America in the latest Palaeocene, where they underwent
a brief radiation in the Early to Middle Eocene
before becoming extinct. Early Eocene tillodonts are
also found in Europe, but so far none have turned up
in South America (Lucas and Schoch 1998a).
Tillodonts varied from small, with a skull length
of only 5 cm, to large mammals such as the bearlike
Trogosus (Fig. 7.9(a)), which has a 35 cm skull
and an estimated body weight of about 150 kg. The
tillodont dentition (Fig. 7.9(b)) is highly distinctive,
particularly due to the enlarged, chisel-like second
incisors in which the enamel is restricted to the
anterior edge. The other incisors, canines and
anterior premolars tended to be lost, leaving only
the square, hypsodont molars with a dilambdodont
pattern of crests, which rapidly wore down. The teeth
are carried in powerfully built jaws, and the body as
a whole was heavily built, with well-developed and
recurved claws. All the indications are of a ground
grubbing mammal feeding on roots and tubers.
At one time tillodonts were regarded as derivatives
of ‘condylarths’, but there are no clear ungulate
characters. The similarity of the dilambdodont
molars of tillodonts and pantodonts has suggested
to several authors that the two are sister groups
(Chow and Wang 1979; Lucas 1993), though the
evidence is not very convincing: the similarities may
be convergent in two groups independently derived
from palaeoryctid-like ancestors. The biogeography
of the group has been investigated by Schoch and
Lucas (1982; Schoch 1986), who argue that it probably
had an Asian origin. The similarity between the
Chinese Meiostylodon and the North American
Esthonyx can be explained by a dispersal into North
America by the Late Palaeocene. They also suggested
that the relatively early demise of the group
in North America was due to competition from the
similarly adapted taeniodonts.
Dinocerata
The dinoceratans include the largest of all the
archaic Palaeocene–Eocene herbivores, ranging in
estimated body weight from 175 kg to 4.5 tonnes.
The earliest record of this short-lived group is
Prodinoceros from the latest Palaeocene of both
China and North America (Lucas and Schoch 1998),
and none survived beyond the Middle Eocene.
Although massively built, Prodinoceros (Fig. 7.9(c))
was relatively small for the group and lacked the
bony protuberances of the skull so characteristic of
the later members, such as the huge mid-Eocene
Uintatherium (Fig. 7.9(d)), which occurred in North
America and Asia, although not in Europe. The
dentition of dinoceratans shows a tendency to
reduce and eventually lose the upper incisors, but to
retain and enlarge the upper canine. The premolars