ASPECTS OF ONTOGENY IN THE STUDY OF 

 AMMONITE EVOLUTION 



A. E. TRUEMAN 

 University College of Swansea, Swansea, Wales 



It is well known that the development of an individual generally 

 recapitulates to some extent the stages passed through during the 

 evolution of its ancestors, or in other words, that "ontogeny repeats 

 phylogeny." In the paleontological work of the last quarter of a 

 century this principle of recapitulation has frequently been used in 

 determining the relationships of fossils, and its general truth has 

 been demonstrated among many groups of Invertebrates; for 

 example, by Mr. R. G. Carruthers and others among the Rugose 

 corals,^ by Professor Grabau among the Gastropods,^ and by 

 numerous workers, particularly Mr. S. S. Buckman, among the 

 Ammonites.^ 



That ontogeny is not always merely a recapitulation of phy- 

 logeny, however, has been recognized by zoologists for many 

 years; the development is often modified to fit the embryo for a 

 special mode of life, as in many modern Arthropods, so that ances- 

 tral stages are obscured to a greater or less extent. In other 

 cases on account of the individual hurrying through its development 

 in order to attain maturity as early as possible, certain phylo- 

 genetic stages are "skipped" in development. 



Numerous paleontologists have recognized the importance of 

 this skipping of stages in development; Mr. S. S. Buckman dealt 

 with it some years ago and has pointed out several examples."* 



^ See, for instance, R. G. Carruthers, "The Evolution of Zaphrensis delansuei," 

 Quart. Jour. Geol. Soc, Vol. LXVI (1910), p. 523. 



2 A. W. Grabau, "Phylogeny of Fusus and Its Allies," Smithsonian Misc. Coll., 

 No. 1417 (1904), pi. xviii, fig. 2. 



3 See, for instance, "Yorkshire Type Ammonites." Introduction to Vol. I 

 (1909-12). 



" S. S. Buckman, "Monograph of the Inferior Oolite Ammonites," Palaeont. Soc, 

 p. 289. 



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