The Origin and Evolution of Mammals - Moodle

CHAPTER 1
Introduction
There are about 4,600 species of animals today that
are called mammals because, despite an astonishing
diversity of form and habitat, they all share a long
list of characters not found in any other organisms,
such as the presence of mammary glands, the single
bone in the lower jaw, and the neocortex of the forebrain.
This makes them unambiguously distinct
from their closest living relatives, and their unique
characters together define a monophyletic taxon,
the class Mammalia. Three subgroups are readily
distinguished amongst the living mammals. The
Monotremata are the egg-laying mammals of
Australasia, consisting only of two species of
echidna and a single platypus species; for all their
primitive reproductive biology, monotremes are
fully mammalian in their general structure and
biology. The Marsupialia, or Metatheria are the
pouched mammals, whose approximately 260
species dominate the mammalian fauna of Australia,
and also occur as part of the indigenous fauna of the
Americas. By far the largest group of living mammals
are the Placentalia, or Eutheria with about 4,350
species divided into usually eighteen recent orders.
It is virtually unanimously accepted that the closest
living relatives, the sister group, of mammals
consists of the reptiles and the birds. The only serious
dissent from this view in recent years was that of
Gardiner (1982) who advocated that the birds alone
and mammals were sister groups, the two constituting
a taxon Haemothermia, defined among other
characters by the endothermic (‘warm-blooded’)
temperature physiology. Gardiner certainly drew
attention to some remarkable similarities between
birds and mammals, notably the details of the
endothermic processes, the enlarged size and surface
folding of the cerebellum, and a number of more
superficial morphological features. There was also
some molecular sequence data supporting the
Haemothermia concept, including the beta-globin
gene and 18S rRNA. Gardiner’s view briefly became
a cause célèbre in part for its sheer heterodoxy, but all
concerned have since rejected it on the grounds that
a careful, comprehensive analysis of the characters
supports the traditional view (Kemp 1988b), particularly
if the characters of the relevant fossils are
taken into account (Gauthier, Kluge, and Rowe
1988). Nor does it receive any significant support at
all from the mass of nucleotide sequence data now
available.
The definition of Mammalia
The formal definition of Mammalia is simple as far
as the living mammals are concerned, because of
the large number of unique characters they possess.
However, the fossil record makes the situation a
good deal less clear-cut. The first part of this book
is about the extraordinarily good fossil record of
animals that were, to varying degrees, intermediate
in grade between the modern mammals and the
last common ancestor between them and their
living sister group. They are known informally as
the ‘mammal-like reptiles’, and more formally as
the non-mammalian Synapsida. By definition, a
mammal-like reptile possesses some, but not all the
characters that define living mammals. A semantic
difficulty arises about defining a mammal because
a decision has to be taken on which, if any of the
mammal-like reptiles should be included. The earliest,
most primitive ones have very few mammalian
characters, just a small temporal fenestra in
the skull and an enlarged canine tooth in the jaw. In
contrast, there are later forms that possess almost
all the modern mammalian skeletal characters,
lacking only a few of the postcranial skeleton ones
like a scapula spine, and fine details of the ankle
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