ON THE BIOLOGICAL IMPROBABILITY OF ZAHNREIHEN AS EMBRYOLOGICAL UNITS.

Abstract

In a series of odontological essays Boll (1922) observed that in the polyphyodoni dentitions of elasmobranchs and other nonmammalian vertebrates teeth were replacec alternately. In order to explain the evolu. tion of mammalian diphyodonty from rep. tilian diphyodonty he proposed that on( alternating set of teeth in a reptile corresponded with the whole deciduous dentition together with the permanent molars ol a mammal, and that the other alternating set corresponded with the permanent incisors, canine and premolars. Parrington (1936) extended the observations of Bolk when he demonstrated that in the mammal-like reptile Thrinaxodoon the postcanine teeth were replaced in waves which swept through alternate tooth positions from the back to the front of the jaw rather than the strict alternation described by Bolk. Romer and Price (1940) demonstrated the same condition in the Pelycosaurs and Edmund (1960), in what has now become the classic work in this field, documented the condition in a wide variety of fossil and recent reptiles. In order to explain the ontogeny of the sequences he had described, Edmund (1960) introduced the Zahnreihe theory which is the subject of the present paper. His theory linked together several types of tooth replacement phenomena and he extended it to include the evolution of mammalian dentitions. Since this time numerous workers have documented the wave replacement of alternate teeth by studying tooth replacement in individual dentitions over periods of many months (for example Cooper, 1963) .