TRILOBITES — ^WHITTINGTON 411 



One might expect that, occasionally, a dead individual trilobite 

 would be found associated with such a trail— the remains of an animal 

 tliat had died, or of one that was overwhelmed by a sudden inrush of 

 sediments or some other catastrophe. Yet, so far, no such dramatic 

 proof of this scientific detection seems to have been found. Thus, the 

 interpretation of these trails, although reasonable, is not positive. In 

 almost all cases, fossils are the remains of animals that possessed hard 

 parts (skeletons impregnated with mineral matter) that could be 

 preserved. Yet in these ancient seas there were, in all probability, 

 many inhabitants that lacked such hard parts. Conceivably, some 

 of these fossil trails are the enigmatic traces of just such soft and now 

 vanished animals. 



A different type of track, from Lower Cambrian rocks of Pakistan, 

 has recently been described by Dr. A. Seilacher, University of Got- 

 tingen, Germany. This track, Dimorphichniis^ is abundant on the 

 surfaces of the sandstone layers in which the remains of trilobite 

 shells are rare. Nevertheless, the size and nature of the track make 

 it probable that it was made by the tips of the appendages of a trilo- 

 bite (pi. 7). Dr. Seilacher considers that the animal held itself diag- 

 onally to its direction of progression, and that it dug in the walking 

 legs on one side to make deep, short scars, while raking over the sur- 

 face with the legs on the other side to form longer scraping marks. 



Thus, compilation of all available knowledge of the trilobite body, 

 combined with interpretations of the tracks and trails, affords a picture 

 of how some trilobites may have lived. Those like Isotelus^ smooth- 

 shelled, and with the tail similar to the head in size, or like Dipleura^ 

 which had a narrower body and more thoracic segments, are pre- 

 smned — because of their smooth, elongate form — to have burrowed 

 into the sediments. There does not seem to be any obvious correlation 

 between the type of exoskeleton and the habit of rakmg the surface of 

 the sediments or making shallow excavations in it. Such a mode of 

 life seems reasonable for such different trilobites as PtycTioparia^ 

 Flexicalymene^ GTyptolithus^ or Cordania. The broad, pitted fringe 

 around the head of Cryptolithus and the long, backwardly directed 

 spines may have served to prop the animal up on the sea floor with 

 its thorax extended above it, so that its appendages could have stirred 

 up the mud. The broad border around and behind the head of Cor- 

 dania may have supported the animal in a similar way. Despite this 

 possible similarity in habit, Cordania^ which had eyes, facial sutures, 

 and many more thoracic segments, can be only very distantly related 

 to Gryptolithus. 



Such spinose trilobites as Ceratocephala (fig. 3) and Miraspis (pi. 

 3, fig, 1) can hardly have burrowed or dug into the sea bottom. They 

 may, however, have rested the level front and side edges of the head 



