The Origin and Evolution of Mammals - Moodle

the lower molars is relatively higher (Fig. 5.3(e)).
The wear facets on the teeth indicate that there was
a radical difference in the way in which occlusion
occurred. Those on the lower teeth are almost vertical,
but those on the upper tooth almost horizontal.
This means that as a lower tooth worked against an
opposing upper, the lower jaw must have rotated
about its long axis to a much greater extent than in
Morganucodon, causing the outer surface of the lower
tooth to face almost upwards as it moved in a
medial direction. The morphology of the lower jaw
was modified in several ways to accommodate this
rotation. First, the symphysis was mobile and almost
horizontally orientated to permit a degree of differential
movement between the paired jaw rami.
Second, the ventral edge of the dentary is complex,
with a process referred to by Jenkins et al. (1983) as
a pseudoangular process lying well in front of the
jaw hinge. There is also a flange developed below
the jaw hinge, which may be equivalent to the true
angular process of later mammals. These modifications
indicate an elaboration of the masseter musculature
in order to exaggerate the tendency that
muscle already has to rotate the lower part of the
jaw laterally.
Docodonta
Docodontans are characterised by molar teeth
whose crowns have expanded to form complex
occluding surfaces (Fig. 5.5(c)). Indeed, they are
comparable to the tribosphenic teeth of the modern
mammal groups, which achieved similar functional
ends by entirely separate means. Docodontans are
believed to occur only in Jurassic sediments, the
earliest from the Middle Jurassic of the Isle of
Skye (Waldman and Savage 1972), and the latest
from several Upper Jurassic localities of North
America and Europe. Pascual et al. (2000) described
a jaw fragment bearing three teeth as a latest
Cretaceous docodontan from South America. Named
Reigitherium, it would indicate a remarkably long
Gondwanan radiation of the group in isolation of
the northern continents. However, the pattern of
cusps of its very worn teeth is quite different from
that of other docodontans, and therefore its inclusion
in the group is regarded as very dubious by
Kielan-Jaworowska et al. (2004). As with most
THE MESOZOIC MAMMALS 147
Mesozoic mammal groups, docodontans were until
relatively recently known only from teeth and jaw
fragments. The discovery by Walter Kühne of a
largely complete specimen of Haldanodon, from the
Guimarota lignite mine in Portugal (Henkel and
Krusat 1980) was actually the first discovery of any
Upper Jurassic mammalian skeleton. Its postcranial
bones have not yet been fully described, but
Lillegraven and Krusat (1991) have given a very
detailed account of the skull (Fig. 5.5(a) and (b)).
The molar teeth of docodontans are expanded
across the jaw to increase the area of occlusion
between the uppers and the lowers. The result is
that there is a crushing basin between the anterior
lingual parts of the molars that worked analogously
to the protocone-talonid system of tribosphenic
teeth described later. A comparison of the dentition
of Morganucodon, the primitive docodontan
Haldanodon, and the advanced, Upper Jurassic
Docodon (Fig. 5.5(d)) illustrates how docodontan
teeth may have evolved from a hypothetical primitive
condition (Crompton and Jenkins 1968). What
in effect occurred was that the upper molars evolved
a lingual extension that acted against the inner part
of the crown of the lower molars. Instead of occlusion
and therefore shearing being restricted to the
sides of the main cusps, now the whole of the crown
was involved. Exactly what diet this was adapted
for can only be guessed at. Kron (1979) speculated
that it involved a greater degree of simple crushing
and grinding, associated perhaps with an omnivorous
or frugivorous diet. This suggests that docodontans
were one of the several superficially rodent
analogues of the Mesozoic, incorporating a significant
percentage of high-energy plant material into
their hitherto largely insectivorous food. The limb
bones of Haldanodon are stout and apparently horizontally
oriented, which led Krusat (1991) to suggest
that the animal was adapted for burrowing.
Kielan-Jaworowska et al. (2004) proposed an alternative
interpretation, that it was semiaquatic, living
in a swampy environment that is preserved as the
Guimarota Lignite mine.
Views about the phylogenetic relationships of
docodontans have shifted widely over the years. At
one time the specialised, complex molar teeth were
considered homologous with the triangulated molars
of therians, and at another they were compared to