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INTRODUCTION TO EVOLUTION 



FIG. 9.1. Ammonite shell, showing complex suture lines. (Courtesy of Ward's Natural 

 Science Establishment, Inc., Rochester, N.Y.) 



parable complexity. The Mesozoic, however, saw an "outburst" of am- 

 monite evolution, accompanied by great increase in complexity of suture 

 lines. More than 6000 species of ammonites have been described from 

 Mesozoic deposits. Most of these were relatively small, with shell diame- 

 ters averaging not over 4 inches. Yet some Mesozoic ammonites attained 

 large size; shell diameters of 5 feet were not uncommon, and some species 

 were 10 feet in diameter. 



For the most part each individual turn or volution of the coiled shell was 

 high and narrow in cross section. Lightness of structure characterizing 

 many ammonite shells suggests that their inhabitants were active animals, 

 perhaps good swimmers. A variety of knobs, spines, and ridges orna- 

 mented shells of a number of species. Many had a trap-door-Uke arrange- 

 ment (operculum) by which the opening of the shell could be closed when 

 the body and tentacles were completely withdrawn into the shell. But 

 in numerous ways the most remarkable feature of Mesozoic ammonites 



