The Origin and Evolution of Mammals - Moodle

their presence. The five orders may be described
briefly as follows.
Litopterna. The litopterns were relatively conservative
as far as the dentition is concerned, and
the teeth of the earliest members of the group
(Fig 7.10(c)) resemble those of ‘condylarths’, particularly
the South American group Didolodontidae
(Fig. 7.10(b)). Some authors have even included the
latter in the Litopterna. The crowns are generally
low, and the development of lophs weak. In contrast,
the limbs were the most highly evolved of the ungulates,
with a strong tendency to reduce the side toes
leaving the third digit dominant. Of the two groups
of litopterns, the proterotherioideans were remarkable
for their horse-like build and the Miocene member
Thoatherium (Fig. 7.11(a)) possessed a single-toed,
extraordinarily equid-like condition of its limbs. The
other group, the macrauchenioideans are invariably
described as camel-like, with their elongated neck
and limbs, and broad foot consisting of three almost
equal toes. The group is also represented in the
Eocene of Antarctica by teeth of Victorlemoinea.
The Plio-Pleistocene Macrauchenia (Fig. 7.11(b)) has
its nostril opening high up on its skull, which led to
the belief that there was a proboscis present,
although it might alternatively have been associated
with a swampy habitat, or keeping the brain cool by
evaporation. This genus also has the most derived
molar teeth of all litopterns, having become highcrowned
and lophodont.
Notoungulata. The notoungulates were far the most
diverse of the meridiungulate orders, with more than
100 genera in 13 families evolving in the course of the
Cenozoic (Fig. 7.10(f)). At one time, it was believed
that there were Palaeocene notoungulates in China
and North America, which were placed in a primitive
family Arctostylopidae (page 242). However, the dental
similarities on which the proposed relationship
was based are evidently convergent and the arctostylopids
are properly placed in an entirely unrelated
placental order of their own (Cifelli et al. 1989; Cifelli
and Schaff 1998). This leaves notoungulates as exclusively
South American, with the single exception of the
Pleistocene Mixotoxodon, which briefly extended its
range into central America.
Notoungulates are categorised by a generally
broad, flat skull, and unique features of the auditory
LIVING AND FOSSIL PLACENTALS 245
region. The molar teeth tend to be strongly
lophodont, and full hypsodonty is commonly
developed. The most basal group, the Notioprogonia,
were medium-sized, heavily built, and digitigrade
animals (Fig. 7.12(a)). In the course of the Tertiary
three groups evolved from notioprogonian-like
ancestors. The toxodonts included the largest of all
South American ungulates, with the late surviving
Toxodon (Fig. 7.12(e)) the size and build of a large
rhinoceros. Its incisor teeth were chisel-shaped and
the molars hypsodont, indicating a low-level browsing
and grazing habit. The typotheres (Fig. 7.12(b) and (c))
were rather like giant rodents, with loss of the anterior
dentition except for broad, open-rooted, chisel shaped
incisors, and again hypsodont molars. The third
group, the hegetotheres (Fig. 7.12(d)), resembled the
typotheres in dentition, but tended to develop elongated
hind legs, giving them a rabbit-like appearance.
Astrapotheria. The astropotheres were very large
ungulates, with the best-known genus, the Oligocene
Astrapotherium (Fig. 7.10(e) and 7.11(e)), some 3 m
in length. The upper incisors of astropotheres are
absent and presumably there was a horny pad
against which the lower incisors bit. The upper
canines are huge and tusk like while the lowers are
less enlarged but still prominent. The last two
molar teeth evolved enormous size. The nostrils
have shifted to the top of the skull, suggesting the
possibility of a proboscis as in the litoptern
Macrauchenia. The postcranial skeleton was
extremely peculiar. The forelegs were very stout
and strongly built, but the hindlegs relatively slender.
Romer (1966) suggested the possibility of an
amphibious mode, perhaps comparable to hippos.
Astropotheres survived into the Late Miocene, and
are one of the two ungulate orders represented by
teeth in the Eocene of Antarctica.
Pyrotheria. The pyrotheres (Fig. 7.10(g) and 7.11(d))
were also very large and elephant-like in general
form (MacFadden and Frailey 1984). The similarity
is enhanced by the tusk-like incisors, two upper and
one lower on either side. The six cheek teeth
resembled those of primitive proboscideans in the
transversely expanded, bilophodont condition.
Compared to other South American ungulate orders,
pyrotheres were short-lived, appearing in the early
Eocene and disappearing during the Oligocene.