The Origin and Evolution of Mammals - Moodle

238 THE ORIGIN AND EVOLUTION OF MAMMALS
with five widely spaced digits bearing small hooves,
and probably associated with a semi-plantigrade
stance.
The Phenacodontidae were the most specialised
herbivorous family of all. They appeared in the
Middle Palaeocene of North America and survived
into the Eocene. Phenacodus (Fig 7.7(e) and (f) also
occurs in the Early Eocene of Europe, and there is a
possible specimen of similar age from Morocco
(Gheerbrant et al. 2001), but the group is not represented
for certain in Asia (Thewissen 1990).
Taxonomically, the phenacodontids are not usually
included with the more conservative herbivorous
families, but as a separate group of more progressive
ungulates, possibly related to the Tethytheria
(Prothero et al. 1988; Janis et al. 1998a), a question
returned to later. Phenacodontids show the extreme
of ‘condylarth’ specialisation for herbivory of both
the dentition and the postcranial skeleton. The posterior
premolars are the most molariform of all. The
molars are very low-crowned with the cusps developed
into crescents or lophs of various patterns in
the different genera. The limbs are well-designed
for cursorial locomotion: elongated, slender, and
bearing reduced first and fifth digits. Phenacodus
(Fig. 7.7(e) and (f)), with a presacral body length up
to 2 m, had the least specialised version of
phenacodontid teeth; others such as the 60 cm long
Meniscotherium (Fig. 7.7(g)) reached the zenith of
‘condylarth’ herbivorous dentition, with its fully
expressed pattern of crescents and lophs on the
crowns.
The phenacolophids are a poorly known, but
phylogenetically very important group of Asian
‘condylarths’ because they have been variously
implicated in the origins of the embrithopod,
tethythere, and perissodactyl ungulate orders
(McKenna and Manning 1977; Prothero et al.1988).
They are known from little more than teeth of Mid-
Late Palaeocene age, the molars of which are
dilophodont, having two transverse lophs or crests.
Eocene ‘condylarths’ from Morocco have been
mentioned. There are also Palaeocene ‘condylarths’
from the Ouarzazate Basin of Morocco (Gheerbrant
1995), but the affinities of these specimens
are unclear. Given the molecular evidence for the
origin of the African ungulate orders as part of a
geographically isolated radiation of Afrotheria, it
will be very important to discover whether these
African ‘condylarths’ are members of groups existing
elsewhere, or are actually basal members of
Afrotheria, related perhaps to the Proboscidea and
Hyracoidea.
Taeniodonta
The taeniodonts were omnivorous mammals occurring
exclusively in the North American Palaeocene
and Eocene (Schoch 1986; Lucas et al. 1998). In
body size they vary from 5 kg to over 100 kg. The
most primitive member is the Early Palaeocene
Onychodectes (Fig. 7.8(a)), in which there is a moderately
large canine followed by widely spaced,
interlocking premolars. The molars are high
crowned with low, bunodont cusps and a roughly
rectangular occlusal surface. The postcranial
skeleton is that of a fairly generalised, non-cursorial
mammal perhaps able to climb well, and Lucas
et al. (1998) suggest that it had a mode of life comparable
to that of the Virginia Opossum. Later
taeniodonts, notably Stylinodon (Fig. 7.8(b)), were
larger and had evolved a much shorter, powerfully
built skull. The canines and upper incisors were
greatly enlarged, open-rooted, and the enamel only
covered the anterior faces, giving them a chiselling
function. The anterior premolars had evolved into
cutting blades while the posterior teeth were simplified
crushing pegs of dentine. The whole skeleton
had become heavily built, and the powerful forelimbs
bore large, flattened claws, presumably
adapted for digging out and consuming roots and
tuberous parts of plants.
The relationships of taeniodonts are obscure.
There is no evidence at all to relate them to ‘condylarths’,
but rather to one of the more conservative
primitive insectivorous taxa. McKenna and Bell
(1997) classify them with palaeoryctids. A more precise
possibility is that they are related to the otterlike
pantolestids, which are themselves believed to
have palaeoryctid affinities.
Pantodonta
The pantodonts (Fig. 7.8(c)) include the very first of
the large, herbivorous placental mammals to evolve
after the Cretaceous, although others were much
smaller, less than 10 kg. The earliest and most primitive
forms are from the Early Palaeocene of China,
where Bemalambda and Hypsilolambda occur in the
Shanghuan Formation (Wang et al. 1998). Bemalambda