"In 1981, a large jawbone, 41723-5, from Brewster County, Texas was described as "M. mckinneyi", named after one of the discoverers Billy Pat McKinney, and was found in an area dated to 43 mya in the Late Eocene.The jawbone and tusks were more massive than that of "M. chadronensis", and it could represent either a transitional phase between "Amynodon" and "M. chadronensis", or simply a migrant to the area.