The Origin and Evolution of Mammals - Moodle

134 THE ORIGIN AND EVOLUTION OF MAMMALS
characters are possible in this way. There is a limit
to how much the metabolic rate, or the hearing acuity
can increase in the absence of compensatory
changes elsewhere in the organism. Further progress
in those particular characteristics must await the
time when other attributes have themselves undergone
their own small changes. The overall evolutionary
progression can be likened to a row of people
walking forwards hand in hand. Any one person can
get a little ahead of the rest, but further forward
movement must wait until the rest of the members of
the row have all progressed forwards in their own
good time. The line as a whole moves forwards and
all the individuals remain part of the interconnected
whole, although at any instant some lag slightly
behind and some have pulled slightly ahead.
Correlated progression predicts that in a sequence
of ancestors and descendants reconstructed from the
fossil record, each successive stage will have undergone
changes in several biological systems. No one
system will have evolved very much in the absence
of changes in the others. This is exactly what the
fossil record of the mammal-like reptiles shows insofar
as the preserved characters are concerned. At
every hypothetical stage that can be reconstructed
from known fossils, there are modifications in the
mammalian direction of dentition, jaw structure, and
aspects of the postcranial skeleton. Less detailed evidence
at least suggests that incremental changes to
the ventilation mechanism, sense organs, and brain
size similarly occurred over a succession of stages.
Furthermore, it is implicit that if the evolutionary
sequence of changes in other, non-preservable characters
and processes could be determined, they too
would fit into the scheme. The model also predicts
that, because of the correlated progression of all
characters, over a significant period of time each
recognisable biological system of the organism will
evolve at least approximately at a similar rate, insofar
as such a parameter can be estimated.
What drove the trend towards mammals?
Having postulated what the first mammals were
adapted for, and having concluded that all the
changes that led up to them evolved in a coordinated
fashion, the final question is what drove this
trend from primitive, ectothermic, sprawling-gaited,
simple-toothed amniotes to mammals? Of course,
the trend to mammals is a single lineage picked out
from a highly branched phylogeny for no better
reason than that there is a special interest in mammals
as the taxon containing humans, although as it
happens it is the longest branch on the tree measured
both by the number of relevant grades of
known fossils and the morphological distance
spanned. There is no reason to suppose that trend
culminating in, say, the Upper Triassic dicynodonts
had a different kind of evolutionary cause.
The characters of mammals emerged gradually
over the whole of the 100 Ma history of the mammallike
reptiles, mostly in the context of a habitat
and mode of life rather different from those of the
reconstructed hypothetical ancestral mammal.
Furthermore, at several of the various levels in the
evolving sequence, there was a radiation into a range
of different kinds of animals, large and small, herbivore,
omnivore, and carnivore. However, when the
hypothetical sequence of ancestral forms is reconstructed
from the distribution of characters on the
cladogram, there are two consistent features. All of
them have a carnivorous dentition; and all of them
have a body size that is towards the small end of
the size range of the members of the radiation to
which they gave rise. This is reasonably confidently
inferred for the respective hypothetical ancestors of
the basal pelycosaur grade, sphenacodontid grade,
therapsid grade, therocephalian grade, and basal
cynodont grade. In short, the trend towards mammals
consisted of a sequence of relatively small carnivores
with an ever-increasing level of homeostatic
regulatory ability.
The normal explanation proposed for a morphological
trend that has been revealed in the fossil
record is the neodarwinian one, that natural selection
drove the change in a direction of ever-increasing fitness.
In the present case of the trend towards mammals,
it is supposed that each increase in homeostatic,
regulatory ability adds an increase in adaptedness
to the terrestrial habitat. There is an anomaly
though: if it was indeed true that increasing homeostatic
regulation was advantageous in the habitat
of mammal-like reptiles, why were only relatively
small carnivores evidently selected for it? Over the
timescale of the evolution of the mammal-like reptiles,
increasingly mammalian herbivores are found
to replace one another. In the Early Permian there