The Origin and Evolution of Mammals - Moodle

84 THE ORIGIN AND EVOLUTION OF MAMMALS
considerable increase in the proportion of seed
plants such as gymnosperms and a corresponding
reduction in spore-bearing, fern-like lepidodendrids
and sphenopsids. There was also an increase in
plants with xeromorphic adaptations, such as
leathery leaves, hairy surfaces, and spiny leaves.
Meanwhile, increasingly rich, diverse floras were
developing in the temperate areas (Cuneo 1996). The
actual environments of the two areas in which the
majority of early therapsids have been found,
Russian in the north and South African in the south,
were both moist, temperate zones.
The consequences of these gradual changes in
the environment on the tetrapod fauna are not
surprising. There was a reduction and eventually a
virtual disappearance from the North American
deposits, apart from the extremely few pelycosaurs
and possible, but very doubtful, early therapsids
found. On the other hand, a wide range of
tetrapods spread into the southern and northern temperate
regions, including various kinds of amphibians
and diapsid amniotes (Milner 1993). In fact, there
is considerable taxonomic continuity between the
Early and the Late Permian non-therapsid, terrestrial
faunas (Anderson and Cruickshank 1978; Milner
1993). The pelycosaurs also underwent this biogeographic
shift, for a few relict members of the group
occurred briefly for the first time outside the tropics:
caseids in the north and varanopseids in the north
and the south.
Looking at the whole picture, the transition from
Early Permian to Late Permian did not appear to
involve any abrupt changes. The climate, flora and
fauna all shifted relatively gradually over the course
of the Permian, with one exception: the sudden
appearance of the diverse therapsid fauna 270 Ma.
The absence of any sign of a large change in climate
or biota in the middle of the Permian leads to
speculation that it was the evolution of some biological
innovation that caused the rapid radiation
of the therapsids and their replacement of the pelycosaurs.
The idea of a particular key evolutionary
innovation leading to instant competitive superiority
is notoriously difficult to test (Kemp 1999), as in
reality any change of sufficient magnitude to have
the effect is likely to have involved the subtle interplay
between many attributes of both organism and
environment. Nevertheless a number of interesting
possible pointers can be considered. It has been
suggested earlier that the pelycosaurs played a fundamental
role in the establishment of a modern
style of fully terrestrial ecosystem involving tetrapod
herbivores, and therefore the end of direct
dependence on aquatic productivity. Compared to
the pelycosaurs, the therapsids were undoubtedly
more highly evolved along the same road. The
structure of their skeletons indicates that they were
more active organisms. The carnivorous forms had
much larger canine teeth and indications of more
elaborate musculature to operate the jaws. The herbivores
had a more specialised and presumably
therefore a more efficient mode of dealing with
plant food. The therapsid postcranial skeleton, with
its relatively longer, more slender limbs and less
constraining shoulder and hip joints, indicates
greater mobility and agility. As discussed at length
later, these modifications must have been accompanied
by an incremental shift in metabolic and neurological
physiology, which in turn imply a more
mammal-like organisation, more capable of surviving
the fluctuations of the terrestrial environment.
Two associated consequences might have followed
the origin of therapsid biology. One is that, unlike
most cases of co-existing taxa, the pelycosaurs and
therapsids were direct competitors for essentially
the same niches, those requiring physiological tolerance
of warm and dry conditions, and that the
therapsids were the better adapted. The other is
that the higher level of homeostatic organisation of
therapsids opened up a range of niches not hitherto
available to pelycosaurs, or indeed to any other
contemporary tetrapods, and hence their rapid
radiation into new kinds of terrestrial organisms.
This may be one of the few cases where a convincing
hypothesis of competitive replacement of one
taxon by another may be entertained.
Therapsida and the end-Permian mass
extinction
During the Late Permian, the therapsid radiation is
far and away best illustrated by the great Beaufort
sequence of the South African Karoo and equivalentaged
strata in other southern African areas, such as
in Zambia and Tanzania (King 1990). The environment
consisted of broad, slow-flowing rivers with