THE DECADENCE OF AMMONITES 



By FELIX OSWALD, D.Sc. (Lond.) 



Hardly any class of fossils is better known to the general 

 public than that of the Ammonites ; their beautiful spiral forms, 

 often ornamented with spines and ridges, appeal to every lover 

 of nature. Whilst the majority of Ammonites are rolled up to 

 form spiral discs, it is well known that quite a number of more 

 or less unrolled, turreted, and even straight species frequently 

 occur in rocks of Cretaceous age, previously to the total 

 extinction of the whole group ; the abundance and variety 

 of these abnormal forms is obvious even to the most casual 

 visitor of the exposure of the Gault in the Warren, near 

 Folkestone, where these fossils are so lavishly scattered on 

 the seashore. 



The development of more or less uncoiled species prior to 

 annihilation is manifested, however, not merely in one family 

 but in several ; and it has occurred not only in the Cretaceous 

 period, but in the Jurassic, and even so early as the Trias, 

 when an important division of Ammonites became extinct. 

 It would seem probable, therefore, that this phenomenon of 

 uncoiling in different families may have had the same deter- 

 mining cause, and the present article is an attempt to trace 

 graphically the parallel evolution of these forms and to hazard 

 a speculation as to their origin. 



The table on the opposite page of the range in time of 

 Ammonites will show that the important sub-order of the 

 Discocampyli appeared in the Permian, reached a great develop- 

 ment in the Triassic, and died out completely at the end of 

 that period. 



One of the best known types of the Discocampyli is the genus 

 Ceratitcs of the Muschelkalk (fig. i) ; its comparatively simple 

 lobes and saddles are in marked contrast to the increasing 

 complexity which they display in the Jurassic and Cretaceous 

 Ammonites. The degenerate families of the Choristoceratidce 



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