A Theory of the Evolution of Therian Dental Formulas and Replacement Patterns

Abstract

An explanation of certain dental characteristics of metatherian and eutherian mammals is synthesized from ontogenetical, paleontological, and morphological observations. The two therian dental sets represent the two successive Zahnreihen of an as yet unknown therapsid. The molars are serially homologous with the primary Zahnreihe or milk dentition. Generally, molar number decreased from eight or nine in the ancestral therapsid and primitive pantothere to four in the mid-Cretaceous eupantotherian stock. Occasional reversals of this process took place, the addition (and subtraction) always occurring at the posterior end of the molar row. However, the antemolar number has seldom, and perhaps never, increased. Through the Jurassic and well into the Cretaceous, the maximum incisor number probably remained five above and four below. The maximum premolar number remained four in both jaws, although selective pressures favored a gradual retardation in development time of both milk and replacement first premolars. ...