The Origin and Evolution of Mammals - Moodle

136 THE ORIGIN AND EVOLUTION OF MAMMALS
solely an adaptive response to a particular habitat,
or whether the process of miniaturisation was a
structurally necessary way of evolving some of the
characteristics of mammals.
Several suggestions have been made to the effect
that miniaturisation was structurally necessary for
a mammal to evolve. One discussed earlier is
McNab’s (1978) hypothesis for the origin of endothermy,
proposing that is was the reduction of body
size that necessitated the evolutionary increase in
BMR, and therefore the shift to endothermy.
Another possibility is that the relative increase in
brain size accompanying miniaturisation was necessary
for remodelling the anatomy of the braincase
and its relationship to the rest of the skull in such a
way as to be able to house an absolutely larger
brain in mammals. A further consequence of the
enlarged brain is that the distance between the
quadrate and the fenestra ovalis that is spanned by
the stapes decreased, which may have been a
requirement for the final development of the mammalian
ear ossicle system of hearing. It is very
likely that the relative reduction in the mass of the
limb bones was instrumental in the final stages of
evolution of the characteristic mammalian locomotor
mechanism.
However, none of these speculations are overwhelmingly
convincing, because so many of the
mammalian characters first evolved in earlier
mammal-like reptiles that were not at all miniaturised.
Many eucynodonts, and even some of the
tritylodontids, with their large number of mammalian
features, were medium to large in body size.
It is simpler to accept that the miniaturisation was
simply an adaptive response to the habitat of nocturnal,
insectivorous tetrapod.