The Origin and Evolution of Mammals - Moodle

and Prokennalestes, although it does posses the more
derived number of four premolars. In the skull
there is a large lachrymal and a deep jugal bone. In
the postcranial skeleton Asioryctes is uniquely primitive
in its incompletely co-ossified atlas ring, and
in the absence of superposition of the astragalus on
the calcaneum in the ankle that characterises all
other placentals. Instead, the two bones lie side by
side, so the astragalus has full contact with
the ground (Kielan-Jaworowska 1977; Novacek
1980).
The molar teeth are unusual in being exceptionally
transversely widened and narrow from front to
back, so that the paracone and metacone of the
uppers lie very close together. The trigonid of the
lower molars is shorter than the talonid.
The only closely related form is another Mongolian
genus Ukhaatherium, of which there are also several
skulls and skeletons (Novacek et al. 1997). It differs
from Asioryctes in having less extremely narrowed
molar teeth, in which feature it is presumably more
primitive. An epipubic bone has been found in
Ukhaatherium, indicating that this is a primitive feature
of placentals as well as marsupials.
Leptictida
The North American Gypsonictops is only
known from incomplete jaws and isolated teeth
(Fig. 7.2(c)), although these are relatively common
in the Campanian and Maastrichtian. The dentition
is often taken as close to the unspecialised condition
for Late Cretaceous placentals. The number of incisors
is unknown, but the primitive condition of five
premolars is probably present, the posteriormost of
which tends to be well molarised in form. The molar
teeth are uncompressed from front to back, and
have well separated metacone and paracone.
Kennalestes (Fig. 7.2(b)) is from the Coniacian to
Campanian of Asia, and is a great deal better known
than Gypsonictops, on the basis of complete skulls
although not, as yet, postcranial skeletons. Like,
Asioryctes, the skull is primitive in most respects, and
therefore it is not at all certain that Kennalestes should
actually be included in the Leptictida (Novacek
1986a; Kielan-Jaworowska et al. 2004): However, the
premolar and molar teeth are very similar to those of
Gypsonictops. One derived character of Kennalestes,
unknown for the American form, is the reduction of
LIVING AND FOSSIL PLACENTALS 229
the number of incisor teeth to four uppers and three
lowers, from the ancestral 5/4 condition.
Leptictidans continued into the Palaeocene and
Eocene, when they are represented by complete
skull and skeletal material described by Novacek
(1986a), who considered them to be relatives of the
Lipotyphla on the basis of several cranial characters.
Zalambdalestidae
The earliest known zalambdalestid specimens are the
jaws and teeth of Kulbeckia, from the Coniacian of
Uzbekistan, which is dated at 85–90 Ma (Archibald
et al. 2001; Archibald and Averianov 2003), although
the more familiar members are the two Mongolian
genera, Zalambdalestes and Barunlestes (Fig. 7.2(f)),
which are Campanian in age. There are no known
North American representatives.
The family includes the largest of the Cretaceous
placentals, with a skull length up to around 5 cm.
There are only three incisors, and the first lower
one is greatly enlarged and strongly procumbent.
The lower molars have developed a relatively large,
strongly basined talonid. The postcranial skeleton
resembles superficially that of an elephant shrew,
with elongated hind legs indicating a saltational
ability. Kielan-Jaworowska et al. (1979a) speculate
that they lived in a rocky habitat, and the procumbent
lower incisor could extract insects from
crevices.
As far as the relationships of the group are concerned,
they have often been associated with the
anagalids, which occur in the Asian Palaeocene.
More recently, some authors have claimed a relationship
with the Glires, the supraordinal taxon that
includes rodents and lagomorphs (Archibald et al.
2001; Fostowicz-Frelik and Kielan-Jaworowska
2002). The main character supporting this view is
the similarity of the zalambdalestid lower incisor to
that of the Glires. Both are enlarged, procumbent,
and have the enamel restricted to the labial side.
Palaeoryctida
The North American genus Cimolestes (Fig. 7.2(d)) is
usually accepted as a basal palaeoryctidan, an otherwise
largely Palaeocene group of small, but incipiently
more specialist carnivores (Lillegraven 1969).
They are characterised by the development of very