PALEONTOLOGY 331 



time. Up to this period the septa across the shell were flat and saucer- 

 like, and the sutures, the lines of junction of the septa with the wall of the 

 shell, were nearly straight or only slightly curved. Later the septa 

 became bent in various ways, at least at their edges, so that the sutures 

 were curved or angular (see Fig. 227). Forms whose sutures were of 

 this curved and angular form are called goniatites, and they were abun- 

 dant in the Carboniferous period. These were to a large extent superseded 

 in Triassic time by other genera, still tightly coiled but with sutures thrown 

 into a number of regular curves and saw-teeth, which may be described as 

 "crooked." These forms with crooked sutures are known as ceratites, 

 from a very common genus Ceratites. And finally, in the forms known 

 as ammonites, the sutures became finely crimped in a compound fashion, 

 often producing most exquisite foliaceous patterns. The ammonites were 

 most abundant in the Jurassic to Cretaceous strata. 



Though there were many irregularities and overlappings in the series 

 of tetrabranchiate cephalopods, the fossils show on the whole clear 

 evidence of progress from a straight shell to one tightly coiled, and from 

 nearly straight sutures to sutures that were bent, angular, crooked, and 

 finely lobed. 



Relationships of Living Animals. — The existence of fossil records 

 demonstrating step by step the origin of animals now living from more 

 primitive ancestors can hardly escape throwing light upon obscure 

 relationships of modern groups. Even when series as complete as the 

 ones described above are lacking, fossils of animals intermediate between 

 two living groups may create the presumption that both groups have 

 sprung from a common stem. Thus the reptiles may be traced to an 

 amphibian ancestry through the Stegocephali, huge armored Amphibia 

 of the past. Birds are assigned a reptilian origin through Archaeopteryx 

 (Jurassic), a reptile-like animal which, however, had feathers. Mammals 

 have been derived from reptiles, as indicated not only by fossils but also by 

 the facts of structure and embryonic development. The paleontologist 

 is not called upon to decide only the relationships of the major groups of 

 animals. He is most successful, it is true, in establishing the connections 

 between phyla, classes, and orders; but attempts to relate families by 

 means of fossils are not uncommon, and problems of generic and specific 

 rank are essayed with as little hesitation as in the case of living animals. 



Extinction of Groups. — Everywhere in the rise of the animal groups 

 of today there have been tragedies more thrilling than the memorable 

 catastrophes of historical times among men. Races have come into 

 being, have passed through a series of changes, and then perished. Some- 

 times what may be regarded as the main line of development was pre- 

 served, while only the offshoots were lost. At least, in those cases, the 

 modern animals represent as complex an attainment as the lateral lines 

 which disappeared. In other cases, the climax of the series was extin- 



