The Origin and Evolution of Mammals - Moodle

Xenungulata. Only two Late Palaeocene genera are
presently included in the xenungulates. The best
known, Carodnia (Fig. 7.10(h) and 7.11(c)), has at
least superficial similarities to the pyrotheres (Cifelli
1993b). It was large, with chisel-like incisors, and
basically bilophodont cheek teeth.
Palaeanodonta
The palaeanodonts are a Late Palaeocene and
Eocene group of North American mammals. They
were small to moderate in size and had robustly
built skeletons, with heavily built limbs and strong
claws, clearly adapted for a fossorial life. The teeth
were reduced in number, size, and enamel, trends
most highly expressed in Eocene members such as
Metacheiromys (Fig. 7.13(b)). All these features suggest
that the palaeanodonts were an early placental
group adapted for ant-eating, though whether the
resemblances to xenarthrans and pholidotans indicate
relationship or convergence has been much
debated. Rose and Emry (1993) suggested that,
while they may be related to pholidotans, there is
certainly no good evidence for a relationship with
xenarthrans. As far as the origin of palaeanodonts is
concerned, Rose and Lucas (2000) described a relatively
complete skeleton of Escavadon (Fig. 7.13(a)), a
0.5–1 kg specimen from the Middle Palaeocene of
North America, and concluded that it is the most
primitive palaeanodont known. It has a number of
postcranial similarities to other members of the
group, but the dentition is much more primitive
compared to a typical palaeanodont. In the latter
respect, the authors note certain similarities to the
Palaeocene otter-like pantolestids.
There is also a very early Chinese form, Ernanodon
(Fig. 7.13(c)), which was regarded as a palaeanodont
by Radinsky and Ting (1984). However, the similarities
to any other placental anteaters are believed by
Rose and Emry (1993) to be no more than superficial
convergences for a fossorial mode of life.
Carnivorous mammals: Creodonta and Carnivora
The earliest mammals to have increased in body
size and adapted for carnivory were amongst the
‘Condylarthra’ discussed above, culminating in the
mesonychids. In addition to these, members of two
more or less exclusively carnivorous placental orders
also made their appearance during the Palaeocene,
LIVING AND FOSSIL PLACENTALS 247
although members of neither actually achieved large
size until later. The two orders are the Creodonta,
stigmatised as archaic because they did not survive
beyond the Miocene, and the Carnivora, which
eventually radiated to become the dominant terrestrial
carnivores of today. Most accept that the two
are related as a monophyletic group termed Ferae,
although almost equally as often admit that the
characters supporting the relationship are not very
impressive (Flynn et al. 1988; Wyss and Flynn 1993).
They include restriction of the specialised, shearing
function of carnassial teeth to a limited part of the
postcanine dentition, although the actual teeth
involved differ from group to group. In the
Carnivora it is invariably the upper PM4 and lower
M1 that are the major carnassials. In creodonts
there tends to be some shearing along the entire
molar row, but with emphasis on either the upper
M1 and lower M2 as generally in oxyaenids, or
the upper M2 and lower M3 as generally in the
hyaenodontids. These differences indicate convergent
evolution of the specialised carnassial teeth.
Other shared characters of carnivorans and
creodonts are a bony lamina, called the osseous
tentorium, dividing the cerebellar from the cerebral
hemisphere regions of the cranial cavity, certain
details of the tympanic bulla and the course of the
internal carotid artery, and aspects of the structure
of the ankle joint.
The possibility of a sister group relationship
between Carnivora and Creodonta became less likely
with the description of teeth of Early Palaeocene
carnivoran mammals from Canada by Fox and
Youzwyshyn (1994). Pristinictis (Fig. 7.14(d)) is
described as a primitive member of the basal group
Viverravidae, and there is a similar form,
Pappictidops, from the Early Palaeocene Shanghuan
Formation of China. Ravenictis is more primitive than
any other known member of the Carnivora. These
specimens lack any dental features shared uniquely
with the Creodonta, and therefore the Carnivora
must have evolved from a form even more primitive
in its molar structure than creodonts. This conclusion
accords with that of Wyss and Flynn (1993), who
pointed out a number of braincase similarities
between primitive members of the Carnivora, the living
insectivorous Eulipotyphla, and the early
Cenozoic insectivorous Leptictida (Novacek 1986a).