The Origin and Evolution of Mammals - Moodle

86 THE ORIGIN AND EVOLUTION OF MAMMALS
a brief increase in the presence of preserved spores
in the sediments, known as a fungal spike (Visscher
et al. 1996). A handful of presumably particularly
hardy land plants also survived. Among animals, it
must be assumed that despite the breakdown of the
ecosystem worldwide, some species had sufficiently
cosmopolitan diets, and were capable of occupying
sufficiently protected microhabitats to withstand
extreme seasonal conditions. Perhaps some were
already adapted by having evolved the ability to
aestivate.
The effect on the terrestrial environment is most
clearly and dramatically seen in the transition from
the latest Permian Dicynodon Assemblage Zone to
the overlying basal Triassic Lystrosaurus Assemblage
Zone of South Africa, where the � 13 C shift has been
recorded (MacLeod et al. 2000). There is a marked
change in the palaeoenvironment at this time. The
latest Permian geography consisted of slow flowing,
meandering rivers with extensive seasonal
floodplains, laying down greenish-grey mudstones.
In the immediately overlying basal Triassic, the
river systems were much less sinuous and faster
flowing, and the floodplains had largely dried up
(Smith 1995). The deposits are now dominated by
reddish sandstones indicative of extreme drought
conditions and faster flow of the rivers. The change
can be interpreted as the consequence of a severe loss
of vegetation, reducing the forests and therefore
removing the stabilising effect of the plant life.
Erosion would increase and the flow of the river systems
be less checked. The shift in sediment types
coincides with a dramatic change in the vegetation,
with the disappearance of the Glossopteris dominated
flora, and its replacement by a much lower diversity
flora consisting of the seed fern Dicroidium and a few
genera of conifers and lycopods.
The accompanying change in the fauna was also
stark. The Dicynodon Assemblage Zone contains
approximately 44 reptilian genera of which only
three occur in the Lystrosaurus Assemblage Zone
(Rubidge 1995). Of these, 36 are therapsids, and
two occur in both of the adjacent Assemblage
Zones, namely the dicynodont Lystrosaurus and the
therocephalian Moschorhinus. A few other lineages
must also have survived the end-Permian because
their descendants occur in the Triassic, although no
Permian specimens of these genera have yet been
found. At present, the full list of known or inferred
surviving lineages includes four dicynodont ones,
Lystrosaurus itself, the kannemeyeriids which,
although related to Lystrosaurus, must have differentiated
from that genus before the end of the Permian,
the diictodontoid Myosaurus which occurs in the
Lystrosaurus Assemblage Zone, and the kingorioid
Kombuisia that occurs in the succeeding Cynognathus
Assemblage Zone. Of the therocephalian lineages,
there was Moschorhinus, several baurioids, for example
Regisaurus, from the Lystrosaurus Assemblage
Zone, and the possibly separate lineage that includes
the Cynognathus Assemblage Zone Bauria. Finally,
two cynodont lineages made the transition. These
are the galesaurids represented by Cynosaurus in the
latest Permian and Galesaurus in the Lystrosaurus
Assemblage Zone, and Thrinaxodon from the latter
zone. Set against this dozen or so survivors was the
disappearance of all the gorgonopsians, the great
majority of the hugely diverse dicynodonts, the last
of the large-bodied therocephalians, and the procynosuchian
cynodonts.
Smith and Ward (2001) have studied highresolution
stratigraphic sections in the South African
Karoo which include the actual Permo–Triassic
boundary. As well as confirming the lithological
change from mostly greenish-grey mudstone to
mostly reddish sandstone dominance, the section
contains a non-fossiliferous layer within which is
found the � 13 C shift that marks the actual boundary.
Below this layer, the fossil record indicates a geologically
abrupt extinction event, and the only
taxon they found that occurred both below and
immediately above the Permo-Triassic boundary is
Lystrosaurus, already abundant in the latter. Other
Lystrosaurus Assemblage Zone forms do not appear
until higher levels in the zone. It is difficult precisely
to estimate the time represented by the unfossiliferous
transitional bed, but at most it was 50,000 years
and could have been considerably less, especially if
associated with an increased sedimentation rate
consequent upon the loss of vegetation; by geological
standards the change in biota was undoubtedly
a catastrophic and not a gradual process.
The surviving taxa found in the Lystrosaurus
Assemblage Zone must have been able to tolerate
very arid conditions. Lystrosaurus was probably an
adept digger, capable of protecting itself by