The Origin and Evolution of Mammals - Moodle

230 THE ORIGIN AND EVOLUTION OF MAMMALS
high, sharp cusps and crests on the molar teeth. The
uppers are transversely expanded and the paracone
is markedly taller than the metacone. In the lowers,
there is a very marked difference in height between
the trigonid and the talonid. This tendency to
develop transversely oriented shearing edges suggests
that the teeth were designed for coping with
relatively hard insects, or small vertebrate prey.
Zhelestidae
The family Zhelestidae (Fig. 7.2(e)) consists of a series
of small mammals that have incipiently ungulate
molar teeth (Nessov et al. 1998). The majority of the
numerous zhelestid genera recognised, for example,
Zhelestes and Aspanlestes, are from the Turonian-
Coniacian of Central Asia, approximately 88 Ma
(Averianov 2000). Younger, Maastrichtian-age genera
occur in Western Europe (Gheerbrant and Astibia
1994), and also in North America where they extend
from the Late Cretaceous into the Palaeocene
(Nessov et al. 1998).
There are five upper premolars. The crowns of the
upper molars are broad, and subrectangular in
shape. This was achieved by antero-posterior expansion
of the protocone region of the tooth, and a
wide separation of that cusp from the paracone and
metacone. In the lower molars, the trigonid is relative
lower, and so is more nearly equal in height to
the talonid, while the talonid is expanded to about
the same antero-posterior length as the trigonid.
Zhelestids were presumably tending to omnivory
in habit, their teeth adapted for high-energy plant
food as well as insects.
Although none are yet known from more than
teeth and jaw fragments, zhelestids have assumed
great significance as possible basal members of a single
radiation of ungulate mammals, culminating
ultimately in such orders as artiodactyls, perissodactyls,
elephants, and even whales. Indeed, Nessov
et al. (1998) concluded from their cladistic analysis
of dentitions that Zhelestidae is paraphyletic, and
contains basal relatives of all these later forms.
It was thought for a long time that there are true
ungulate mammals in the latest Cretaceous of North
America, and in particular the renowned Bug Creek
Protungulatum (Fig. 7.6(a)). In fact, while its ‘condylarth’,
and therefore ‘ungulate’ credentials are
not in much doubt, and it is more advanced than
zhelestids, Protoungulatum is now believed to be Early
Palaeocene in age (Archibald and Lofgren 1990).
Palaeocene fossils: the archaic
placentals
The mass extinction event at the end of the
Cretaceous Period that marked the demise of almost
all the North American marsupials had far less
effect on the placentals; indeed quite the opposite.
The handful of Maastrichtian placental genera on
that continent increased dramatically in the Early
Palaeocene (Alroy 1999b), the result of a combination
of survival, evolutionary radiation, and immigration.
The picture is less clear elsewhere for want
of fossil beds that span the K/T boundary. Rich
Early Palaeocene placental faunas occur in the
Shanghuan Formation of China (Lucas 2001), and at
Tiu Pampa in Bolivia, the latter in association with
the marsupials found there. In Europe, mammals
are unknown until the Middle to Late Palaeocene of
Hainin in Belgium and Cernay in France (Agustí
and Antón 2002). The African Palaeocene is even
more poorly represented, being largely restricted to
the modest fauna of the Ouarzazate Basin of Morocco
(Gheerbrant 1992, 1995).
Cretaceous survivors
Many of the new genera were members of preexisting
Late Cretaceous groups, notably leptictidans
and palaeoryctidans. Small, insectivorous leptictidans
occur abundantly in North American Early
Palaeocene mammal faunas, and persisted through
the Eocene and into the Oligocene. The skull of the
Late Eocene Leptictis, which has been described in
full detail by Novacek (1986a), illustrates the conservative
structure. The group is also found in
Europe, where again they survived into the
Oligocene. There are superbly well-preserved, complete
skeletons of Leptictidium in the Eocene
Lagerstätten at Messel in Germany (Koenigswald et
al. 1992). It was a relatively large form (Fig. 7.3(a)),
up to 90 cm in presacral length, and possessed very
long hind legs and short fore legs, indicating a
saltational habit resembling modern jerboas. There
is one family present in the Chinese Palaeocene,
Didymoconidae, that is regarded by McKenna and
Bell (1997) as a constituent of Leptictida.