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.
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).
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:
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):
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.
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).
(Benton, 2005).
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:
- Secondary palate
- Mammal-like skull architecture
- Reduced phalange formula
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):
- Lower jaw opens and moves back (a)
- Moved forwards by sliding at the joint (b)
- Food to the tips of the jaw and closed completely (c)
- Jaw slide back firmly: very powerful retraction phase (d)
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 |
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).
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