The Origin and Evolution of Mammals - Moodle

186 THE ORIGIN AND EVOLUTION OF MAMMALS
Like insects these also tend to occur as small, discrete
items requiring individual discovery and collection.
Small body size is again a prerequisite for
surviving on this diet, but with the complication
that teeth adapted to some degree of grinding, and
hence subjected to a greater degree of wear must
evolve. A number of Mesozoic mammals occupied
this role, probably the haramiyids and the docodontans,
certainly the multituberculates, and also the
Late Cretaceous placental zhelestids.
Predaceous carnivory is a mode of life that readily
provides enough food, provided there is a source of
suitable sized prey. During the Mesozoic this would
have been a problem for a large mammal. There
were no medium-sized to large herbivorous mammals,
and the dominant herbivores, the dinosaurs,
were mostly too large in body size to be prey items
for all except very large predators indeed. Of the
other terrestrial animals present and forming a
potential food source, some tended to be too large
such as crocodiles, and others such as lepidosaurs
too small to provide an adequate diet for a middle to
large mammal carnivore. Therefore, no large carnivorous
mammals evolved. As it happens, the largest
Mesozoic mammals that did exist were the predaceous
members of the Eutriconodonta, and their
body size of up to 1.5 kg is consistent with a diet of
amphibians, small lizard, and other mammals.
The fourth basic diet to consider is leaf-eating,
browsing herbivory, the diet followed by the majority
of large herbivorous mammals today. This mode
of nourishment creates special problems because of
the relatively low energy content of foliage, and its
low rate of assimilation due to the need for a gut
fermentation flora. Large body size is necessary so
that the basal metabolic rate is relatively less, the
storage capacity of the gut is larger, and the potential
foraging area is greater. Small mammals cannot
collect or assimilate this kind of food at a rate that
is high enough to satisfy their relatively higher
metabolic demands, a problem exacerbated in a
female committed to lactation.
From these comments, simple as they are, it follows
that the fundamental point was the inability of
Mesozoic mammals to adapt to the large herbivore
habitat because of the absence of, or inability to
acquire food at a high enough rate for the endothermic
metabolism. A possible reason for this is a den-
tition unable to cope with the abrasion involved.
As it happens, the first appearance of the form of
enamel found in modern chewing mammals was
in the Early Palaeocene herbivorous mammals.
Koenigswald et al. (1987) showed that the arrangement
of the enamel prisms into what are termed
Hunter–Schreger bands was present in certain
condylarths. The apparent function of this organisation
of the enamel is to reduce the tendency of
teeth to split when actively masticating plant food.
Since similar enamel is found in most large herbivore
mammal taxa thereafter, it must have evolved
convergently several times, implying that it was a
morphological adaptation in response to a common
ecological opportunity in different lineages. Precisely
what that opportunity consisted of is unclear.
To conclude, there are two seriously proposed
physiological constraints that might have been
responsible for the persistent small body size of
Mesozoic mammals: over-heating in the absence
of adequate cooling adaptations, and undernourishment
in the absence of adequate folivorous
adaptations.
End of the era: the K/T mass extinction
and its aftermath
Sixty-five million years ago, an episode of massextinction
affected the world’s biota to the extent
that some 65–75% of the species disappeared in,
geologically speaking, an instant. Such events have
occurred every few tens of millions of years
throughout the history of life and this one was not
even the largest. That accolade goes to the end-
Permian mass extinction which was experienced by
the therapsid ancestors of mammals 250 Ma while
even the Late Triassic mass extinction of 210 Ma,
around the time of the origin of mammals and
dinosaurs, was at least of comparable magnitude
(page 87). Nevertheless, the end-Cretaceous, or
K/T event was the mass-extinction most significant
in the story of the origin and evolution of mammals.
It marks the transition from the ecologically
very limited Mesozoic mammal radiation to the
Tertiary radiation that culminated in today’s mammalian
fauna with its huge variation of body size
and habitat. Whether the mass extinction acted
by removing the dinosaur competitors of