The Origin and Evolution of Mammals - Moodle

lakes and swamps in the low-lying levels and
forested higher levels. The climate was cool and
seasonal, with dry and wet seasons. The flora
is described as the Glossopteris flora after the hardy
seed-fern of that name which was the most abundant
form. It was a genus of woody plants, the
largest species being 4 m in height and others lower
and shrubbier. Other seed ferns, a variety of horsetails,
and primitive conifers were also present. It was
in this habitat that the dicynodonts flourished, radiating
into dozens of species, adapted in different
ways to exploit different aspects of this rich flora,
during the times of the successive Assemblage Zones
of the Late Permian (Fig. 2.2(c)). Gorgonopsians
and the larger therocephalians preyed on them, and
smaller carnivores, the baurioid therocephalians and
the rare primitive cynodonts occupied insectivore or
small-prey carnivore roles. The other elements of the
fauna were far less diverse. There were pareiasaurs,
which were large-bodied herbivores superficially
like the by then extinct tapinocephalid dinocephalians,
and the small-bodied herbivorous, procolophonids,
both probably related to the turtles. A few superficially
lizard-like diapsid reptiles, and a handful of
temnospondyl amphibians more or less complete
the tetrapod faunal list. During the course of the Late
Permian, there certainly were faunal changes. The
dinocephalians as a whole did not survive beyond
the Tapinocephalus Assemblage Zone, and neither did
the carnivorous lycosuchid and scylacosaurid therocephalians.
At the other extreme, whaitsiid therocephalians
and cynodonts did not appear until the
Dicynodon Assemblage Zone, just before the end of
the Permian. However, this degree of faunal
turnover is within the normal, background rates for
terrestrial tetrapods generally.
The same cannot be said of the Permo–Triassic
boundary. The end-Permian marked the largest
mass extinction in the Earth’s history with an estimated
loss of 90–95% of all species; the terrestrial
biota was as much affected as the marine. The discovery
of a large, sharp shift in the ratio of the stable
carbon isotopes 13 C to 12 C (expressed as a negative
shift of the � 13 C value) in several parts of the world,
coincident with the biotic changes, indicates that
the event in question was synchronous worldwide,
and on the geological time scale was at least relatively
brief if not actually catastrophic (Erwin et al.
EVOLUTION OF MAMMAL-LIKE REPTILES 85
2002). The cause of the end-Permian mass
extinction continues to be extensively discussed
(Erwin et al. 2002; Benton and Twitchett 2003). One
of the most significant environmental features is the
negative shift in the � 13 C value itself, indicating a
large rise in organically derived carbon in the atmosphere.
This is presumably due at least in part to
decreased levels of plant productivity, but estimates
suggest that the level of CO 2 was too high for
that alone to explain it. One possibility is that there
was a massive release of methane from methane
hydrates trapped within the polar ice sheets
because of a severe greenhouse warming of the
environment due to the initial increase of CO 2 . A
positive feedback process would have followed,
leading to a rise of the Earth’s average surface temperature
by as much as 6ºC. Indeed, there is direct
evidence for such a global warming episode,
including changes in the oxygen isotope ratios, and
the nature of the floral changes. Other geochemical
signals of the time indicate that anoxic conditions in
both deep and shallow water settings occurred. A
change in the strontium isotope ratios indicates a
possible increased rate of weathering of continental
rocks. Concerning possible triggers for these
changes, there is some evidence quoted for an
extraterrestrial impact, but this is limited and
ambiguous, and does not approach the convincing
nature of the evidence that exists for an end-
Cretaceous bolide impact. In contrast, there is
increasingly convincing evidence for a volcanic
trigger. The Siberian Traps are a huge deposit of
basalt produced by vulcanism. They have an estimated
volume of 3 � 10 6 m 3 and a thickness of up to
3,000 m. The date these rocks began to form coincides
with the end-Permian, 251 Ma. Hallam and
Wignall (1997; Wignall 2001) have developed a
plausible sequence of events beginning with the
eruption of the Siberian Traps that accounts for all
the geochemical and biotic signals described. The
volcanic outgassing increased CO 2 levels, causing
global warming, and this in turn released methane
trapped in gas hydrates in the polar ice sheets. At
the same time, the release of SO 2 and chlorine
caused acid rain. The effect on the biota was devastating,
both directly and by the effects of increasingly
anoxic conditions as photosynthesis levels
fell. Survivors were fungi and algae, as indicated by