The Origin and Evolution of Mammals - Moodle

206 THE ORIGIN AND EVOLUTION OF MAMMALS
time ago in Eocene deposits of Seymour Island
(Woodburne and Zinsmeister 1982, 1984). More
recently, specimens identified as Polydolops, and
also Perrodelphys which is a member of another
polydolopoid family Prepidolopidae, have been
found (Reguero et al. 2002).
The Caenolestoidea is the superfamily that contains
the only living paucituberculates. Like the
polydolopoids this group also developed quadritubercular
upper molar teeth, but in a different way,
namely by enlargement of the metaconule rather
than development of a new cusp, the hypocone. A
second dental character is the procumbent nature of
the first or second lower incisor, accompanied by
reduction of the first two premolars. With one
controversial exception, caenolestoids did not appear
in the fossil record until the Oligocene, much later
than the other South American superfamilies. The
possible but doubtful exception is the Itaboraí form
Carolopaulacoutoia (�Sternbergia), as interpreted by
Szalay (1994). However, it is known only from its
fairly generalised dentition (Fig. 6.7(d)), which lacks
the characters of caenolestoids just noted. The family
Caenolestidae (Fig. 6.1(c) and 6.7(c)), the shrewopossums
of the modern South American fauna,
retained the basic caenolestoid dentition, but used
it for a generally insectivorous habit.
During the Miocene, two caenolestoid families
evolved teeth adapted for a more specialised diet.
The Abderitidae (Fig. 6.7(e)) possessed a pair of
heavily ridged shearing teeth very similar to those
of multituberculates and of several of the Australian
diprotodonts such as Phalanger. In the case of the
abderitids, these specialised cutting teeth are the
last upper premolar and the first lower molar teeth.
Judging from the nature of the wear facets, and by
comparison with the similar system in Phalanger,
Dumont et al. (2000) inferred that the function of the
shearing system was to deal with both relatively
hard food such as nuts, straw, and particularly
tough insects, and also more pliant material that
tends to stretch and bend rather than sever. From
this they concluded that abderitids were adapted
for a broader, more cosmopolitan diet than
caenolestids. The Palaeotheniidae constitute the
second specialised Miocene family. They also had a
modified last upper premolar and first lower molar
forming a specialised shearing system. However,
they differ from abderitids in the details of the tooth
anatomy, and in having molar teeth also specialised
for a shearing function (Bown and Fleagle 1993).
The final superfamily of the Paucituberculata is
Argyrolagoidea, which includes two highly specialised
families whose closeness of relationship
is doubtful, but which do share certain characters
related to a rodent-like, small herbivore diet. The
argyrolagids (Fig. 6.8(a) to (c)) were superficially
similar to small hopping rodents with greatly
elongated hindlegs (Simpson 1970). The snout is
elongated, possibly indicating a mobile proboscis,
and there is a prominently procumbent pair of
lower incisors which worked against two or three
pairs of sharp-edged upper incisors. These are
followed by a rodent-like diastema. The molar teeth
are high-crowned, hypselodont, and according to
Sánchez-Villagra and Kay (1997) they were adapted
for particularly abrasive plant material, possibly
seeds in the main. Argyrolagids have been found
from the Late Oligocene through the Plio-
Pleistocene (Sánchez-Villagra et al. 2000).
The genus Groeberia (Fig. 6.8(e)) is an extremely
peculiar little mammal from the Late Eocene/Early
Oligocene of Argentina, whose very marsupial status
has been doubted in the past, although the palatal
vacuities, inflected angular process of the dentary,
and four molar teeth seem to settle the question.
It shares little apart from some superficial aspects
of the dentition with argyrolagids, or even with
paucituberculates generally, and Pascual et al.
(1994) regard it as a member of a quite separate
order of presumably ameridelphian marsupials,
which they term Groeberida.
Groeberia was a small animal with the dentition
and jaw musculature highly modified for dealing
with hard food. The rostrum is extremely short and
deep, and there is a bony platform, unique amongst
mammals, extending posteriorly from the fused
symphysis of the two lower jaws, and so providing
a solid floor to the oral cavity. There are two
upper incisors on either side, the first one being
enormous, and described as gliriform in that the
roots extend within the skull as far as the orbit. The
enamel is restricted to the front edge as in rodents,
giving it a self-sharpening ability. The single lower
incisor is similarly constructed. Unlike the rodents,
there is no diastema and the incisors are followed