The Origin and Evolution of Mammals - Moodle

22 THE ORIGIN AND EVOLUTION OF MAMMALS
and a presumably correlated tendency towards a
forward tilt of the very front of the pointed snout.
However, they are highly modified for herbivory
(Reisz and Sues 2000). The skull is short and low,
the temporal fenestra large, and the jaw articulation
depressed below the level of the tooth row. These
are features relating to enlargement of the adductor
musculature to increase the force of the bite. The
dentition consists of a homodont marginal dentition
in which each individual tooth is stout, blunt,
and somewhat bulbous. The palate bears several
rows or areas of small palatal teeth which, since
there are no comparable teeth on the inner surface
of the lower jaw, presumably worked in conjunction
with a tough, muscular tongue, an interpretation
supported by the presence of a particularly welldeveloped
hyoid apparatus in the floor of the
mouth. The skeleton of caseids is also characteristic
of a herbivore (Fig. 3.4c). Proportionately the skull
is small and the neck vertebrae reduced. The body
is barrel-shaped, as indicated by the ribcage, implying
that there was a very large alimentary canal
with a fermentation chamber somewhere along its
length. There is a wide size range in caseids (Olson
1968). Casea broilii, for example, had a body length
of around a metre, while the largest species known
is Cotylorhynchus hancocki with a length of at least 4
m and a body weight estimated to have been in
excess of 500 kg.
Langston (1965) suggested that Oedaleops was a
good model for the ancestry of the caseids. The
Caseidae appear relatively late in the fossil record,
not until into the Early Permian, but on the other
hand were one of the last surviving pelycosaur
groups, being known from the Late Permian rocks of
the San Angelo and Flowerpot Formations of Texas
(Olson 1968). They are also amongst the few pelycosaur
taxa with representatives from other parts of
the world. There is a beautifully preserved Early
Permian specimen named Casea rutena (Fig. 3.4b)
from France (Sigogneau-Russell and Russell 1974),
and Ennatosaurus is from the Kazanian Zone II of the
Late Permian of Russia (Ivakhnenko 1991).
Eupelycosauria: Varanopseidae
All other pelycosaurs are included in the Eupelycosauria,
characterised by elongation and narrowing
of the snout, the frontal bone forming at least
one-third of the dorsal orbital margin, the supratemporal
bone narrow and set in a groove, and the pineal
foramen set relatively further back in the parietal
bone (Reisz 1986; Reisz et al. 1992). The members of
the family Varanopseidae are the least derived of the
eupelycosaurs. Relative to other pelycosaurs, they
were small, long-limbed, agile animals with a long
row of sharp teeth and little development of caniniforms,
and presumably they led an active, predatory
mode of life. The best known are Varanosaurus
itself and Aerosaurus (Langston and Reisz 1981),
both from the Early Permian (Fig. 3.4(d)). However,
the family is actually the longest lived and most
widely distributed of all pelycosaur families. Some
specimens of Aerosaurus also date from the Upper
Carboniferous, while other taxa share with the
caseids the distinction of surviving into the Late
Permian. Varanodon occurs in the Chickasha
Formation of Oklahoma, and there is also a Russian
Late Permian form, Mesenosaurus (Reisz and Berman
2001). Even more remarkably, a genus of varanopseid
occurs in the southern hemisphere. Elliotsmithia
(Fig. 3.4e) has long been known from a poorly
preserved skull from the Late Permian Tapinocephalus
Assemblage Zone of the South African Karoo, and
Dilkes and Reisz (1996; Reisz et al. 1998) confirmed
the impression of several earlier authors including
Romer and Price (1940) that Elliotsmithia is indeed
properly referred to the varanopseids. Modesto et al.
(2001) have described a second specimen further
confirming this conclusion. So far this is the only
well-established record of pelycosaurs from
Gondwana, although another form, Anningia, has
been claimed to be a South African pelycosaur.
Unfortunately, the sole specimen is a skull which is
too incomplete and poorly preserved for confirmation
to be possible (Reisz and Dilkes 1992).
Eupelycosauria: Ophiacodontidae
The traditional taxon Ophiacodontia was regarded as
the most primitive group of pelycosaurs by Romer
and Price (1940), but they included the eothyridids as
well as the eupelycosaurian family Ophiacodontidae,
and it was therefore paraphyletic. Taken alone,
the latter family is a monophyletic group of eupelycosaurs
containing a number of markedly long
snouted forms (Berman et al. 1995). The earliest
adequately known pelycosaur, Archaeothyris