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. He was also noted for his research into the extinction of the dinosaurs at the Cretaceous–Paleogene boundary (K–Pg or K-T boundary). 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.
Selected publications
Clemens, William A. (1997). "Characterization of enamel microstructure and application of the origins of prismatic structures in systematic analyses", pages 85–112 in W. V. Koenigswald (ed.) Tooth enamel microstructure. Rotterdam: A. A. Balkema.
Clemens, William A. (2001). "Patterns of mammalian evolution across the Cretaceous-Tertiary boundary". Mitteilungen aus dem Museum für Naturkunde in Berlin, Zoologische Reihe77:175–191.
Clemens, William A. (2002). "Evolution of the mammalian fauna across the Cretaceous-Tertiary boundary in northeastern Montana and other areas of the Western Interior." Geological Society of America, Special Paper 361: 217–245.
Clemens, William A. (2006). Ecological diversification of mammals during the Mesozoic, the Age of Dinosaurs. McGraw-Hill.
