Feeding habits in trilobites

We briefly review the various types of feeding habits in marine arthropods, and suggest that the trilobites adopted a range of different feeding strat egies. We show that much of the variety of trilobite exoskeletal developmen t, particularly in the cephalon, can be explained as a response to the adop tion of specific feeding modes. We regard the primitive mode as having been predatory/scavenging, both from morphological grounds and by out-group com parison, but this habit had a long subsequent history in the group. Predato rs/scavengers included those trilobites with rigidly braced and attached co nterminant or impendent hypostomes, which often developed posterior forks o r rasps used by the animals for manipulating prey after it had been grasped by the 'gnathobases'. Advanced predators often acquired expanded anterior glabellar lobes which are associated with the ingestion of bulky food; conc omitantly, the lamest trilobites of all had predatory morphology. Associate d trace fossils are of the Rusophycus type in which impressions of limb bas es and rarely the hypostome can be seen. Detritivors were derived from pred ators by detachment of the hypostome from the doublure in natant mode; it i s significant that the hypostome of such feeders exhibited little change th ereafter. The typical detritivor morphology is of the 'generalized' ptychop ariid type, common in outer shelf habitats, with rectangular or tapering gl abellas and small to moderate overall size. It is suggested that in some sp ecies the hypostome may have functioned as a 'scoop' directly to aid ingest ion of sediment. Trace fossils of Cruziana semiplicata type have been assoc iated with sediment ploughing in this feeding mode. Filter feeders evolved a vaulted cephalic chamber of trinucleimorph type, and elevated thoraces, o ften flanked by extended genal spines. Where it is known, the hypostome is curved up inside the cephalic chamber, within which sediment stirred into s uspension by the limbs was sorted for edible particles. Filter feeding tril obites are typically small, and are uncommon outside muddy habitats. Bean-l ike Rusophycus are the associated trace fossils. In trinucleids ingress of the feeding current was alongside the thorax and out through the fringe pit s. The combination of different feeding modes with adaptation for different prey and/or particle sizes goes some way to account for the variety of tri lobites cohabiting in a single site (alpha diversity). We do not claim that the model accounts for all morphological variation displayed by the group.