TRILOBITES — WHITTINGTON 415 



The evolution of many new kinds of trilobites in the Ordovician 

 perhaps reflects adaptation to new environments in response to chang- 

 ing conditions. Yet it may be seen (fig. 2) that, toward the end of this 

 period, the rate of extinction became greater than the rate of evolution 

 of new forms. This is a pattern that continued through the animals' 

 remaining history. Only a single group persisted through the Car- 

 boniferous and into the Permian. This decline — and the ultimate 

 total extinction of trilobites — cannot readily be explained. 



One of the mysteries of the evolutionary process is why such a fate 

 should overtake a group of animals that, for millions of years, were 

 well adapted to their surroundings and continued to evolve new species 

 until near the close of the Paleozoic era. Phrases that imply "over- 

 specialization" or "the senescence of the trilobite race" are neither apt 

 nor meaningful. G eratocephdla has been regarded as a highly "spe- 

 cialized" trilobite, yet its exoskeleton is known from rocks ranging 

 from Ordovician to Devonian age — a period of some 100 million years. 

 This is clear evidence that tj^pes of animals well adapted to a par- 

 ticular environment may exist for an extremely long time without 

 significant morphological change. 



The competition with other groups for food may have played a part 

 in the trilobites' demise. In addition, the Devonian fishes — among 

 which jaws evolved for the first time — may have become trilobite 

 predators. At present, however, there is no acceptable theory that 

 explains the reasons for extinction of the trilobites. 



New kinds of trilobites are constantly being found on all continents 

 (although new information on trails and on appendages collects much 

 more slowly). As the store of knowledge from new discoveries and 

 improved techniques of investigation accumulates, we should be able 

 to outline more precisely the natural history of these remarkable 

 arthropods. For the present, we may agree with the late Prof. Percy 

 Raymond that perhaps the greatest contribution that trilobites have 

 made to our world "is the aesthetic pleasure the contemplation of their 

 elegant shells has given to countless collectors and students of fossils." 

 But paleontology is a science that does more than enjoy its raw ma- 

 terial : it also tries to bring extinct animals back to life. To me, it is 

 far more exciting to try to casualize these "elegant shells" as parts of 

 living animals, inhabiting their particular niche in nature at a time 

 so long ago that the vertebrate animals had yet to evolve. 



625325—62 28 



