- « "Small and ugly in a kind of cute way." »
- — Connor Temple on Sid the Diictodon[src]
Diictodon was a small, herbivorous synapsid, which measured approximately 45 centimetres in length. They possessed a cylindrical body with strong legs, and a large head which ended in a small beak with a pair of tusks protruding from the upper-jaw. Diictodon also had large round eyes, and white and brown patterns on their skin. Diictodon were notable for their ability to and habit of chewing up practically anything they could - ranging from papers and clothing to cables, cage mesh and electrical wiring. Diictodon were not very aggressive creatures, and were highly playful and could be considered 'cute' with each-other, and were also willing to socialize with other small herbivores such as Coelurosauravus. They also seemed slightly afraid of human contact, but not very much.
- Sid and Nancy - A duo of Diictodon who came through an Anomaly to a London hospital in the present, and were marooned in the present when the Anomaly closed and were afterwards taken in by Connor Temple and Abby Maitland. (Episode 3.3) They spent a brief time living in Abby's flat, before Jack's arrival forced Connor to move out and take Sid and Nancy with him. (Episode 3.4) They initially lived in Connor's lab at the Anomaly Research Centre, (Episode 3.5) then moved with Connor to James Lester's flat. (Episode 3.6, 3.8, 3.9) After Connor and Abby became stranded in the Cretaceous, (Episode 3.10) Sid and Nancy were taken into the ARC's custody and placed in the Menagerie.
- Approximately twelve other Diictodon who came through the same Anomaly as Sid and Nancy to the London hospital at the same time, and proceeded to burrow into the hospital's walls and chew through the electrical wiring there. Using one of the Diictodon's distress calls, all the others were lured back to the Anomaly, and all of them save Sid and Nancy managed to return through before it closed. (Episode 3.3)
Diictodon lived from approximately 260 to 254 million years ago in what is now Africa and Asia (particularly South Africa), and would have shared many features with modern-day mammals. Diictodon made burrows in the earth of a depth of up to 5 feet (1.5 metres), and generally nested near flood plains, and they were generally solitary despite having burrows within close proximity to each other. Diictodon are believed to have probably fed on desert shrubs and/or roots in their burrows. Diictodon had no known rival or predator species in its niche, so it is thought that they probably competed with other species for food.
- Although Sid and Nancy haven't been seen since Episode 3.9, it was confirmed by the producers that they were put in the Menagerie after Connor and Abby disappeared.
- The Diictodons in Primeval resemble the Diictodons from Walking with Monsters.