The Origin and Evolution of Mammals - Moodle

262 THE ORIGIN AND EVOLUTION OF MAMMALS
The Equoidea also radiated in the Eocene,
particularly in Europe where Palaeotherium was a
common, superficially tapir-like browsing animal.
However, a second major radiation followed in the
Miocene, centred in North America but with representatives
throughout Europe, and dominated by
grazing forms with hypsodont teeth culminating in
the modern genus Equus (McFadden 1992, 1998).
The third major lineage of perissodactyls are the
Moromorpha, which is the most diverse. The chalicotheres
were another group of bizarre perissodactyls
(Coombs 1998). Although present in the
Eocene, their major radiation occurred in the
Miocene, when large body size evolved, along with
retractable claws in place of hooves. Moropus was
like a very large horse with its forelimbs somewhat
longer than the hindlimbs. This trend reached its
culmination in the European and African
Chalicotherium Fig. 7.19(c)), in which the forelimbs
were about 50% longer than the hindlimbs and the
animal must have been capable of standing on its
hindlegs to pull down branches from quite large
trees. Chalicotheres survived into the Pleistocene
of Asia and Africa and indeed there have been
occasional, unsubstantiated reports from Kenya
of sightings of a living chalicothere (Savage and
Long 1986).
Rhinocerotoids were far more diverse from the
Eocene through the Miocene than they are now. In
addition to the familiar horned rhinoceroses of
today, they included small, agile, dog-sized
browsers, semiaquatic hippo-like grazers, tapirlike
forms, and gigantic forms capable of browsing
on high trees (Prothero and Schoch 1989). Hyrachyus
is the earliest and most primitive rhinocerotoid,
abundant throughout North America and Eurasia
during the Early Eocene. The indricotheres were a
purely Oligocene Asian group of huge mammals,
amongst which is included Paraceratherium, variously
referred to in the past as Indricotherium and
Baluchitherium. It was a strictly quadrupedal,
graviportal animal but with a height reaching over
6 m, a 1.3 m long skull, and a body weight estimated
at around 30 tonnes, it was capable of browsing
on leaves and twigs well above the reach of any
other mammals of its day. By the Late Oligocene
and through the Miocene, the true rhinoceroses
radiated and became an abundant part of the
ungulate population of all the northern continents
and Africa (Heissig 1989).
The final group of perissodactyls to mention are
the tapirs, the sister group of the rhinocerotoids. As
with perissodactyls as a whole, the paucity of modern
species, no more than four in this case, is a small
echo of a once much more diverse group (Schoch
1989; Holbrook 2001). The greatest diversity of
tapiroids occurred during the Eocene, when there
was an abundance of genera across Eurasia and
North America. They remained conservative,
browsing forms and their only remarkable feature
was a tendency to evolve a proboscis, or short
trunk.
Cetartiodactyla
Despite the difference between living whales and
the even-toed ungulates, there is overwhelming
molecular evidence that the former are taxonomically
nested within the latter. As it happens, the fossil
evidence is also tending to point to this
conclusion. The first known cetartiodactyls are the
Early Eocene ‘Dichobunidae’, almost certainly a
paraphyletic group including relatives of more than
one of the subsequent lineages (Gentry and Hooker
1988; Stucky 1997). Diacodexis (Fig. 7.20(c)) is the
most completely preserved of these. It was a rabbitsized
mammal occurring in Eurasia and North
America. Despite its small size, the long, slender
limbs, and digitigrade stance indicate a cursorially
adapted form and the single most characteristic
artiodactyl feature (Fig. 7.20(b)), an astragalus
with pulley-shaped articulating surfaces for the
tibia above and the rest of the foot below was present
(Rose 1982, 1987). This arrangement effectively
defines what is termed the paraxonic foot, in which
the main axis runs between digits three and four.
The dentition, and indeed the skull generally of
Diacodexis, was little advanced from the ‘condylarth’
grade and gives little away about the origin
of the artiodactyls. An earlier proposal by Van
Valen (1971) that they were related to the arctocyonid
‘condylarths’ has received little support (Rose
1987; Prothero et al. 1988; Thewissen and Domning
1992).
From this Early Eocene start, the artiodactyls
radiated into around 20 families (Fig. 7.20(a)),
including the ten that are usually recognised