The Origin and Evolution of Mammals - Moodle

CHAPTER 5
The Mesozoic mammals
The expression ‘Mesozoic Mammals’ refers to more
than simply the mammals of that particular period
of time; it also stands for an extraordinary and quite
mysterious concept. From the first appearance in
rocks of Late Triassic times of the small, obviously
highly active, large-brained animals thought of as
mammals, through the following 145 million years
of life on earth culminating in the great end-
Cretaceous mass extinction that saw the end of the
dinosaurs, these animals remained small. Although
probably far from rare at the time, the great majority
of species of Mesozoic mammals were of the size
of shrews, rats, and mice. A tiny handful managed
to evolve to the body size of foxes or beavers, but
there were no representatives at all of mammals the
size of the prominent mammals of today, the herbivorous
horses, antelopes, and elephants, the lions
and wolves that feed upon them, or the specialist
apes, whales, and anteaters. Two points highlight
just how odd this restriction in body size is. The first
is that the Mesozoic mammals represent no less
than two-thirds of mammalian evolution from their
origin to the present, so there was plenty of time for
evolution, and an extensive radiation did indeed
occur producing a plethora of taxa. The second is that
somewhere along the line, the potential for evolving
large body size certainly existed because within,
metaphorically speaking, moments of the end of the
Mesozoic Era, middle-sized and soon thereafter large
mammals had arisen and were flourishing.
Since their very earliest recognition by Dean
William Buckland (Buckland 1824) from the Middle
Jurassic Stonesfield Slate of Oxfordshire, Mesozoic
mammals have generated controversy (Desmond
1985). Transformationists like Robert Grant denied
that they were mammals, because it disturbed their
accepted temporal sequence of Mesozoic reptiles
preceding the exclusively Tertiary mammals. On
the other hand, establishment figures like Buckland
himself and Sir Richard Owen welcomed this
apparent refutation of transformationism and had
no doubt that they were indeed opossum-like
mammals. In the end, the true nature of these fossils
was accepted, and by 1871, a good number of
undoubtedly Mesozoic localities had yielded
undoubtedly mammalian fossils. In that year, Owen
(1871) published his monograph covering all the
known forms, dating as they did from Late Triassic
through to latest Cretaceous, and derived from
several localities in Europe and North America. Yet,
while clearly more abundant than hitherto imagined,
every single specimen consisted of no more
than at best an incomplete jaw and teeth, or the odd
isolated postcranial bone. In fact, little had changed
by the time G. G. Simpson re-monographed all the
world’s material in two volumes (Simpson 1928,
1929) and the definitions of the groups and the
interpretation of their interrelationships were still
necessarily based solely on characters of the teeth
and dentary bone. The prevailing view was that the
mammals had been very rare, tiny animals totally
dominated numerically and taxonomically by the
dinosaurs. Simpson also epitomised the widely
held opinion that the early mammals were highly
polyphyletic, with anything up to five lineages
having achieved the mammalian grade independently
from the mammal-like reptiles (Simpson 1960).
This concept was due to the practical necessity of
relying on the divergent characters of the teeth,
coupled with a faith in the efficacy of natural selection
to drive similar kinds of organisms to considerable
lengths in the same evolutionary direction.
Historically, rather little happened in the study of
Mesozoic mammals until the remarkable discovery
by Walter Kühne (1950) of abundant remains of latest
Triassic mammals contained in clay-filled fissures
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