The Origin and Evolution of Mammals - Moodle

184 THE ORIGIN AND EVOLUTION OF MAMMALS
modern vertebrates, many lizards and birds have
equally acute hearing, despite their predominantly
diurnal habits. The evolution of tactile vibrissae in
mammals is also suggestive of sense organs adapted
for night-time use. A second argument for nocturnality
is the indirect one, that the nocturnal insectivore
habitat was potentially available, and that the
great majority of small, sharp-toothed mammals
occupy it today. The only radical adaptive shift
from the insectivore habitat was towards at least
omnivory if not full herbivory. Three taxa independently
evolved broader, crushing dentitions suitable
for dealing with plant material such as seeds and
tubers. The earliest were the haramiyidans, and later
the Jurassic docodontans possessed teeth that functioned
in a manner that included crushing as well as
shearing. The most specialised and diverse of the
omnivore/small herbivores were the multituberculates
whose superficially rodent-like dentition indicates
that they had the sort of cosmopolitan diet
found in modern murids. Despite this apparent
shift in diet, the members of all three groups nevertheless
remained small in body size and presumably
nocturnal.
The continuous occupation of this fundamentally
small body size, nocturnal habitat by varying taxa
of mammals throughout the whole history of the
group from the Late Triassic to the present day is
not surprising. What is surprising is the virtually
complete restriction of mammals to this way of life
throughout the Mesozoic. The absence of any mammals
at all of larger body size for the extraordinarily
long period of time of about 140 million years, the
entire length of the Jurassic and Cretaceous Periods
together, is not easy to explain. Yet, whilst the post-
Mesozoic fauna continued to include large numbers
of small insectivorous, omnivorous, and herbivorous
mammals, several groups of larger body-sized
mammals appeared almost immediately after the
end of the Cretaceous, and continued to be a highly
conspicuous element of the terrestrial biota ever
since. Why were there no Mesozoic equivalents of
the foxes, lions, bears, the antelopes, elephants, and
kangaroos, the monkeys, anteaters, and sloths?
Two alternative explanations have been offered for
their absence, neither of which is overwhelmingly
convincing. The commoner is that the dinosaurs
constituted the larger body-sized terrestrial tetrapods
and that by competition for resources they excluded
the mammals from evolving into those niches. In this
view, the end-Cretaceous extinction of the dinosaurs
simply removed their influence, thereby freeing the
mammals to fulfil a pre-existing biological potential
to evolve increased body-size. The second explanation
is that during the Mesozoic phase of their existence,
the physiological or structural design of
mammals imposed a constraint on how large the
body-size could be. In the light of this interpretation,
the end-Cretaceous extinction event either created
new environmental conditions within which
mammals could evolve larger size, or else happened
to coincide with an adaptive breakthrough
in mammalian design that overcame the constraint.
The competitive exclusion hypothesis. Both the mammals
and the dinosaurs originated in the Upper
Triassic. The earliest known member of the Mammalia
occurs in the Carnian, although only as the
single, poorly preserved specimen of Adelobasileus.
The other early taxa, morganucodontans, haramiyidans,
and kuehneotheriids, appeared during the
Norian-Rhaetian (Lucas and Hunt 1994). The earliest
dinosaurs are also found in deposits of Carnian age,
but they too are rare and accompanied by a range of
more primitive ‘thecodont’ archosaurs (Olsen and
Sues 1986; Benton 1994). Again like the mammals,
several more dinosauran taxa were added during
the Norian-Rhaetic prior to the close of the Triassic.
Clearly, the timing of the origin and initial radiation
of the two groups supports well the hypothesis that
competitive exclusion by the dinosaurs was the
mechanism by which mammals of large body size
were prohibited. It must have been set in place at
the very time of origin of the two groups. The pattern
of extinctions and originations at the Cretaceous-
Tertiary boundary also supports the concept of competitive
exclusion. There were members of several
families of multituberculates, placentals, and marsupials
that survived the K/T mass extinction to be
present at the start of the Palaeocene. But, as far as
is known, not a single species of dinosaur survived
the transition. Within the first few million years of
the Palaeocene, a minimum of three and almost
certainly rather more of the surviving mammalian
lineages had produced species of substantially
increased body size.
Against this argument, the great difficulty with
the competitive exclusion hypothesis lies in the lack