The Origin and Evolution of Mammals - Moodle

(a)
Leptictidium
Palaeoryctidans were also common in the North
American Palaeocene and Eocene, and present in
Europe. None have yet been found in China,
although there are teeth of a family of primitive
insectivores, Micropternodontidae, which Butler
(1988) regarded as derived from palaeoryctids.
Others dispute this relationship (e.g. McKenna and
Bell 1997). The shrew-like Eoryctes, described from
a complete Early Eocene skull (Thewissen and
Gingerich 1989), is a typical micropternodontid.
Of the other Cretaceous placental groups, a few
zhelestid teeth have been found in the Early
Palaeocene of North America, but neither asioryctidans
nor zalambdalestids appear to have survived.
In addition to pre-existing groups, there were
several new lineages that were related to, but taxonomically
distinct from the Cretaceous groups. The
majority of these still consisted of relatively small,
basically insectivorous or omnivorous mammals.
However, within as little as 2 or 3 million years
after the start of the Palaeocene, several groups of
radically new kinds had arisen, and entered habitats
not previously occupied by mammals during
any of their previous 145 Ma of existence. There
(b)
Buxolestes
Figure 7.3 Two Early Eocene mammals from the Messel Oil
Shales. (a) The leptictidan Leptictidium (Koenigswald et al. 1992).
Presacral length approx. 30 cm. (b) The pantolestidan Buxolestes,
with occlusal view of upper molars and lateral view of
dentition. Total length of body approx. 80 cm (Savage
and Long 1986).
LIVING AND FOSSIL PLACENTALS 231
were middle to quite large-sized herbivorous groups,
and dog-sized to small bear-sized carnivores.
Anagalida
The anagalidans were an Asian group, appearing in
considerable diversity in the Early Palaeocene of
China. They were small herbivores or omnivores,
with somewhat enlarged, procumbent lower
incisors (Fig. 7.4(a)). The premolar and molar teeth
are broad, low-crowned, and hypsodont. Their
evolutionary affinities are not clear, but several
authors accept the relationship with the Asian
Cretaceous zalambdalestids that was proposed by
Szalay and McKenna (1971).
Mixodonta
The mixodonts are also an exclusively Asian group of
small omnivorous mammals, also possibly related to
zalambdalestids. They are notable for their fully gliriform
incisor teeth, and for this reason are widely
regarded as related to the origin of the super-order
Glires (rodents and lagomorphs). Li and Ting (1993)
go so far as to claim that different families of
mixodonts are relatives of lagomorphs and rodents
respectively. A mimotonid, such as the Early
Palaeocene Mimotona (Fig. 7.4(b)), has the dental
formula, 2/2: 0/0: 3/3: 3/3, along with several
cranial characteristics of lagomorphs. In contrast, the
early eurymylid Heomys (Fig. 7.4(c)) has a rodent-like
dentition including a single upper incisor and only
two lower premolars. There are also characters of the
postcranial skeleton that resemble those of rodents.
Pantolestida
The pantolestidans are almost, if not exclusively
North American and European in occurrence,
appearing in the Middle Palaeocene and surviving
into the Oligocene (Koenigswald 1980). Otter-like in
general size and proportions, the canines were well
developed and while the molars of the earlier forms
had sharp cutting edges, they tended to evolve a
lower, broader form coupled with powerful adductor
musculature suggestive of mollusc eating. The
Eocene Buxolestes (Fig. 7.3(b)) from Messel has fish
remains preserved in its stomach (Koenigswald et al.
1992). The relationships of pantolestidans are unclear,
though it is generally assumed that they are a highly
derived lineage related to palaeoryctidans.