The Origin and Evolution of Mammals - Moodle

are forms such as Casea itself, varanopseids and
certain amphibians. Only in one locality may the
caseid chronofauna have been sampled directly,
Fort Sill in Oklahoma. This is an upland fissure-fill
deposit in the Arbuckle Limestone, probably of
Arroyo Formation age, and containing fragmentary
remains of several small pelycosaurs, some apparently
caseid-like. Olson’s view is that as increasingly
seasonally arid conditions spread during the
Early Permian, so this hypothetical caseid chronofauna
expanded and by the end of the Early
Permian it had replaced the pre-existing Permo-
Carboniferous fauna that had been adapted to wetter
conditions. From this argument, it would follow
that the caseids and varanopseids were physiologically
adapted for drier conditions, and it would
also follow that the sphenacodontids underwent a
parallel physiological change, since they spanned
the transition from coal measure to full-blown red
bed environments.
Thus pelycosaurs were the major faunal element
in the development for the first time of a fully terrestrial
vertebrate ecosystem that was no longer
embedded directly in freshwater productivity. The
palaeoecological evidence leads to the conclusion
that the important physiological features ultimately
characterising mammals made their initial, incipient
appearance within the pelycosaur grade. The relevance
of this possibility to the question of the origin
of the therapsids will be discussed next.
Biogeography and palaeoecology of the origin
of the Therapsida
Irrespective of whether or not the origin of the
taxon Therapsida lay in the earliest Permian as
very doubtfully suggested by Tetraceratops (page 27),
the fossil evidence is of a rapid radiation of therapsids
about 270 Ma, soon after the start of the
Late Permian, or Guadalupian. Within a very narrow
time band, a considerable variety of ecotypes
appeared, carnivores and herbivores varying considerably
in body size. This radiation coincided with
the decline almost to extinction of pelycosaurs. It
was also associated with a major increase in biogeographic
range. The restriction of pelycosaurs to
the tropical region, within 30º latitude of the
palaeoequator is markedly contrasted by the initial
EVOLUTION OF MAMMAL-LIKE REPTILES 83
therapsid radiation, which included the temperate
zones between 30º and 60º, in both the northern and
the southern hemispheres (Fig. 3.27(b)). Admittedly,
few tetrapod bearing beds have been discovered
yet within the tropics from this time period
apart from the extremely poor North American
Guadalupian, so it is not known whether the early
therapsid distribution really was disjunct, or
whether it spanned the intervening equatorial
zone. The taxonomic similarity between the respective
northern fauna of Russia and the southern
fauna of South Africa strongly suggests that the
two could readily interchange, and therefore that
there was a more cosmopolitan distribution that
included the intervening tropics (Parrish et al. 1986;
Milner 1993).
The question of what could have triggered this
initial radiation of the therapsids, apparently at the
expense of the pelycosaurs, hinges on whether there
were any environmental changes at the time that
opened up new opportunities for the nascent group,
or whether therapsids possessed some evolutionary
innovation in their biology that both created a
competitive advantage over pelycosaurs and allowed
their radiation into areas hitherto unavailable to
synapsids.
There are no indications in the geological record of
any large, or abrupt palaeo-geographical or climatic
events at this time. The start of the Late Permian
coincided approximately with the attachment of the
eastern landmasses of Kazakhstan, Siberia, North
China, and South China to Pangaea, increasing to a
small extent the area potentially available for a cosmopolitan
fauna. More importantly, it also coincides
with the withdrawal of the glaciation that had
covered much of Gondwana from the Carboniferous
through the Early Permian, although this was
merely the continuation of a process that had been
going on through much of the Early Permian.
Zeigler et al. (1997) reconstructed the climate and climatic
zones of the Permian, and according to their
analysis, the transition from the Early Permian to the
Late Permian was not remarkable apart from this
glacial retreat to high altitudes and latitudes. The
palaeoclimate of the tropical regions increased
in aridity as illustrated in most detail by the
southwestern USA sequence (Behrensmeyer et al.
1992). The flora indicates a gradual but eventually