Diarthrognathus ("Two joint jaw") is an extinct genus of tritheledontid cynodonts, known from fossil evidence found in South Africa and first described in 1958 by A.W. Crompton. The creature lived during the Early Jurassic period, about 200 million years ago. It was carnivorous and small, slightly smaller than Thrinaxodon, which was under 50 centimetres (20 in) long. The articular and quadrate bones evolved to become two of the middle-ear bones in mammals. The transition exemplified by Diarthrognathus suggests that natural selection favoured animals with a more powerful bite.