A Natural History of Trilobites' 



By H. B. Whittington 



Museum of Comparative Zoology, Harvard College 



[With 8 plates] 



Trilobites are arthropods, one class of a phylum of invertebrate, 

 segmented animals with jointed legs that contains among its living 

 representatives such diverse forms as insects, spiders, centipedes, 

 crabs, and lobsters. They are found in the oldest rocks of the Paleo- 

 zoic era, at the beginning of the Cambrian period, some 600 million 

 years ago. These animals, abundant in the early part of the Paleozoic 

 era, decreased during the middle part of that era, and were even fewer 

 in its younger rocks. The last survivors are found toward the end of 

 the Paleozoic, about 250 million years ago. Trilobite fossils occur in 

 a wide variety of sediments — sands, silts, muds, and limestones. The 

 other types of fossils found with them suggest that these sediments 

 were laid down in relatively shallow, marine waters. 



In the process of preservation, the original mineral matter in the 

 trilobite's shell may be preserved, or there may be some addition to — 

 or replacement of — this matter. Or the original material may be dis- 

 solved away altogether after burial and consolidation of the rock, leav- 

 ing only an impression or mold of the shell (pi. 5, fig. 1). Study 

 of the well-preserved shells of trilobites shows that the animal's body 

 comprised a head region, followed by a thorax (a series of segments 

 that articulated with each other) , and a tail. The tail region is formed 

 by the fusion of several segments like those in the thorax ; it may be 

 smaller, equal in size, or larger than the head. The shell covered the 

 back of the body, and, on the underside, extended inward only a short 

 distance from the margin. Since it is outside the body, it is called 

 an exoskeleton. 



The trilobite's body was bilaterally symmetrical; it had a raised 

 region extending lengthwise down the middle and was flattened or 



^ Repriuted by permission from Natural History, vol. 70, No. 7, August-September 1961. 



405 



