The Origin and Evolution of Mammals - Moodle

Cretaceous, (Aptian or Albian) of Mongolia. Apart
from an isolated molar, Kielantherium (Fig. 5.16(c)
and (d)) has been described from a partial lower
jaw with the posteriormost four teeth in place.
While the full dental formula cannot be determined
from this specimen, the similarity of all the preserved
teeth to one another suggests that they are
all molars. Further forwards there are alveoli for
four more teeth, all presumably premolars, and
Dashzeveg and Kielan-Jaworowska (1984) argue
that these are the second to fifth premolars, with the
section of the jaw that housed the first premolar
missing. Therefore they proposed that the basic
tribosphenidan tooth formula consists of five premolars
and four molars. The trigonid of the lower
molars is still much the dominant part of the tooth,
with a very tall protoconid and the metaconid higher
than the paraconid. The relatively small talonid
bears only two cusps, but unlike Peramus is in the
form of a completed basin.
There is a possible candidate for an even earlier
tribosphenidan than these. Tribotherium (Fig. 5.19(c))
is known as isolated mammalian teeth of Berriasian
age, right at the base of the Cretaceous, from Morocco
(Sigogneau-Russell 1995).
Aegialodon, Kielantherium, and possibly Tribotherium
are placed in the family Aegialodontidae, which
accordingly represents the most primitive group of
the Tribosphenida, a taxon formally defined by
the presence of a protocone on the lingual side of the
upper molar working against a basined talonid on the
posterior side of the corresponding lower molar. (Luo
et al. 2001a; Kielan-Jaworowska et al. 2004) have
replaced this name with a new name, Boreosphenida,
on the grounds that they have removed a very small
number of certain tribosphenid-like taxa and placed
them in a completely different group they name
Australosphenida, a move not universally accepted
yet, and not very good grounds for abandoning a
familiar, long-established name.
The subsequent evolution of the Tribosphenida
involved a complex radiation of mammals (Fig. 5.20),
whose teeth show further degrees of enlargement
of the protocone and talonid, the addition of minor
cusps and crests to various parts of the teeth, and
modifications to the primitive dental formula. The
precise interrelationships of the several lineages are
obscure, particularly because many are still only
THE MESOZOIC MAMMALS 169
based on isolated teeth. It is clear that the best
known groups of tribosphenidans, namely the placentals
(eutherians) and marsupials (metatherians)
respectively, represent only two of these lineages,
albeit the two that eventually survived into the
Tertiary to diverge into the mammalian fauna of
today. Other lines evolved independently, survived
briefly, and disappeared without further trace by the
close of the Cretaceous (Clemens and Lillegraven
1986; Kielan-Jaworowska and Cifelli 2001).
Placentalia
There is some discrepancy between the use of the
respective names Placentalia and Eutheria. McKenna
and Bell (1997) regard them as synonyms and prefer
the former. Others, particularly Kielan-Jaworowska
et al. (2004) discriminate between the two, using
Placentalia for the living groups and their fossil
relatives, and Eutheria for the more inclusive group
consisting of these plus their stem-group. The problem
is that the relationships of the various early fossil
members to living placentals is far from safely
established, and therefore Placentalia in Kielan-
Jaworowska et al’s sense may be a paraphyletic
group. McKenna and Bell’s terminology is therefore
adopted in the present work.
Placentals are recognisable from the presence of a
semi-molariform last premolar, only three molars,
and the cusp pattern (e.g. Fig. 5.17(c)). As described
in more detail in a later chapter (page 226), the earliest
recorded placental is Eomaia from China (Fig.
5.17(a) and (b)), which is believed to be Barremian
in age and therefore 125–130 Ma (Ji et al. 2002).
Murtoilestes from Russia (Averianov and Skutschas
2001) is believed to be only 4 or 5 younger, occurring
as it does around the Barremian–Aptian boundary.
Younger Early Cretaceous placentals occur in
Aptian-Albian rocks, in the form of Prokennelestes
(Fig. 5.17(c) and (d)) from Mongolia, and probably
Montanalestes (Fig. 5.17(e)) from North America
(Cifelli 1999).
Once into the Late Cretaceous, placentals started
to diversify in both Asia and North America, as discussed
in detail in a later chapter. Following 40 years
on in the footsteps of the celebrated American
Museum expeditions of the 1920s (Colbert 2000),
Zofia Kielan-Jaworowska and her colleagues collected
numerous skulls and skeletons of a range of