The Origin and Evolution of Mammals - Moodle

time, which suggests that it was on its shores that
Cetacea originated (Thewissen 1998; Gatesy and
O’Leary 2001). These early whales are referred to as
the Archaeoceti, which is probably a paraphyletic
group because it is based on ancestral characters.
The dentition is relatively differentiated, and
includes molar teeth recognisable as modified
ungulate in form. Hindlimbs are still present to various
degrees, which casts light on both the relationships,
and the mode of evolution of these most
highly specialised of mammals.
The earliest specimen yet collected is of
Himalayacetus, from the 53 Ma Early Eocene of
northern India (Bajpai and Gingerich 1998).
Unfortunately it consists only of a fragment of
lower jaw with the second two molar teeth in
place, and nothing at all is known of its postcranial
skeleton. The 5 Ma younger pakicetids are reasonably
well known and they possess the least modified
and therefore the most terrestrially adapted of
whale skeletons. Pakicetus (Fig. 7.21(c)) is identifiable
as a cetacean from the structure of the braincase
and ear region, and various features of the
cheek teeth such as reduction of the paraconid and
metaconid, and presence of only a single talonid
cusp of the lower molars. Most spectacular, however,
is the postcranial skeleton, which was still
fully adapted for terrestrial life (Thewissen et al.
2001). Pakicetus was about the size of a wolf, with
full length, slender limbs of terrestrial ungulate
proportions and unmodified pectoral and pelvic
girdles. The vertebrae are variable in size, and the
zygapophyses indicate a stiff thoracic and lumbar
column, as in modern cursorial ungulates. This
structure of the skeleton, and the absence of fisheating
adaptations of the dentition, or a specialised
aquatic ear suggest that Pakicetus was no more
adapted for an aquatic habitat than many terrestrial
ungulates.
Other archaeocetes had become adapted for an
amphibious mode of life. The Middle Eocene
Ambulocetus (Fig. 7.21(d)) had reduced limbs, but
they were still large enough to support the body
weight on land, and the pelvis was attached to the
vertebral column (Thewissen et al. 1994, 1996).
However, the feet were large and could probably
have paddled effectively, and though there was a
normal tail so could not have been a tail fluke, the
LIVING AND FOSSIL PLACENTALS 265
structure of the vertebral column indicates that it
was capable of extensive dorso-ventral flexion and
extension.
The protocetid genera Artiocetus (Fig. 7.21(a)) and
Rodhocetus (Fig. 7.21(e)) had comparable sized
limbs to Ambulocoetus, but larger fore and hind feet,
which were probably webbed (Gingerich et al.
2001). Like Ambulocetus, they would still have been
able to move on land, but very clumsily and
presumably more infrequently, after the style of
a modern seal.
Basilosaurus (Fig. 7.21(f)) is a Middle Eocene
Egyptian whale, in which a minute hindlimb is still
present (Gingerich et al. 1990). The individual bones
are recognisable, although several of them have
fused. The tiny pelvis, like that of modern whales,
has lost its attachment to the vertebral column. The
forelimb is also small, and could only have functioned
in the control of undulatory swimming.
Thus, Basilosaurus must have possessed a tail-fluke,
and the modern mode of cetacean locomotion had
finally evolved.
The subsequent fossil history of cetaceans is relatively
uncontroversial (Uhen 1998). The radiation
from the Middle Eocene onwards of the basilosaurid
whales includes the origin of the two modern
groups, Mysticeti (baleen whales) and Odontoceti
(toothed whales). Both appear as fossils in the
Oligocene, and share numerous characters indicating
their monophyly.
The limbs of the primitive archaeocete whales
possess the artiodactyl structure (Fig. 7.21(b)). Even
the highly reduced hindlimb of Basilosaurus can be
seen to be paraxonic in structure, although it is too
modified to reveal whether it had evolved from a
fully artiodactyl form, or is consistent with an
origin from the more primitive, mesonychid ankle
(Gingerich et al. 1990). However, the much less
reduced, and still functional ankle joints of
the other genera have the trochleated facet of
the astragalus for the navicular, and the unique
arrangement of the astragalus–calcaneum joint
that unambiguously define Artiodactyla (Rose
2001). Furthermore, the principal axes of the
feet are clearly distinguishable. In the forefoot, the
third digit is the largest, indicating a mesaxonic
type of foot. In the hindfoot, the third and fourth
digits are equally large, which is the paraxonic