 Most of the units are based on "assemblage zones", layers of strata that contain three or more distinctive fossils.In 1941, a committee chaired by Horace E. Wood II compiled a list of 19 "provincial ages" for North America, later called "North American Land Mammal Ages" (NMLAs).An example of an NMLA is the Rancholabrean, named after the Rancho La Brea fossil site.