The Origin and Evolution of Mammals - Moodle

166 THE ORIGIN AND EVOLUTION OF MAMMALS
One is confirmation of the absence of a broad
trough for the postdentary bones, thereby distinguishing
spalacotheriids from the more primitive
Kuehneotherium, although, as in the latter, there is
no angular process on the dentary. The postcranial
skeleton of Zhangeotherium has a very modern style
of shoulder girdle, with scapula spine and large
supraspinatus fossa in front of it.
Sigogneau-Russell and Hahn (1995) described isolated
teeth of a very peculiar, and very early possible
‘symmetrodontan’. Woutersia (Fig. 5.14(b)) is
from latest Triassic, Rhaetian deposits in France. It
has the obtuse-angled ‘symmetrodontan’ pattern of
main cusps, but also greatly enlarged inner cingulum
cusps, a single one on the upper molar, and two
on the lower. Woutersia may be the earliest ‘symmetrodontan’
which would be a considerable extension
backwards of the time range of the group or,
equally likely, an unrelated, specialised holotherian
lineage evolved from a Kuehneotherium-like ancestor.
At the other end of the timescale, symmetrodontans
survived far longer in South America, where
isolated molar teeth of Bondesius (Fig. 5.14(c)) occur
in the Late Cretaceous Los Alamitos Formation of
Patagonia (Bonaparte 1990, 1994). The anterior and
posterior cusps and the cingulum are all reduced in
size, and their relationships are presently obscure.
Shuotherium
In 1982, Chow and Rich described a lower jaw of an
extraordinary Upper Jurassic mammal from China,
which they named Shuotherium (Fig. 5.13(e) to (g)).
The jaw is slender, and very reminiscent of that of
Kuehneotherium, complete with a well-developed
trough that would have housed postdentary bones.
There are three premolar and four molar teeth, or
four and three respectively, according to Kielan-
Jaworowska et al. (2004), and it is the molars that are
so remarkable. Each one has a trigonid of three main
cusps of normal holotherian form, but in addition
there is a large basin extending from the front of
the tooth. This resembles remarkably closely the
talonid of the tribosphenic lower molars of
advanced holotherians, except that there the development
is at the back of the tooth. Wang et al (1998)
subsequently found what appears to be an isolated
upper molar of Shuotherium from the same locality
(Fig. 5.13(f)). Not unexpectedly, it possesses a new,
large, lingual cusp, which would have occluded
with the basin of the corresponding lower molar.
Functionally, the molar teeth of Shuotherium had a
crushing mechanism between a pseudo-protocone
on the upper molar against a pseudo-talonid at the
front of the lower, which is analogous to the mechanism
in true tribosphenic teeth, but quite independently
evolved (Fig. 5.13(g)).
Shuotherium teeth have also been identified in
the Upper Jurassic of England (Sigogneau-Russell
1998). Few have doubted the therian affinities of
Shuotherium, but within that context there has been little
agreement in detail because of the combination of
uniquely specialised molars with a primitive
mandible. Recently, Luo et al. (2002) have raised interest
in the genus by proposing that it is related to those
Gondwanan tribosphenids, and monotremes that
they have somewhat controversially combined as
Australosphenida, an issue discussed at more length
later (page 178).
‘Eupantotheria’
Traditionally, the group Eupantotheria has been used
for those basal therians which had evolved a definite,
but single-cusped, un-basined talonid on the back of
the lower molar teeth, but lack a protocone on the
uppers. As such, these teeth are structurally intermediate
between the simpler Kuehneotherium and symmetrodontans
on the one hand and the tribosphenids
with their fully developed talonid and protocone
on the other. There are also several grades within
the group. As so constituted, ‘eupantotheres’ are a
diverse paraphyletic group. However, as their precise
interrelationships are still uncertain, it remains a useful
informal taxon.
The ‘eupantotheres’ with the least derived dentitions
constitute the several families of the Dryolestida
(Fig. 5.15(a)). The molar teeth are short from front
to back and transversely widened, and the talonid
is small (Fig. 5.15(b)). The family Paurodontidae
includes the only described complete skeleton of a
‘eupantothere’, Henkelotherium from the Late
Jurassic Guimarota mine of Portugal (Krebs 1991).
It was a mouse-sized animal (Fig. 5.15(c)) with a
remarkably modern postcranial skeleton. The
shoulder girdle lacks a procoracoid and interclavicle,
has a reduced coracoid, and there is a fully
developed supraspinous fossa. In the pelvis, the