The Origin and Evolution of Mammals - Moodle

28 THE ORIGIN AND EVOLUTION OF MAMMALS
the presence of large bosses on the skull-roofing
bones. Romer and Price (1940) proposed that it was
allied to the basal pelycosaur group Eothyrididae, a
view subsequently discarded by Reisz (1986) on the
grounds that the two taxa shared no derived characters
that he could discover. Recently, Laurin and
Reisz (1996) re-studied the specimen and far from
regarding it as merely a peculiar pelycosaur, they
came to the conclusion that it is, in fact, the most
basal therapsid known. Their interpretation is all
the more remarkable because the specimen comes
from the Clear Fork deposits of Texas, which are
Early Permian, Leonardian, in age. Thus
Tetraceratops predates the first of the Russian and
South African therapsids by as much as 10 million
years. They listed several characters shared with
therapsids, including an enlarged temporal fenestra
with signs of a broad, fleshy muscle attachment to
its upper edge. In the palate, the interpterygoid
vacuity is reduced in size and bounded posteriorly
by the meeting of the pterygoid bones. The quadrate
is reduced in size, as is the base of the epipterygoid.
The braincase is attached to the back of the skull in
a therapsid manner. In other respects, Tetraceratops
has the primitive characters of sphenacodontids and
other pelycosaurs, such as a large lachrymal bone,
unfused basipterygoid articulation, and differential
sizes of the premaxillary teeth.
As interpreted by Laurin and Reisz (1996),
Tetraceratops is an extremely illuminating fossil that
illustrates an intermediate stage in the evolution of
the definitive set of therapsid characters. It is all the
more unfortunate, therefore, that it is known from
such limited and poorly preserved material and so
it is hard to avoid the suspicion that Tetraceratops is
actually a highly specialised member of one of the
pelycosaur-grade taxa that has a number of superficial
similarities to therapsids.
The diversity of early Therapsida
Leaving aside the very dubious possibility that
Tetraceratops is a therapsid, a most remarkable feature
of the origin of therapsids is the high diversity
of taxa present at their earliest appearance in the
fossil record. At present, the stratigraphic resolution
(Fig. 2.2) is inadequate to distinguish the early Late
Permian dates of the Russian Tatarian-Kazanian, the
South African Eodicynodon Assemblage Zone, or the
Chinese Xidagou Formation, all of which have produced
very early therapsids. For the time being the
respective faunas must be considered at least
approximately contemporary, a position supported
by their similarity. At least nine lineages are apparent,
five of which are identified as basal members of
groups that achieved prominence later, as described
later in the chapter. The others are representatives of
short-lived taxa that did not survive beyond this
short period of time. This initial therapsid radiation
included a variety of carnivores and herbivores, and
at least one possible insectivore.
Biarmosuchia (page 31)
The biarmosuchians are the most sphenacodontidlike
therapsids in appearance due to the strongly
convex dorsal margin of the skull and the short,
broad intertemporal region. The single canine is
very much larger than the other teeth and the postcanines
are reduced in relative size. Several specimens
including postcranial skeletons have been
found in the Eshovo (Ocher) and Mezen’ faunas of
Russia, although not yet in South Africa until
younger levels, or in the Chinese sediments.
Brithopian Dinocephalia (page 37)
The earliest dinocephalians were relatively large carnivores,
which had retained very prominent canines,
and dorso-ventrally elongated temporal fenestrae.
Brithopians occur in all the three areas yielding early
therapsids, Russian, South African, and Chinese.
Anomodontia (page 39)
Several basal genera of small to medium-sized primitive
members of what was to become by far the most
diverse of all therapsid herbivores have been found.
Unlike the later forms, incisor teeth are still present,
but the characteristic shortening of the skull and
inferred rearrangement of the adductor jaw musculature
was under way. Primitive anomodonts occur in
the Russian and the South African early faunas.
Gorgonopsia (page 52)
Some poorly preserved remains of gorgonopsians,
the dominant carnivorous group of the later
Permian, have been recorded in the South African
Eodicynodon Assemblage Zone (Rubidge 1993;