The Origin and Evolution of Mammals - Moodle

side of the head can be in action at any one time,
but this permitted the forces from the adductor
musculature of both sides of the head to be applied
simultaneously to the teeth on only one side,
thereby increasing the potential bite force available,
as is discussed elsewhere (page 100). Neither of the
more primitive mammalian dentitions described so
far, haramiyidans or Sinoconodon, had evolved this
mode of occlusion, but all phylogenetically more
derived mammals had, or were descended from
forms that had such a system. In this respect morganucodontans
represent a very important step in the
evolution of fully mammalian biology.
The mode of tooth replacement of morganucodontans
also exhibits for the first time a state
ancestral to all subsequent mammals. Amongst the
many hundreds of Morganucodon fragments of the
fissure-fill material studied by Parrington (1971),
he found specimens of jaws containing replacement
teeth for incisors, canines, and premolariform teeth
(Fig. 4.12(c)). He found none at all for the posterior
three molariform teeth. Consequently, he inferred
THE MESOZOIC MAMMALS 145
that Morganucodon had achieved full mammaliantype
diphyodonty, with only a single replacement
of the deciduous anterior teeth, and no replacement
of what can properly be termed molars. Crompton
and Luo (1993) have expressed some doubt about
whether Parrington’s sample was really adequate
to support this conclusion, although it seems certain
that at the very least tooth replacement was greatly
reduced in morganucodontans and approached the
definitive mammalian condition. The functional
relationship of diphyodonty, growth pattern, and
lactation in these early mammals is mentioned elsewhere
(page 120).
The postcranial skeleton of morganucodontans
(Fig. 5.4) was the first of any Mesozoic mammal to
have been properly described, on the basis of both
the excellently preserved Morganucodon fragments
from the Welsh fissures, and the complete though
less finely preserved skeletons of the South African
genera Megazostrodon and Erythrotherium (Jenkins
and Parrington 1976). While characteristically mammalian
in most features, morganucodontans still
Figure 5.4 Postcranial skeleton of Megazostrodon rudneri. Presacral length approx. 7 cm (Jenkins and Parrington 1976).