The Origin and Evolution of Mammals - Moodle

138 THE ORIGIN AND EVOLUTION OF MAMMALS
of the limestone quarries of South Wales. Although
extremely fragmentary, these specimens were the
singularly well-preserved bones of little animals,
not only mammals but several kinds of reptiles as
well, that apparently fell into the potholes and
underground streams of the limestone hills of that
time. These, plus one or two complete skulls and
skeletons of contemporaneous specimens from elsewhere
allowed for the very first time the full
description of the dentition ( Mills 1971; Parrington
1971), skull (Kermack et al. 1973, 1981), and postcranial
skeleton (Jenkins and Parrington 1976) of
one of the very earliest kinds of Mesozoic mammals,
Morganucodon. Modern study and understanding of
Mesozoic mammals dates from this period.
The next important overview was the edited volume
of Lillegraven et al. (1979), by which time a reasonably
clear picture of the main groups and their
supposed interrelationships had emerged. Most
authors had come to regard the mammals as monophyletic,
having achieved their grade via a single
lineage from one or other of the advanced cynodont
taxa. However, interpretation of the interrelationships
of the various groups was by this time dominated
by a belief in a single, early dichotomy of the
mammals into two groups. On the one hand were the
therian mammals, which include the modern marsupials
and placentals along with their more primitive
Mesozoic relatives; on the other hand were the nontherian
mammals, which included the living monotremes
amongst many fossil groups (e.g. Hopson and
Crompton 1969; Crompton and Jenkins 1978).
Kemp (1983, 1988a) first questioned this simple
phylogenetic division at the base of mammal evolution
on the grounds of the inadequacy of the characters
used to support it, and before long it had
been largely abandoned. Since then it has become
increasingly clear that the evolution of mammals in
the Mesozoic produced a very complex phylogenetic
pattern. Several quite new kinds of Mesozoic
mammals have been discovered, and the detailed
knowledge of several known forms has increased
enormously with the description of a number of virtually
complete skeletons. This phase of study has
culminated in the publication of the three-authored
monograph of Kielan-Jaworowska et al. (2004).
Although the story is a good deal clearer as a
result of this new information and several detailed
cladistic analyses, a number of important aspects
of the interrelationships are still very much debated.
Meanwhile, the underlying conundrum of the
group remains unsolved: why were the Mesozoic
mammals so diverse in taxa, yet so conservative in
body size and form?
The diversity of the Mesozoic mammals
In Chapter 3, it was established that a well-corroborated
group Mammalia could be defined by a
number of unique characters, which excludes the
tritylodontids, tritheledontids, and other ‘nearmammalian’
forms. Omitting them is necessary for
clarity because their precise interrelationships are
still unclear. On this basis, the main diagnostic
characters of Mammalia are the dentary condyle
articulating in a glenoid fossa of the squamosal; the
formation of a large percentage of the side wall of
the braincase by the anterior lamina of the petrosal,
which encloses the foramina for the maxillary and
mandibular branches of the trigeminal nerve; and
several detailed features of the periotic region of
the braincase, and occipital condyles. Using these
characters, ten major groups of Mesozoic mammals
can be recognised, although, as will become clear,
some of them are not strictly monophyletic. The
nomenclatural issue of whether the term Mammalia
or Mammaliaformes should be applied to the monophyletic
taxon including all of these ten groups was
also addressed earlier, when it was decided to follow
Luo et al. (2002; Kielan-Jaworowska et al. 2004) by
adopting the former.
Adelobasileus
The earliest contender to be a member of the strictly
defined Mammalia is an incomplete and poorly
preserved specimen from the Upper Triassic of Texas
(Lucas and Luo 1993). It is from rocks of Carnian age,
which dates it at about 225 Ma, some 10 Ma earlier
than any other known mammal. Unfortunately,
only the hind part of this little skull is actually present,
and even that is badly crushed (Fig. 5.1(a)).
There is no indication at all of the jaws or dentition.
Nevertheless, the preserved portion is certainly
mammalian. An anterior lamina of the periotic
enclosing the foramina for the V 2 and V 3 branches
of the trigeminal nerve is present, there is at least an
incipient promontorium for the cochlea, and the