The Origin and Evolution of Mammals - Moodle

26 THE ORIGIN AND EVOLUTION OF MAMMALS
nostril. The keel of the angular does have the deep
form characteristic of the group, but it is not reflected
laterally as occurs in other sphenacodontians. The
skull is relatively short snouted, and the dentary
and incisor teeth only moderately enlarged.
Haptodus is also a small form, with a skull length
around 10 cm in most species described, although
the English species Haptodus grandis may have
reached about 28 cm (Paton 1974).
All other sphenacodontians can be included in
the family Sphenacodontidae (Reisz et al. 1992;
Laurin 1993), in which there is a more powerful
dentition. The snout is longer, the lachrymal bone
shorter, and the maxilla has a strongly convex
margin. The canines are very large and all the teeth
develop anterior and posterior cutting edges.
Dimetrodon (Fig. 3.6(d)) is notorious for its elongated
neural spines, presumed to support a sail involved
in thermo-regulation in what must be assumed to
be an ectothermic animal. Several authors have
attempted to model the system and calculate the
effect such an increase in surface area would
have had, and generally agree that it would have
made a very significant difference to the rate of heat
exchange with the environment. Possible functional
advantages are to achieve more quickly a preferred
body temperature in the morning, to lose heat during
the day, or to maintain a more or less stable temperature
throughout (Bennett 1996). Dimetrodon is
abundantly preserved in the Early Permian of North
America, and also extends into the beginning of the
Late Permian in Texas. Some members of the genus
achieved very large body size, with Dimetrodon
grandis about 320 cm in body length and a weight
estimated by Romer and Price (1940) to be around
250 kg. Sphenacodon (Fig. 3.6(c)) is similar to
Dimetrodon but has less strongly developed canine
teeth. It also lacks the dorsal sail, although the
neural spines of the vertebrae are still quite long.
Secodontosaurus (Fig. 3.6(b)) is a specialised
sphenacodontid with a low skull and a long, slender
snout. The anterior lower teeth are procumbent,
and none of the lower teeth are enlarged. Even the
upper canine is modest in size. It was long considered
to be a secondarily fish-eating animal because
of its generally crocodile-like skull. However, Reisz
et al. (1992) showed that there was a fully developed
dorsal sail which they regard as improbable in
a semi-aquatic animal. They proposed instead that
it was adapted for catching small tetrapods hiding
in crevices and burrows.
The origin and early radiation of the
Therapsida
The San Angelo and overlying Flower Pot Formations
of Texas, along with the more or less contemporary
Chickasha Formation of Oklahoma represent
the final stage of the fossil-bearing Permian sequence
in North America. They are dated as Guadalupian,
which is equivalent to Middle Permian, and pelycosaurs
still occur in the form of herbivorous caseids,
Dimetrodon, and also, in the Chickasha, the varanopseid
Varanodon. Olson (1962, 1986) claimed that these
beds also contain the first fossil record of the more
advanced synapsid group Therapsida. The specimens
are in fact extremely scarce and consist only of
fragments of braincases, vertebrae, and limb bones,
and there is considerable doubt about whether any
of them at all are actually the remains of therapsids
rather than of pelycosaurs, probably caseids (Reisz
1986; King 1988; Sigogneau-Russell 1989). However,
approximately contemporaneous continental sediments
occur in the cis-Uralian region of Russia (Fig
2.2(a)), referred to as the Kazanian and early part of
the Tatarian stages (Sigogneau-Russell 1989;
Golubev 2000), and contain undisputed primitive
therapsids (Ivakhnenko 2003). They belong to several
taxa, existing alongside a few lingering caseid
and varanopseid pelycosaurs (Modesto and
Rybczynski 2000). The fossil record of therapsids in
the South African Beaufort Series also extends
downwards to beds equivalent to the early
Kazanian, and also yields primitive members of
therapsid groups (Rubidge 1995; Rubidge et al. 1995).
This fauna is referred to as the Eodicynodon
Assemblage Zone. A third area for very primitive
therapsids comparable to the previous two has been
described in northern China, the Xidagou Formation
in Gansu. Although the exact age of these deposits is
uncertain, they are presumably approximately the
same as the Russian and South African records
(Li and Cheng 1995; Li et al. 1996). Thus by about
270 Ma, a therapsid fauna had evolved and
expanded its range to encompass Gondwanan as
well as Laurasian parts of Pangaea.