The Origin and Evolution of Mammals - Moodle

2 THE ORIGIN AND EVOLUTION OF MAMMALS
structure. If the definition of a mammal is based
rigorously upon possession of all the characters of
living mammals, then some fossil forms that are
extremely mammalian in anatomy, and by inference
in their general biology, are excluded. If,
on the other hand, a mammal is defined as an
animal that possesses any of the modern mammal
characters, then some extremely non-mammalian
forms, primitive, sprawling-limbed, and no doubt
scaly, ectothermic creatures must be included. If a
compromise is sought by using certain selected living
mammal characters as the basis of the definition, then
it becomes an arbitrary decision as to exactly which
characters are to be given defining status.
After rather a lot of fruitless discussion during
the last half-century about this issue generally, two
alternative approaches emerged. Developing one of
Willi Hennig’s (1966) proposals in his original prescription
for cladistics, Jefferies (1979) and Ax
(1987) proposed distinguishing a ‘crown’ group for
the living members of a taxon from a ‘stem’ group
for all the more basal fossil forms possessing some
but not all the characters of the crown group. Rowe
(1988, 1993) adopted this solution and formally
defined the taxon Mammalia as those organisms
possessing, or presumed to have possessed, all the
characters of living mammals. All those fossil
groups that possess at least one, but less than all of
the characters of living mammals can be referred
to as stem-group Mammalia, a paraphyletic, but
informally recognisable taxon.
Developments of both technology and fashion in
systematics have since overtaken the simple concept
of crown versus stem groups based respectively
on all, or less than all the defining characters
(Benton 2000). The ability of computer programs
such as PAUP to handle vastly more amounts
of information has led to computer-generated
cladograms involving large numbers of taxa and
characters. Even in the maximally parsimonious
cladogram, many of the characters are inevitably
homoplasic, occurring independently in different
groups. Therefore, it is impossible to read from the
cladogram simple lists of characters that define the
various contained groups. How much more so is
this true of phylogenetic trees based on molecular
sequence data, which can involve computation of
several thousand nucleotides? The formal creation
of taxon definitions has therefore shifted from definitions
based on lists of characters, to labelling
nodes in the cladogram: a taxon is defined as the
clade that includes the common ancestor of subgroups
X and Y. The evidence for the existence of
a clade so defined is that it occurs in the bestsupported
cladogram. In this rarified sense,
Mammalia is the clade that includes the common
ancestor of monotremes, marsupials, and placentals.
The shift in fashion that has simultaneously affected
the naming of groups, whether character- or nodebased,
is a reluctance to accept the vagueness of
paraphyletic stem groups, but as far as reasonable
to name every monophyletic group in it. Thus
Rowe (1988, 1993) introduced the formal names
Mammaliamorpha and Mammaliaformes at two of
the nodes on the stem lineage below Mammalia, and
two more above it, Theriomorpha, Theriiformes,
before the node Theria that represents the common
ancestor of marsupials and placentals.
An altogether different perspective on defining
Mammalia is based on traditional palaeobiological
practice (e.g. Simpson 1960; Kemp 1982). An arbitrary
decision is made about which characters to
select as defining characters, and therefore which
particular node on the stem lineage to label
Mammalia. Characters deemed appropriate are
those reflecting the evolution of the fundamental
mammalian biology. The essence of mammalian life
is to be found in their endothermic temperature
physiology, greatly enlarged brain, dentition capable
of chewing food, highly agile, energetic locomotion,
and so on. The organisms that achieved
this grade of overall organisation are deemed to be
Mammalia, and consequently those characters that
they possess are the defining characters of the
group. In this view, missing an odd few refinements
such as, free ear ossicles or the details of the
ankle joint is insufficient justification for denying
mammalian status to a fossil that is otherwise mammalian
in structure. Around the end of the Triassic
Period, about 205 Ma, a number of fossils are found
of very small animals that have the great majority
of the skeletal characters of modern mammals. The
brain is enlarged and the postcranial skeleton differs
from that of a modern mammal only in a few
minor details. Their novel feature is the jaw mechanism.
The dentition is fully differentiated with