The Origin and Evolution of Mammals - Moodle

204 THE ORIGIN AND EVOLUTION OF MAMMALS
3 m in height, and bearing a huge head with ferocious,
raptorial beak. The maximum borhyaenid
diversity was reached in the Miocene, with some
two dozen genera described (McKenna and Bell
1997). Prothylacinus was about 80 cm in body length
(Fig. 6.6(e)), Borhyaena jaguar sized, and
Proborhyaena said to be larger than a grizzly bear. At
the other extreme Cladosictis (Fig. 6.6(c)), for example,
was no larger than an otter. From their Miocene
peak onwards, borhyaenids declined in both diversity
and maximum body size, perhaps in part due
to the radiation of the phorusrhacid birds during
this period of time. By the time the new carnivorous
placental immigrants arrived in South America in
the late Pliocene, borhyaenids had already disappeared,
with the one exception of Thylacosmilus
(Marshall and Cifelli 1990).
The genus Thylacosmilus (Fig. 6.6(d)), the marsupial
sabre-tooths, were the most extraordinary
borhyaenids of all, with their remarkable convergence
on the placental sabre-toothed cats. The
upper canine is enormous and, unlike the placental
equivalent, it was continuously growing. When the
jaws were closed it was protected by an equally
large ventral extension of the jaw. At the other end,
the jaw articulation was modified to permit the necessary
gape of 90� and more. Thylacosmilus lived
through the Miocene and Pliocene and were the last
borhyaenids to survive, lasting until about the time
of the entry of placental carnivores from North
America.
Paucituberculata
The third of the ameridelphian orders includes the
living caenolestids or shrew-opossums, but there is
a very much broader range of extinct forms, constituting
about 12 families. Because of the width of the
radiation, there are not many characters that universally
define the group and there is indeed considerable
disagreement about the membership. The main
character is the distinctive molar teeth, which are
relatively broad and bunodont. Two of the stylar
cusps of the upper molars, identified as cusps B and
D, have greatly enlarged to increase the crushing
action of the outer part of the crown. A second trend
in the group is the development of procumbent
lower incisors and canines. The cladistic analysis
of Sánchez-Villagra (2001) is based mainly on the
dentition, and it supports the monophyly of two of
the paucituberculate families, Caenolestidae and
Argyrolagidae. However, no other paucituberculate
families were included.
Reflecting the difficulties of understanding the
interrelationships of the paucituberculates, there is
divided opinion about which are the most basal
members. Several authors regard the superfamily
Caroloameghinioidea as occupying this position,
with the ludicrously named Palaeocene Tiupampan
species Roberthoffsteteria nationalgeographica as the
earliest South American member (Aplin and Archer
1987; Marshall 1987). The North American Late
Cretaceous genus Glasbius has very similar molars,
is usually included in the same superfamily, and
therefore offers an important link between the
faunas of these two continents around the end of the
Cretaceous. Other authors, however, do not even
accept that the caroloameghinioids belong within
Paucituberculata, but argue that they are basal to
the marsupials as a whole, as discussed by Marshall
et al. (1990). Caroloameghina (Fig. 6.7(a)) is known
from complete jaws and dentitions, and exhibits
well the molar characters of its group. In the
uppers, the cusps are bunodont and bulbous, and
the stylar shelf large with equal-sized, enlarged
stylar cusps B and D. In the lowers, the talonid is
short, broad, and of about the same height as the
trigonid. These adaptations were presumably for
an omnivorous, and perhaps specialised frugivorous
diet in a small mammal (Marshall et al. 1983), a
style that was to be inherited or repeated in the rest
of the polydolopoids. Whatever their true relationships,
the caroloameghiniids were a short-lived
group, being unknown after the Early Eocene.
The Palaeocene polydolopoids are the earliest
undisputed members of the Paucituberculata.
Epidolops (Fig. 6.7(b)) has been identified in the
Itaboraí fauna, and Polydolops in the San Jorge
Formation. They are diagnosed by reduction of
the incisors to 3/2, procumbent lower canines, and
development of quadrilateral upper molar teeth
associated with evolution of a new cusp, the
hypocone. Polydolopoids radiated during the first
half of the Eocene but had dwindled practically to
extinction by the Early Oligocene. During this time,
members of the family Polydolopidae also existed
in western Antarctica. Antarctodolops was found some