The Origin and Evolution of Mammals - Moodle

adjacent lower molars. A small extension, or cusp at
the back of one molar fits into a corresponding concave
area at the front of the molar behind.
Presumably this was a way of helping to stabilise
an individual lower tooth, while it occluded powerfully
against an upper. Triconodon itself is typical
(Fig. 5.7(a)). It occurs in the Upper Jurassic of Great
Britain, and was amongst the larger of the Mesozoic
mammals. Although not known from more than
fragmentary material, there is a complete lower jaw
of Tricondon ferox, which is about 8 cm in length,
suggesting that it was a carnivorous animal about
the size of a modern genet.
While the great majority of tricondontids are
known only as isolated jaw and dental fragments, a
complete skeleton from the Late Jurassic or Early
Cretaceous of China, Jeholodens, has come to light
(Ji et al. 1999). It is one of several beautifully preserved
kinds of mammal skeletons found in the
extraordinary Yixian Formation in Liaoning
Province, which is renowned for early bird skeletons
(Lucas 2001; Zhou et al. 2003). The skeleton of
Jeholodens (Fig. 5.8(b)) is that of a small animal,
(a)
(b)
Jeholodens
Gobiconodon
THE MESOZOIC MAMMALS 151
a mere 8 cm in presacral length with a long, slender
tail. Its limb and foot structure suggest it was
ground dwelling rather than arboreal in life,
because there is no sign of marked pedal flexibility
or long, curved claws. The structure of the molar
teeth (Fig. 5.7(c)) differs from typical triconodontids.
Each molar, upper and lower, has the triconodont
condition of three major cusps on the crown
arranged linearly from front to back, but in this case
the middle cusp is distinctly higher than the anterior
and posterior accessory cusps. The cingulum is
weak and incomplete in the upper molars, and both
upper cingulum and the better-developed lower
cingulum lack cuspules. These are probably primitive
features of the teeth, but Jeholodens does have
the interlocking arrangement of the lower molars
found in tricondontids. The postcranial skeleton of
Jeholodens reveals an unexpected mosaic of characters,
most notably a combination of primitive features
of the hindlimb with characters otherwise
found in more derived mammals in the forelimb.
The pelvic girdle is very similar to that of morganucodontans,
there is only a weakly developed
Figure 5.8 Eutriconodontan
skeletons. (a) Gobiconodon ostromi.
Presacral length approx. 33 cm
(Jenkins and Schaff 1988).
(b) Jeholodens jenkinsi. Presacral
length approx. 4 cm (Qiang et al.
1999).