Each species of mammal-like reptile that has been found appears suddenly in the fossil record and is not preceded by the species that is directly ancestral to it. It disappears some time later, equally abruptly, without leaving a directly descended species…’
Tom Kemp, ‘The reptiles that became mammals’ New Scientist, vol.92, March 4, 1982, p.583.

Kemp is Curator of Zoological Collections at the Oxford University Museum in England.