The Origin and Evolution of Mammals - Moodle

152 THE ORIGIN AND EVOLUTION OF MAMMALS
patella groove on the end of the femur, and the two
proximal ankle bones, astragalus and calcaneum lie
side by side, rather than the astragalus superposed
on the calcaneum to create a joint for supination of
the foot. However, the shoulder girdle has evolved
a true supraspinatus fossa anterior to a scapular
spine, a character of mammal groups more progressive
than morganucodontans and docodontans.
Amongst the original jaws known from the classic
Middle Jurassic Stonesfield Slate of Oxfordshire,
first described early in the nineteenth century, there
are two kinds that have the three cusps of the molar
teeth in a line, but as in Jeholodens, the middle one is
higher than the anterior or posterior ones. They
have symmetrical premolars, and lack the tongue
and groove interlocking between the molars that is
found in triconodontids. Thus Amphilestes (Fig. 5.7(b))
and Phascolotherium, are placed in the separate family
Amphilestidae.
Forms with similar teeth have been described
from North America, including two incomplete
skeletons of Gobiconodon (Jenkins and Schaff 1988).
Gobiconodon (Fig. 5.8(a)) was a relatively very large
Mesozoic mammal, even more so than Triconodon.
The skull length was about 10 cm, and the presacral
body length 35 cm and quite robustly built. Like the
amphilestids, the central cusp of the molar teeth
of Gobiconodon is taller than the accessory cusps
(Fig. 5.7(d)), but differs in being slightly offset medially
from the latter, causing a slight degree of obtuse
triangulation of the three cusps. The adjacent lower
molars have evolved a different interlocking system
from that found in triconodontids, for here the back
of one molar tooth bears a cingulum cusp that
inserts between a pair of cingular cusps on the front
of the next tooth. Isolated jaw fragments and teeth
attributed to Gobiconodon have been found in
Mongolia (Kielan-Jaworowska and Dashzeveg 1998),
and a somewhat younger, Early Cretaceous Chinese
genus, Repenomamus, has been described that is very
similar to Gobiconodon but was of even larger body
size, with a 11 cm long skull (Wang et al. 2001). As a
whole, this group seems to have consisted of the
nearest thing to specialised carnivores that evolved
amongst the Mesozoic mammals. With a body size
that of a smallish cat and sharp almost carnassiallike
teeth, they were no doubt capable of capturing
and consuming other, smaller mammals.
The third group of triconodont mammals to
consider is represented only by Austrotriconodon,
from Late Cretaceous times in South America
(Bonaparte 1994). At present they are known from
isolated teeth that occur quite commonly in the
Los Alamitos Formation of Patagonia, which is
probably Campanian in age. The molariform teeth
(Fig. 5.7(e)), while basically triconodont in form, have
the middle cusp very much larger, and the other
cusps relatively very small. Despite the obvious biogeographical
interest, it is not possible to be at all
confident about their relationships to other triconodont-toothed
mammals.
Multituberculata
Multituberculates are unambiguously recognisable
as a specialised, monophyletic group defined by
very clear-cut characteristics of the skull, dentition
and, as more specimens are described, by the postcranial
skeleton as well. They are superficially
rodent-like in structure and inferred habits, and
range in size from that of a small mouse to that of a
marmot. They make their first appearance in the
fossil record relatively late for a major Mesozoic
mammal group, in the Middle Jurassic of Europe
and Upper Jurassic of North America, but having
remained abundant throughout the Cretaceous,
several lineages survived the end-Cretaceous and
occur well into the Tertiary. Indeed the group
reached considerable diversity in the Palaeocene,
while the youngest record is from the North
American Late Eocene (at one time believed to be
Oligocene, Krishtalka et al. 1982, but see Swisher
and Prothero 1990). Geographically, the vast majority
of fossils are from Europe, North America, and
Asia. Gondwanan representation is limited to one
or two possible fragments from the Late Cretaceous
of South America, and isolated teeth from the Early
Cretaceous of Morocco (Sigogneau-Russell 1991).
Krause et al. (1992) described some highly
unusual, isolated teeth from the Late Cretaceous
(Campanian) Los Alamitos Formation of Argentina,
which they named Ferugliotherium, and regarded as
a specialised group of multituberculates. However,
when Pascual et al. (1999) described another form,
the Palaeocene Sudamerica (5.9e), in which they
showed that there are four molars teeth, and no