The Origin and Evolution of Mammals - Moodle

228 THE ORIGIN AND EVOLUTION OF MAMMALS
and Skutschas 2001) and the Mongolian
Prokennalestes (Fig. 5.17(c) and (d)) is Aptian or
Albian in age, making it perhaps 10 Ma younger
still (Kielan-Jaworowska and Dashzeveg 1989).
Although it is generally supposed from their
early appearance in Asia that this was the area of
origin of placentals, Cifelli (1999) described an associated
pair of dentaries from the Cloverley Formation
of Montana, which is also dated as Aptian-Albian.
Montanalestes (Fig. 5.17(e)), as this specimen is
named, has a submolariform last premolar followed
by only three molars. However, the teeth differ in
several respects from Prokennalestes (Fig. 5.17(d)),
and until material of upper molars is found, a placental
attribution of Montanalestes remains a
biogeographically intriguing, but not totally established
possibility. Another possible, though even
more dubious, very early placental has been described
from isolated molar teeth (Fig. 5.19(c)) from Morocco
and named Tribotherium (Sigogneau-Russell 1995).
It is dated as ?Berriasian, which is the very base of
the Cretaceous. The teeth do have some placental
characters such as the narrow labial shelf of the
upper molar, but lack others such as a third talonid
cusp on the lowers. Were Tribotherium actually to
prove to be a stem placental, it would be important
because it would put the fossil date of divergence of
Placentalia from Marsupialia back to at least the
start of the Cretaceous around 135 Ma. However,
the information available is presently far too sparse
to make such an assertion with confidence.
During the Late Cretaceous, a radiation of placentals
occurred. All remained small in body size,
and fairly conservative in dental and, where known,
cranial structure to such an extent that there is limited
agreement on their classification. Relatively
small differences have been used for ordinal and
even higher level separation, but had they all been
living today most if not all the Late Cretaceous placentals
might well have been incorporated into no
higher a taxon than a superfamily. At one time it
was in fact customary to include them in a single
suborder Proteutheria, as a constituent of the order
Insectivora (e.g. Romer 1966; Kielan-Jaworowska
et al. 1979a). Inevitably, the taxon Proteutheria has
long since disappeared from usage due to its manifestly
paraphyletic nature, based as it necessarily
had to be solely on ancestral dental characters.
Kennalestes and Asioryctes from Asia, and Gypsonictops
and Cimolestes from North America are among the
best-known representatives of the Cretaceous placentals,
and the level of taxonomic uncertainty surrounding
them is well illustrated by a very brief,
recent history of the classification of these four
forms.
● In the definitive review of the time, Kielan-
Jaworowska et al. (1979a) placed Gypsonictops and
possibly Kennalestes in a super-family Leptictoidea.
Asioryctes and Cimolestes were placed together in a
family Palaeoryctidae that was part of another
super-family, Palaeoryctoidea.
● In their comprehensive classification of all mammals,
McKenna and Bell (1997) placed Asioryctes
into a group basal to most of the placental mammals,
Kennalestes and Gypsonictops together in the
same family, Gypsonictopidae, of a super-order
Leptictida, and Cimolestes in an entirely separate
Order Cimolesta.
● Most recently, Kielan-Jaworowska et al. (2004)
have associated the two Mongolian genera Asioryctes
and Kennalestes in separate families of the same
super-order Asioryctitheria. Gypsonictops is classified
in a different super-order Insectivora as a leptictidan,
and Cimolestes in yet a third super-order Ferae.
Many other less comprehensive proposals of relationships
concerning these forms have been made
over the years, and there have also been a number
of suggestions of relationships between specified
Cretaceous placentals and particular groups of living
mammals, considered later.
Irrespective of the disputatious details of possible
interrelationships, there are five main groups of Late
Cretaceous placentals, the primitive form Asioryctes
and its possible relatives, the leptictidans, the
zalambdalestids, the sharper-toothed palaeoryctids,
and the incipiently bunodont-toothed zhelestids.
Asioryctida
Asioryctes has been described from about 10 skulls
and several fragmentary skeletons from the Coniacian
and Campanian of Mongolia and elsewhere in Asia
(Fig. 7.2(a)). It has retained several primitive placental
characters that are modified in all other
known Late Cretaceous forms, including the five
upper and four lower incisors that occur in Eomaia