The Origin and Evolution of Mammals - Moodle

urrowing and subsisting on a diet of subterranean
roots and tubers. The other surviving therapsid taxa
were small in size, and may have existed initially in
less harsh habitats such as uplands and fed primarily
on insects. Of particular interest, the one known
and the one inferred lineages of cynodonts that
lived through the end-Permian event may perhaps
have owed their survival to a higher level of homeostasis
combined with small body size, and possibly
burrowing (Damiani et al. 2003).
The Triassic decline of the Therapsida
The vegetation soon recovered from the effects of
the extinction and during the Lower Triassic an
ecosystem basically structured like that of the
Permian had been re-established. There was an
increase in diversity of various primitive conifer
groups, but seed ferns remained abundant. On the
whole, the flora reflected a warm, seasonal climate
(Behrensmeyer et al. 1992). Of the therapsid survivors
of the end-Permian mass extinction, only
two lineages were of much significance. One is the
kannemeyeriid dicynodonts and the other the
eucynodontian cynodonts. Both groups radiated
through the Lower and Middle Triassic worldwide,
but therapsids never again dominated as they had
in the Late Permian. They shared the terrestrial
amniote world from the Lower Triassic onwards
with increasing numbers of diapsid reptiles,
notably the herbivorous rhynchosaurs and a variety
of thecodontan archosaurs. Then during the
Upper Triassic, they disappeared. Correlation of the
ages of terrestrial Triassic strata is notoriously difficult,
but the main extinction appears to have
occurred during or at the end of the Carnian stage,
by about 220 Ma. This included the disappearance
of the kannemeyeriids and the chiniquodontid
EVOLUTION OF MAMMAL-LIKE REPTILES 87
cynodonts, while the herbivorous traversodontid
cynodonts were greatly reduced in diversity,
although a few occur in the following Norian Stage
(Benton 1994).
There has been a long dispute about whether the
disappearance of the therapsids and the radiation
of the dinosaurs in the Upper Triassic resulted from
competitive interactions in which the dinosaurs
were somehow superior, or from a change in the
environment that caused a decline in therapsids and
allowed an opportunistic radiation of dinosaurs into
the large terrestrial tetrapod niches (Benton 1986).
The traditional view was the competitive one, and a
number of authors speculated on what might have
been the basis for the dinosaurs’s better general
adaptation. Charig (1984), for example, believed
that dinosaurs simply evolved a more effective
locomotory ability, while Bakker (1968) argued that
dinosaurs evolved endothermy before the mammallike
reptiles. Benton (1986, 1994) argued forcefully
that there was actually a significant mass extinction
in the late Carnian, correlated with an environmental
change. There is evidence for an increase in
the extinction rate of plants at least approximately at
the same time (Boulter et al. 1988; Simms et al. 1994).
From this perspective, the decline of the therapsids
resulted from the environmental change associated
with this extinction event, and the increase in
dinosaur diversity from the Norian onwards
occurred as the latter opportunistically invaded the
habitats vacated by the former.
Whatever the truth of the matter, the last part of the
Triassic record of therapsids consisted only of the
highly specialised tritylodontids, the tritheledontans
and the small, relatively rare, and insignificant mammals
(Lucas and Hunt 1994). The phase of synapsid
evolution represented by the Mesozoic mammals
was about to commence.