The Origin and Evolution of Mammals - Moodle

transversely occluding molar teeth that functioned
in the unique manner of basic living mammals, and
there is a new jaw hinge between the dentary bone
of the lower jaw and the squamosal of the skull that
permitted a much stronger bite. Such animals were
undoubtedly mammalian in the biological sense
although not strictly members of the crown-group
Mammalia. The most primitive of these forms is
called Sinoconodon, and most palaeobiologists believe
that they should formally be members of the clade
designated Mammalia (e.g. Crompton and Sun
1985; Kielan-Jaworowska et al. 2004). The definition
of Mammalia thus becomes: synapsids that possess
a dentary-squamosal jaw articulation and occlusion
between lower and upper molars with a transverse
component to the movement. This has exactly the
same membership as the clade that includes the
common ancestor of Sinoconodon, living mammals,
and all its descendants.
This is the concept of a mammal that is used in
the chapters that follow, in the belief that conceding
a degree of subjectivity in the choice of what is a
mammal is a small price to pay for allowing the
focus of the work to be on the origin and evolution
of the quality of ‘mammalness’ just as much as on
the origin and evolution of the taxon Mammalia.
A sketch of the plot
The story of the origin and evolution of mammals
as told by the fossil record falls into three distinct
phases, the first of which led to the origin of mammals
as such. The lineage of amniotes that culminated
in the mammals made its first appearance in
the fossil record of the Pennsylvanian (Upper
Carboniferous) about 305 Ma. The best-known
form from this time is Archaeothyris, which existed
very soon after the initial appearance of the terrestrially
adapted amniote animals. From a hypothetical
ancestor comparable to Archaeothyris, the
radiation of mammal-like reptiles commenced and
occupied the succeeding 100 Ma. Taxa appeared,
flourished, and went extinct in a complex kaleidoscopic
pattern of successive groups of small-,
medium-, and large-sized carnivores and herbivorous
groups, which over time exhibited increasing
numbers of mammalian characters superimposed
on their particular specialisations. In the latest
INTRODUCTION 3
Triassic Period, 210 Ma, one particular lineage of
small carnivores culminated in the first mammal.
The second phase of the unfolding story consists
of what are referred to as the Mesozoic mammals.
These were the numerous subgroups that radiated
from the mammalian ancestor during the 140 Ma
duration of the Jurassic and Cretaceous Periods, the
time during which the world’s terrestrial fauna was
dominated by the dinosaurs. A good deal of evolution,
especially dental evolution occurred, but none
of them ever evolved the medium or large body
size found in so many modern mammals. For twothirds
of the whole of their history, mammals
remained small animals with the largest being
barely larger than a cat, and the vast majority of the
size of living shrews, mice, and rats. With hindsight,
the most important evolutionary event was
the origin of the modern mammalian kind of molar
tooth, known as the tribosphenic tooth, and with it
the roots of the two major modern taxa, marsupials
and placentals.
Sixty-five million years ago, the mass extinction
marking the close of the Mesozoic Era saw the end
of the dinosaurs along with the extinction or severe
depletion of many other taxa. Several mammal lineages
survived this event, and within a mere 2–3 Ma
had radiated explosively to produce a plethora of
new small forms but also, for the first time, mammals
of middle to large body size. This was the
commencement of the great Tertiary radiation of
placental and, to a lesser but equally interesting
degree, marsupial mammals during which many
unfamiliar, often quite bizarre kinds evolved, flourished,
and disappeared. The Tertiary world has
been a period of dramatic biogeographical and climatic
change, against which is set this extraordinary
evolutionary pageant. The old, single supercontinent
of Pangaea occupied by the mammal-like reptiles
had long since broken into Laurasia in the
north and Gondwana in the south, and by the early
Tertiary, Gondwana itself was breaking up. Africa
and India for a short while, and South America and
Australia for most of the era were island continents
with extremely limited biotic contact with each
other and with the northern land masses. Even
amongst the Laurasian continents of North America,
Europe, and Asia, interconnections formed and
broke from time to time as land masses shifted, sea