The Origin and Evolution of Mammals - Moodle

310–320 Ma. By this time, the great southern supercontinent
of Gondwana had drifted northwards
and all but completed its coalescence with Laurasia,
forming the huge land mass of Pangaea, most of
which lay in the southern hemisphere and extended
over the South Pole. Only the small landmasses of
Siberia, Kazakhstan, North China, and South China
were separate, forming an arc of islands on the eastern
side (Scotese and McKerrow 1990). The contact
of Laurasia and Gondwana had created a major
range of mountains right across the middle of
Pangaea, almost exactly following the equator, and
of the order of size of the modern Himalayas
(Zeigler et al. 1997). The climate of the Westphalian
world is indicated by several kinds of characteristic
sediments preserved in different regions (Parrish
et al. 1986). Coal measures are found in the equatorial
regions either side of the mountain range, indicating
hot, humid, non-seasonal conditions, covered
in freshwater swamps with their associated rich
flora. This plant life was still dominated by the sporebearing
pteridophytes and lycopsids, although seedbearing
plants, conifers, and cordaites in particular,
were already begin to increase in prominence
(Behrensmeyer et al. 1992). To the north and the south
of this equatorial band there are evaporite deposits
extending to a latitude of around 30º, which are
indicative of warm conditions with at least seasonal
and possibly permanent aridity. Finally, much of the
southern regions from 30º south to the pole have
tillites, which are associated with glaciation. A very
large ice-sheet must have covered a significant
proportion of the southern hemisphere.
All the Pennsylvanian and Early Permian pelycosaurs
so far found have come from the equatorial
region, either side of the mountain range (Berman
et al. 1997), and were therefore part of the equatorial
coal measures habitat, which indicates that they
were adapted for hot, permanently wet conditions
(Fig. 3.27(a)). In fact, this particular ecosystem was
quite different from any that has existed since,
because there were very few herbivorous terrestrial
tetrapods. The energy flow was still evidently
strongly dependent on freshwater productivity,
with fish a major element in the ecosystem (Milner
1987, 1993). On land, the main primary consumers
were the terrestrial invertebrates. Under these
conditions, the tetrapods had three broad ecological
EVOLUTION OF MAMMAL-LIKE REPTILES 81
roles. One was as insectivores, carried out by
various small amphibian groups and the protorothyridid
amniotes. The second was as piscivores,
a guild that included larger amphibians and
also the ophiacodontid pelycosaurs. The third was
as secondary carnivores, able to consume the
smaller amphibians and the protorothyridids, as
manifested mainly by sphenacodontid pelycosaurs.
Terrestrial tetrapod primary consumers did appear
in the Pennsylvanian in the form of the pelycosaur
Edaphosaurus, but only as a relatively insignificant
component of the community: the evolution of
what was to become the fundamentally important
position of tetrapods as fully terrestrial herbivores
had to await a change in environmental conditions,
though when it did happen in the Early Permian,
pelycosaurs were profoundly implicated.
By the lowest part of the Early Permian, conditions
in this equatorial region of the world had
begun to change (Olson and Vaughn 1970). The coal
measure deposits gradually disappeared to be
replaced increasingly by the Red Beds that form the
huge fossiliferous area of Texas and adjacent states,
and in which the vast majority of pelycosaur specimens
have been found. Red Beds indicate a drier,
and seasonally arid climate, and the increase to dominance
of seed-bearing plants over the spore-bearing
groups reflects the environmental shift. Initially
there was little change in the fauna, but gradually
during the course of the Early Permian, some of the
elements disappeared and others appeared (Milner
1993). Fossilised lungfish burrows similar to those
of modern forms strongly support the idea of seasonal
aridity, and different groups of amphibious
tetrapods, perhaps better adapted to periodic dry
circumstances, appeared. A fauna distinctly less
dependent on freestanding bodies of water all the
year round seems to have evolved. The pelycosaurs
evidently reflect these changing circumstances, and
in the process became the dominant tetrapod component
of the fauna. Edaphosaurids, primarily the
genus Edaphosaurus, were the first terrestrial tetrapod
herbivores to became abundant, and therefore
the first to adopt a role continuously filled thereafter
by a succession of taxa, culminating in the deer and
antelopes of the present day. The sphenacodontids
radiated to be the correspondingly dominant large
carnivores as this source of large prey increased.