TRILOBITE S — WHITTINGTON 413 



The nature of the rocks that contain trilobite fossils suggests deposi- 

 tion in waters not more than a few hundred feet deep. Thus we have 

 no direct evidence that trilobites inhabited deep oceanic waters. Yet, 

 extremely similar genera {Ptychagnostus (pi. 3, fig. 2) and Dicranu- 

 nis, for example) have been shown to have a worldwide distribution. 

 Does this mean that these and other kinds of trilobites inhabited the 

 surface waters of the oceans, feeding on the microscopic floating plants 

 or animals that constituted the Paleozoic plankton ? Did they browse 

 amid floating mats of seaweed, like those of the Sargasso today ? If 

 we assume this mode of life, the molts and dead bodies of such animals 

 might have come to rest in widely separated localities, and have been 

 included, in consequence, in very different types of sediments. 



Vfe know, however, that newly hatched trilobites formed their 

 first shells when they were half a millimeter or so in length. These 

 tiny creatures probably floated, like the larvae of today's crustaceans. 

 The young may have existed in this stage for days or weeks and, in 

 that time, could have drifted far from tlie point where the eggs were 

 laid. At a size of less than 1 centimeter in length in most species, 

 trilobites became bottom dwellers in shallow water, and probably spent 

 the remainder of their lives within a limited area. Thus, the wide 

 geographical dispersion of particular trilobites may be explained as 

 taking place during the larval stages, the adults dwelling on the sea 

 bottom — not drifting in the ocean's surface water. 



It has been said that spiny trilobites like Ceratoce'phdla and 

 Miraspis were floating forms even in the adult stage, the spines 

 inhibiting their sinking. However, we know nothing of the append- 

 ages of these trilobites and, as mentioned, the possession of a spiny 

 exoskeleton does not preclude the possibility of bottom dwelling. 



Some modern arthropod species exhibit sexual dimorphism — that is, 

 male and female forms that differ in size or in other characters. More 

 than a hundred years ago, Barrancle (in that great study of trilobites 

 from Czechoslovakia already mentioned) observed a broad and a 

 narrow form in certain species. Today, we consider these differences 

 to be the result of distortion that the fossils sufl'ered when the rocks 

 enclosing them were subjected to various stresses. Other such exam- 

 ples among fossils are well known. Not all the cases of two closely 

 similar forms coming from the same rocks can be so explained, how- 

 ever, and it may be that sexual dimorphism did occur in trilobites. If 

 so, however, it was not universal : the cases are equivocal. 



Although during the 100-million-year period of the Cambrian, trilo- 

 bites were the dominant animals of the shallow seas in kinds, numbers, 

 and sizes, they did not have these seas to themselves. There were other 

 aquatic arthropods in existence— types that, unlike the trilobites, were 

 armed with pincers. However, the rarity of these arthropods as fossils 



