412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



on the surface of the sediment, with thorax stretched out and limbs 

 stirring up the mud in search of food. 



The large head plate — hypostome — of many trilobites was firmly 

 braced against the remainder of the head, thus affording both protec- 

 tion for the main organs and points of attachment for muscles. The 

 posterior edge of the hypostome was sharply folded, and in some 

 species it bore spines, so that these trilobites could have dug in the 

 mud with their hypostomes by walking backward. However, evidences 

 of this behavior, in the form of trails produced by such activity, have 

 not yet been recognized by paleontologists. 



Trilobites of a particular body form, or of an otherwise related 

 group, are in most cases not found exclusively in any one type of sedi- 

 mentary rock. Smooth-shelled forms like Isotelus^ for example, are 

 found in reef limestone, shale, siltstone, and sandstone, but so is the 

 spiny C eratocephala. Some of these occurrences may result from the 

 burial of the animal's shell in a sediment that was laid down in an 

 environment quite different from the one in which the living animal 

 resided. If the trilobite exoskeleton is not disarticulated and is well 

 preserved, however, we may presume that it probably was buried 

 close to where it lived. 



Thus, clues to the ancient environment may properly be sought 

 from the enclosing rock. Many cases in which this procedure has 

 been followed suggest that particular species of trilobites possessed a 

 wide tolerance for such environmental variables as depth of water, 

 amount of light, temperature, and type of bottom sediment. Other 

 species or groups of species seem to have favored one environment, 

 although they were not confined to it. The Upper Cambrian ancestors 

 of Triarthrus, for example, are abundant in dark shales, deposited 

 in stagnant waters that were probably deficient in oxygen. Again, 

 Dipleura and its close relatives are found commonly, but not exclu- 

 sively, in sandy sediments in which they probably dug. 



Trilobite remains are abundant in Middle Paleozoic reef rocks, and 

 it has been claimed that one smooth-shelled form inhabited the rough- 

 water zone of a particular reef, clinging to rock surfaces like a modern 

 chiton. Other examples are known of related but distinct species that 

 are abundant in reef rocks of different ages and wide geographic 

 separation. 



Thus there is evidence that certain trilobites were adapted to life in 

 particular ecological niches in the ancient seas, but little evidence that 

 most were adapted to a restricted environment. The possession of 

 large eyes (in Remopleurides^ for example) or absence of eyes (in 

 Cryptolithus) has been held to suggest a life spent in muddy or deep, 

 dimly lighted waters. Analogy with living arthropods, however, 

 does not point to any positive conclusions. 



