The Origin and Evolution of Mammals - Moodle

24 THE ORIGIN AND EVOLUTION OF MAMMALS
that Ophiacodon was an aquatic, fish-eating animal
(Romer and Price 1940), with its long, narrow snout,
numerous sharp teeth, and absence of a distinct
caniniform. The orbits are high up in the side of the
skull. The digits are flattened suggesting that the
animal indulged in some degree of aquatic paddling.
Varanosaurus is closely related to Ophiacodon
(Berman et al. 1995), but differs in the flattened top
and sides of the snout, creating a box-like effect. Its
axial skeleton is most unusual, for the neural arches
of the vertebrae are swollen, the neural spines alternate
in height, and the zygapopophyseal surfaces are
close to horizontal, much more so than in other
pelycosaurs (Sumida 1989). Thus the vertebrae
resemble those of several primitive tetrapod groups
such as seymouriamorphs, diadectids, and captorhinids,
undoubtedly as a result of convergence.
Sumida (1989) proposed that the arrangement of
the vertebrae in Varanosaurus permitted greater
dorso-ventral flexion of the column as well as lateral
flexion, giving the animal an enhanced degree
of agility of aquatic locomotion.
Eupelycosauria: Edaphosauridae
Edaphosaurids are mainly specialised herbivores
which superficially resemble caseids, and indeed
were for long classified with them as a taxon
Edaphosauria. However, there are significant differences
in skull structure between members of the
two, and also primitive edaphosaurids lacking the
full set of adaptations for herbivory are known.
Both these observations indicate that caseids and
edaphosaurids independently evolved convergent
herbivorous adaptations.
Edaphosaurus itself (Fig. 3.5(d) and (e)) is highly
distinctive. It is known from the Late Pennsylvanian
well into the Early Permian, and occurs in Europe as
well as North America. Species vary in body length
from as little as 100 cm up to around 325 cm. The
skull is very small compared to the body size, and is
short, strongly constructed and has a deep lower
jaw indicating powerful adductor musculature.
Each of the marginal teeth possesses a cutting edge
and is set obliquely across the jaw. The tooth row as
a whole would have functioned in cropping vegetation,
as indicated by the development of wear facets
(Modesto 1995; Reisz and Sues 2000). A second dental
mechanism is created by large tooth plates bear-
ing numerous teeth, occurring on both the palate
and the opposing inner sides of each lower jaw,
which would have had an effective crushing action.
The postcranial skeleton is most distinguished by
the sail along the back, supported by hugely elongated
neural spines from neck to lumbar region.
Unlike the comparable sail in Dimetrodon (page 26),
here there are lateral tubercles or short crosspieces.
While usually assumed that the sail of pelycosaurs,
where present, had a thermoregulatory role, there
has been some doubt about this interpretation in
Edaphosaurus, because the area of the sail does not
scale with body size as predicted (Modesto and
Reisz 1990). However, Bennett (1996) conducted
wind-tunnel experiments on a model sail and
showed that the effect of the transverse projections
is to increase the turbulence of airflow and therefore
the rate of exchange of heat across the surface of the
sail membrane. A relatively smaller sail could therefore
remain effective as a heat exchanger in a larger
animal.
The Upper Pennsylvanian Ianthasaurus (Fig. 3.5(c))
is a member of the family Edaphosauridae by virtue
of possessing elongated neural spines with transverse
tubercles, and also a lateral shelf above the orbit
formed from an enlarged postfrontal bone (Reisz and
Berman 1986; Modesto and Reisz 1990). However, it
lacks the specialisations for herbivory that make
Edaphosaurus so distinctive, retaining sharp, slightly
flattened and recurved teeth and little enlargement of
the temporal fenestra. It is also one of the smallest
pelycosaurs known, with a skull length of about 8 cm
and a presacral body length of about 35 cm. Such an
animal was presumably an insectivore. Modesto
(1994) also re-described the Early Permian form
Glaucosaurus, known only from a single, incomplete
skull (Fig. 3.5(b)). It is structurally intermediate
between Ianthasaurus and Edaphosaurus, suggesting
perhaps that it was omnivorous.
Eupelycosauria: Sphenacodontia
The sphenacodontians include the large carnivorous
pelycosaur Dimetrodon, which is much the best
known and most studied of all the pelycosaur genera.
There is a distinctive dentition in which
the upper canine is enlarged and supported by
a buttress on the inner surface of the maxilla. Less
than five premaxillary teeth are present, of which