There are a variety of successive preservation zones
within the fossil record, highlighting the development and evolution of
the dicynodont form.
The Taphinocephalus zone provides fossils of the first dicynodont
forms. The forms are small but had developed many of the features known
in the higher dicynodont. There land was swampy, interrupted by
rivers, lakes and forest. The climate was seasonal and therefore
herbivores would have to have a mixed diet of fruit, leaves and
groundcover or else hibernate (see burrowing
) (King, 1990).
This was an early South African form from the Taphinocephalus
zone. It had not developed the more complex masticatory systems
of later forms. It had good back and forth movement of the jaw,
however, the secondary palette is small as the premaxilla
expanded. As a result, Eodicynodon had no extensive contact processes or muscle
guidance system (King, 1990).
Present in South Africa but at a later date found in Zambia and China.
Very abundant in numbers within the fossil record. Diictodontidae were
higher dicynodonts with the following features:
- Loss of premaxillary teeth
- Extension of palatine bones
- Loss of marginal post canine teeth
- Maxilla has embayment in front of canine
In Diictodon, there is a complete loss of post canine teeth, and less
of a reliance on the palatine for mastication. A distinctive notch at
the front of the mouth may have
functioned in order to chop very resistant plant material (King, 1990).
Diictodon produced helical
burrow systems (burrowing
) and had well developed forelimbs. Furthermore, a laterally
flexible vertebral column would have aided excavation and movement
within burrow systems (Ray and Chinsamy, 2003). Diictodon also showed a
high level of sexual dimophism
(tusk ornamentation of males). This is
the oldest claim of sexual dimoprhism
in the fossil record (Sullivan et
Shares many of Diictodon characters such as a notch on maxilla
possesson of canniform process. However, has a moderately wide skull
roof and post canine teeth. Palatines are not relied upon and therefore
reduced. Lower jaw teeth are present and there is a variable amount of teeth
behind the tusk. Robertia retains the sprawling locomotion of the reptilia
and has extensive postural musculature. There is a possibility that
these forms dug with their forelimbs. They also had long, sharp claws - possibly to aid digging (King, 1990).
Figure 2.1. Skeleton of Robertia from the Upper Permian (Benton, 1995).
Emydops has a reducion in teeth on the lower jaw. Teeth are small and
irreguar. Widening of this intertemporal regon is a more primitive
feature, later forms are narrower with down-turned sides. This
broad surface does not have the advantageous sharp sides for muscle
attachment. Later dicynodonts have a narrower region in which
powerful jaw muscles can attach (King, 1990).
Peak in dicynodont diversity, with approximately 35
genera. The larger dicynodonts have now radiated.
There was also a spread in geographic distrubution to the southern
Pangean continents (King, 1990).
The family have a deep, thin walled cleft in the lower jaw for
anchoring the horny platform. They had a very precise cutting and
crushing surface between the beaks, where the rim of the maxilla
pointed downward to fit with the lower jaw symphisis. Oudenodon had no teeth at all. The skull was directed forwards,
therefore, adapted to feeding at levels approximately 20-100cm above the
ground (King, 1990). This shows a new level of diversification, away from substrate-feeding.
This is the type specimen of the dicynodonts as it was the first to be
found. It can be found in South Africa, Zambia, USSR and China. It is a
varied form. The skull has
tusks in the upper jaw but the lower jaw is completely toothless. Sharp
ridge for good attachment of powerful
muscles. Covered in horny platforms. Very generalised postcranial skeleton. Not very quick as
retained the sprawling position of limbs. There is dispute over whether
the genus was aquatic due to the higher position of the nostrils (King, 1990).
This was a substrate feeder with a large dentary shelf and blunt,
shovel-shaped lower jaw beak. Tusks were either present or absent. The
head was directed downwards for feeding on substrate. Hindlimbs
parallel to body and more modified than forelimbs. Hindlimbs adapted
for agility, whereas forelimbs for power (King, 1990).
Reduced palatine and a sharp cutting edge. Occiput is broad and
slightly convex as in present day burrowers (see burrowing adaptations
for the attachment of powerful neck and shoulder muscles. This genera
is very muscular in general (King, 1990).
Spread over South Africa, China, Tanzania, Zambia, the USSR and
Scotland. Dicynodonts seen to be among the substrate-feeders. There is
a slump in diversty with only 7 genera. Oudenodon and Dicynodon became
more common, however, most genera declined to inexistence. The zone has a
very poor fossil record (King, 1990).
Covered all the southern continents apart from South America. In the lower part of the Triassic there were 2 genera:
There is a greater proportion of carnivora and increased impovrishment
for herbivores due to floral changes occurring as climate was warming
as land masses coalesced to form Pangea (King, 1990).
Most specialised of the Triassic forms. Possibly aquatic as the pelvic
girdle looks to be adapted for water. Perhaps it fed in water (King, 1990).