The Origin and Evolution of Mammals - Moodle

animal with interdigitating, but not heeled incisors,
large canines that are circular in cross-section, and a
long postcanine dentition of around 20 small teeth
each with a bulbous crown and serrations on the
front and hind edges. There are also many palatal
teeth. As in dinocephalians, the occiput slopes forwards
so that the jaw articulation is shifted anteriorly.
The skull of Estemmenosuchus bears two pairs of bony
protuberances, a long pair that look like horns on the
roof, and smaller laterally extending bosses below
the orbits. Until recently, Estemmenosuchus was interpreted
as the most basal dinocephalian (Hopson and
Barghusen 1986; King 1988), but the situation is still
unresolved. On the one hand Rubidge and Van den
Heever (1997) have published a cladogram of the
main dinocephalian groups based on 27 characters.
They find that Estemmenosuchus is not only a
dinocephalian, but that it is more derived than
Brithopia and therefore related to the Titanosuchia.
At the other extreme, Ivakhnenko (2000a) pointed
to differences in the design of the temporal fenestra
between Estemmenosuchus in which the enlargement
is postero-dorsal, and dinocephalians in which it
is antero-dorsal. As discussed earlier (page 30), he
also noted similarities in the teeth between
Estemmenosuchus and certain other poorly known,
primitive Russian therapsids. On these rather nebulous
grounds, he proposed that Estemmenosuchus is
a member of a group of primitive herbivorous therapsids,
Rhopalodonta, that evolved independently
of the dinocephalians.
Anomodontia
By several criteria, the most successful group of therapsids
were the anomodonts, highly specialised herbivorous
forms. They consist of the largest number of
taxa, with well over 40 genera, and this number is
still increasing regularly, particularly from descriptions
of new Russian and Chinese material. Once
during the late Permian they were the most numerous
individual therapsid specimens in all fossilbearing
localities in which they occur, and must have
occupied a comparable ecological role to the ungulate
mammals of present times as abundant, often
herd-dwelling primary consumers within the terrestrial
ecosystem. They had a worldwide distribution
being the only therapsids yet found in all continents,
including Antarctica and Australia. Finally, they were
EVOLUTION OF MAMMAL-LIKE REPTILES 39
the longest lived of the major therapsid groups, since
they made their appearance at the start of the Late
Permian and survived for certain until the Upper
Triassic. They may actually have survived for a vastly
longer time, for Thulborn and Turner (2003) have
described cranial fragments of what appears to be an
Early Cretaceous dicynodont from Australia. If correctly
interpreted, it extends the temporal range of
the group by a huge margin of 110 Ma.
The actual term Anomodontia has varied in
meaning. The great majority of forms belong to the
advanced group Dicynodontia, but for a time it was
believed that the dicynodonts and the dinocephalians
were sister groups, and the taxon Anomodontia
included both, for example Romer (1966) and King
(1988). However, virtually all subsequent phylogenetic
analyses have refuted this relationship
and the term Anomodontia is now restricted to the
dicynodonts, plus a number of more primitive taxa
which nevertheless possess some of the dicynodont
characters (e.g. Kemp 1988; Hopson 1991).
The main cranial characters defining the
Anomodontia are the following (King 1988; Modesto
et al. 1999):
● shortening of the preorbital region of the skull
● maxillary teeth decrease in size from front to back
● absence of palatal teeth
● zygomatic arch bowed dorsally
● lateral pterygoid processes reduced
● eminence on the dorsal surface of lower jaw
formed from dentary and surangular bones
● mandibular fenestra present.
The ultimate adaptations of dicynodonts for herbivory
are replacement of most or all of the dentition
by a horny beak rather resembling that of turtles,
and extreme modification of the adductor musculature
of the jaws to produce a powerful backwardly
directed slicing bite, an action permitted by a suitably
specialised jaw hinge. A number of early
anomodonts from South Africa and Russia illustrate
stages in the development of the full-blown dicynodont
condition.
Primitive Anomodontia
Until about 1990, the only primitive, non-dicynodont
anomodonts that had been described were Venyukovia
and Otsheria from Russia and three poorly preserved