CHAPTER 



EVOLUTION AS SEEN IN THE 

 GEOLOGIC RECORD: 

 MESOZOIC ERA 



The 160 million years or more comprising the Mesozoic 

 era are sometimes called the "age of reptiles," since during this time the 

 group of reptiles called dinosaurs held undisputed sway over Uving things 

 on the surface of the earth, while other reptiles dominated the sea, and 

 still others the air. Instead of discussing each of the periods of the era 

 (Table 7.1, p. 137) successively we shall discuss Mesozoic life under four 

 main headings: (1) culmination of cephalopods; (2) evolution of dino- 

 saurs and their relatives; (3) origin of birds; (4) origin of mammals. 



CULMINATION OF CEPHALOPODS 

 Ammonites 



Ammonites were the dominant invertebrates of Mesozoic seas. We re- 

 call that they first appeared in the Devonian (p. 155), probably as de- 

 scendants of the nautiloids, which had existed from Ordovician times (p. 

 152). The nautiloids were characterized by straight or smoothly curved 

 suture lines, formed by junctures of the septa with the side wall of the 

 shell (Fig. 8.9, p. 155). Ammonites, on the other hand, had suture lines 

 of some complexity. The Devonian ammonites (goniatites) had suture 

 lines with a "loop-and-saddle" configuration (Fig. 8.13, p. 159). Their de- 

 scendants in later periods of the Paleozoic retained suture lines of com- 



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