414 ANNUAL REPORT SIVHTHSONIAN INSTITUTION, 1961 



Figure 3. — Ceratocephala, an Ordovician to Devonian trilobite, restored to show the 

 antennules and sensory hairs, x 4. (From Whittington and Evitt, 1954.) 



suggests that they were not formidable enemies of the trilobites. From 

 the earliest Cambrian onward, a succession of new genera and families 

 of trilobites appeared, though the rate of extinction of trilobite groups 

 was also high (fig. 2). 



On balance, the picture is one of great evolutionary activity, of adap- 

 tation to a great variety of environments, expressed in a multiplicity 

 of genera and species. At the end of the Cambrian and during the 

 Ordovician period, this picture begins to change. New kinds of ani- 

 mals appeared. Previously existing ones became more numerous, and 

 these animals must have competed with the trilobites for the food 

 supply on and in the sea floor. Among these forms were the bivalved 

 brachiopods and clams, and the snails. The nautiloids, molluscan 

 ancestors of the modern Nautilus, were not only numerous and larger 

 than trilobites, but probably had grasping tentacles and a powerful 

 jaw. Such predators could have seized and eaten trilobites. But the 

 capacity for enrollment may have afforded the trilobites some protec- 

 tion, and their spines must have made them an awkward mouthful. 

 They may have lain partly buried in the bottom sediment, the pro- 

 jecting or stalked eyes of some species enabling them to detect nearby 

 movement. Vegetation, clusters of marine animals such as sea lilies 

 or corals, and crannies in reefs would also have afforded the trilobites 

 places of concealment. 



