The animal with the earliest sexual differentiation.
Name: diictodon (DY-ik-toe-don) means two teeth
animal: therapsid reptile
size: 45 cm long
diet: herbivore
lived: 256 - 252 mya
found: Africa and Asia
fossil: 
In the late Permian period, the same time as the gorgonopes, these extremely common mammal-like synapsids burrowed deep in the ground living in couples, probably staying with the same partner its entire life. It’s head was large, with a beak that had no teeth except two saber like tusks. The tusks are extremely important as they are the characteristic of the first sexual dimorphism in all of animal history. Only the males were found having the pair of tusks. It lived in deep narrow tunnels protecting it from harsh weather and larger predators - making it an extremely successful animal of its time.
The diictodon were abundant, not only would you find multiple burrows within the same patch of land, their fossils also make up about 1/2 of all backboned animals in South Africa from the Late Permian Period. None of the burrows were interconnected further stressing the theory that the diictodon were strictly monogamous. A small chamber at the end of the burrow would have vegetation for comfort, this is where the diictodon female would have and raise her children.
These underground burrows would have been excellent for times of drought. The burrows would have remained cooler, and had a damp environment. The diictodon could also feed on the roots that were underground. The animals would, however, build their burrows in areas that were not so great, such as river banks. They burrows would often flood killing the animals. The particular fossil above is a pair of diictodon that were cuddled together when flood struck.
The diictodon’s arms and legs did not lack in strength, still they were strictly herbivores. It had an excellent sense of balance, sight and smell. It would use these keen senses to dart in and out of its burrow knowing danger was not near. It would then use its beak to break of vegetation and other plant pieces to eat and use in their housing.
The diictodon is widely studied since it is such an abundant animal. Not only was it the first animal with sexual differentiation, it was also the missing link between reptiles becoming mammals. This little reptile is thus a hugely important link in the evolutionary chain.
The finding of the sexual dimorphism led scientists to believe there was an extremely complex set of social behaviors within the diictodon community. They could have been used to combat other males in an attempt to find a mate, or could have been used ritualistically. These tusks gave rise to unbelievably huge developments in mammals in the near future.
