William Alvin Clemens Jr. (May 15, 1932 — November 17, 2020)[5] was a paleontologist at the University of California at Berkeley.

From 1961 to 1967, he served as faculty in the Zoology Department at the University of Kansas and as the curator of higher vertebrates in their Museum of Natural History.

[7] Clemens' research focussed on the evolution of mammals in the Mesozoic Era, both their origin and diversification as well as the microstructure of the early mammalian jaw and teeth.

Clemens supported a view contrary to the more familiar Alvarez hypothesis model of sudden catastrophic extinction precipitated by an asteroid, which was proposed in part by Walter Alvarez, also at the University of California, Berkeley, at the time.

Clemens' research in western North America suggests that the dinosaurs were already undergoing gradual extinction prior to the end of the Cretaceous and that other groups of vertebrates were not severely impacted by the event.