Exoskeleton, molt stages, appendage morphology, and life habits of the Middle Cambrian trilobite Olenoides serratus

The external surface and morphology of the exoskeleton of Olenoides serratus is described; the exoskeletal layer has been replaced by illite and chlorite. The pleura curves down distally, there is no fulcral process or socket, but a flange assists in articulation. An early moult stage has the exoskeleton unmineralized and wrinkled, and was described as Nathorstia transitans. The type is redescribed; a second specimen is of the exoskeleton only. A new reconstruction of the biramous appendage is given. The outer branch was attached to the coxa so that it was extended close below the ventral cuticle of the body; the function was probably respiratory. The inner and ventral surfaces of the coxa, and the ventral side of the outer branch, were spinose. These limbs were adapted to sieze soft prey, squeeze, and shred it, and pass it forward to the mouth; a manner in which these activities could have been effected is outlined. The probable musculature of the limbs and a possible gait are described; this gait is unlike other modes of progression in trilobites that have been proposed. How O. serratus may have jumped off the bottom and drifted is suggested, the swimming powers were probably feeble, it could have ploughed shallowly in soft sediment. By alternate flexure and extension of pairs of limbs it could have dug in search of prey or for concealment; a Rusophycus type of trace may have resulted from such activity. O. serratus is thus considered to have been a benthonic predator and scavenger. No plausible limb movements by which it could have made a Cruziana-like traces are evident; how this trace was made is an unsolved problem.