Characters and Anatomy



Dicynodonts are members of the Anomodontia within the therapsida lineage. It is thought that herbivory may have evolved within the Anomodontia (Barrett, 2000). Dicynodonts belong to the the synapsida, which include all extinct synapsids and mammals. The temporal opening of dicynodonts is greatly enlarged supporting very powerful jaw muscles.

Types of skull anatomy
Figure 1.1. Four main patterns of temporal fenestrae in amniote skulls a) anapsids, b) synapsids, c) diapsids), d) euryapsids. (j. jugal, p. parietal, p.o. postorbital, sq. squamosal)
(Benton, 2005).
Cladogram

Figure 1.2. Cladogram showing the postulated organisation of groups within synapsida (Benton, 2005).

Dicynodonts are part of the reptilia, they can also be described as mammal-like reptiles and provide a missing link between mammals and reptiles. Mammal-like features include:
(Sullivan et al., 2002)

The group originated in the Permian (260 mya) and became extinct in the Triassic (see Lazarus taxa) after millions of years of success. Dicynodonts diversified into many geographical distributions during their history on earth taking advantage of their new niche. Many of their adaptions are related to their herbivorous lifestyle and their specialised jaw apparatus is thought to be one of the main reasons for their success.

General body plan is short and barrell shaped with strong limbs and a short tail. Size range varies from rat to hippo-sized.



Feeding



Many had a complete absence of teeth apart from their upper canines which were extremely prominent in most. The dentary processes were instead made up of a keratin horny covering on both the upper and lower jaws, forming a beak-like appearance (similar to modern day turtles). Dicynodonts have increasing shutting power due to muscles of their jaw being placed further forwards. Process (figure 1.3):
  1. Lower jaw opens and moves back (a)
  2. Moved forwards by sliding at the joint (b)
  3. Food to the tips of the jaw and closed completely (c)
  4. Jaw slide back firmly: very powerful retraction phase (d)
This process enabled the tearing of food at the front of the mouth. (Benton, 2005.)

Pristerodon skull anatomy

Figure 1.3. Chew cycle of Pristerodon see above process (a-d), (e-g) restoration of jaw muscles (Benton, 2005).



Locomotion



Some forms kept the sprawling posture of reptiles, whilst others were more mammal-like in their movements (Table 2) e.g. Kingoria.

Reptile Mammals
Humerous/femur leaves body horizontally (90º) Leg turned in towards the body
Supported by radula and ulna/tibia and fibula Humerous/femur at an acute angle
Leg moves in a horizontal arc around the centre Two lower bones are no longer vertical
Moves in a vertical arc parallel to the body
Table 2. Locomotory ability of reptiles and mammals. Adapted from King (1990).



Reproduction



It is unknown if dicynodonts laid eggs, as in reptiles, or had developed vivipary, as in mammals. The bony syphilis, between the pubis and ischium, was large enough to allow the birth of live young, however, also being useful in laying large eggs (King, 1990).

Some genera do display sexual dimorphism i.e. canines. Males may have shown territorial behaviour. Duvall (1986) found the vemeronasal organ may have been present. This functions in detecting pheromones and may suggest that dicynodonts showed complex reproductive behaviour. Comparative analysis with modern mammals infers that territorial behaviour and parental care may have been important.  If this were true, it would show one of the first occurrences of a more complex behavior system.



Metabolism and Thermoregulation



There is no evidence of a diaphragm, vibrissae, or a complex brain, which have been put forward as markers for thermoregulation. The only evidence for endothermy is an upright locomotion and the presence of endothermic bone. However, this may be an adaption for inertial homeothermy, maintained by a compact body, possible insulation and a short tail (Geist, 1971).


Characters and anatomy
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