The Origin and Evolution of Mammals - Moodle

20 THE ORIGIN AND EVOLUTION OF MAMMALS
There are numerous characters in which the pelycosaurs
retain the primitive condition compared to the
therapsids. These include the relatively small size
of the temporal fenestra, the absence of distinctive
canine teeth in the upper and the lower jaws, and
the absence of a reflected lamina of the angular bone
of the mandible. In the postcranial skeleton, the limbs
are relatively short and heavily built and the girdles
massive, indicating a lumbering, sprawling gait.
The modern cladistic framework for understanding
the interrelationships of the pelycosaurs was
established by Reisz (1986), who abandoned the
largely paraphyletic groupings of Romer and Price
(1940). This was slightly modified by Berman et al.
(1995) as a result of re-study of some of the critical
genera, and their cladogram is illustrated here
(Fig. 3.3). The biggest difference from earlier understanding
of the taxonomy is the recognition of a
basal group of pelycosaurs which, although tending
to be highly specialised, possess a primitive skull
structure. These, the Caseasauria, have broad, low
skulls, large postorbital and supratemporal bones,
and a very limited exposure of the frontal bone in
the margin of the orbit. The rest of the pelycosaurs
form a group now named the Eupelycosauria.
CASEASAURIA
EUPELYCOSAURIA
Caseasauria: Eothyrididae
Eothyris (Fig. 3.2(c)) is known from a single specimen
of a small skull, only about 6 cm in length,
from the Early Permian Wichita Formation of Texas.
The most striking feature of the skull is its exceptionally
broad, flat shape and unique dentition,
which consists of two very large upper caniniform
teeth on each side. No comparably enlarged lower
teeth are present. Set within a row of otherwise
small, pointed teeth, the upper canines indicate a
specialised, presumably carnivorous mode of life,
but nothing at all is known of the postcranial skeleton,
which might have thrown further light on its
habits.
Langston (1965) described another eothyridid,
Oedaleops, from a single skull (Fig. 3.4(a)), which has
a similar size and proportions to Eothyris but in
which the caniniform teeth are much less prominent.
It dates from the contemporaneous Cutler Formation
of New Mexico but is more primitive than Eothyris by
virtue of its more generalised dentition.
Caseasauria: Caseidae
The caseids (Fig. 3.4(b) and (c)) share with the
eothyridids an enlargement of the external nostrils
Eothyrididae Caseidae Ophiacodontidae Edaphosauridae Sphenacodontidae
Varanopseidae
Figure 3.3 Phylogeny of the families of pelycosaurs (Kemp 1988).