ABSTRACT. We briefly review the various types of feeding habits in marine arthropods, and suggest that the trilobites adopted a range of different feeding strategies. We show that much of the variety of trilobite exoskeletal development, particularly in the cephalon, can be explained as a response to the adoption of specific feeding modes. We regard the primitive mode as having been predatory/scavenging, both from morphological grounds and by out-group comparison, but this habit had a long subsequent history in the group. Predators/scavengers included those trilobites with rigidly braced and attached conterminant or impendent hypostomes, which often developed posterior forks or 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; concomitantly, the largest trilobites of all had predatory morphology. Associated trace fossils are of the Rusophycus type in which impressions of limb bases and rarely the hypostome can be seen. Detritivors were derived from predators by detachment of the hypostome from the doublure in natant mode; it is significant that the hypostome of such feeders exhibited little change thereafter. The typical detritivor morphology is of the 'generalized' ptychopariid type, common in outer shelf habitats, with rectangular or tapering glabellas and small to moderate overall size. It is suggested that in some species the hypostome may have functioned as a 'scoop' directly to aid ingestion of sediment. Trace fossils of Cruziana semiplicata type have been associated with sediment ploughing in this feeding mode. Filter feeders evolved a vaulted cephalic chamber of trinucleimorph type, and elevated thoraces, often flanked by extended genal spines. Where it is known, the hypostome is curved up inside the cephalic chamber, within which sediment stirred into suspension by the limbs was sorted for edible particles. Filter feeding trilobites are typically small, and are uncommon outside muddy habitats. Bean-like Rusophycus are the associated trace fossils. In trinucleids ingress of the feeding current was alongside the thorax and out through the fringe pits. The combination of different feeding modes with adaptation for different prey and/or particle sizes goes some way to account for the variety of trilobites cohabiting in a single site (alpha diversity). We do not claim that the model accounts for all morphological variation displayed by the group.
A rich fauna dominated by trilobites and calcified chordates has been collected in Shropshire from the Arenaceous Beds, the highest member of the Tremadoc Shineton Shale Formation, and hitherto regarded as poorly fossiliferous. This fauna shows that shelf conditions persisted longer in Shropshire than has been supposed. It is likely that even younger Tremadoc is cut out at the unconformity below the Caradoc in the Shineton Inlier. The correlation of the later Tremadoc is reviewed, and the nomenclature of British Tremadoc biozones is revised. The trilobites described here are a mixture of previously known and new forms. The name Shumardia (Conophrys) salopiensis Callaway is revived for British material traditionally assigned to the Scandinavian species Shumardia pusilla (Sars), from which it is distinct. The new taxa Litagnostus meniscus sp.nov., Apatokephalus sarculum sp.nov. and Skljarella cracens sp.nov. are described. The type species of Asaphellus, A. homfrayi and Leptoplastides, L. salteri, are redescribed, and Geragnostus callavei, Pseudokainella impar, and Parapilekia sp. are recorded. New information on the ontogeny of S. (C.) salopiensis and A. homfrayi is given. Litagnostus and Skljarella are recorded from the British Isles for the first time.
The Treatise on Invertebrate Paleontology has been made possible by (1) funding principally from the National Science Foundation of the United States in its early stages, from the Geological Society of America through the bequest of Richard Alexander Fullerton Penrose, Jr., and from The Kansas University Endowment Association through the bequest of Raymond C. and Lillian B. Moore; (2) contribution of the knowledge and labor of specialists throughout the world, working in cooperation under sponsorship of the Geological Society of America, the Paleontological Society, the SEPM (Society for Sedimentary Geology), the Palaeontographical Society, and the Palaeontological Association; (3) acceptance by The University of Kansas of publication without any financial gain to the University; and (4) generous contributions by our individual and corporate sponsors.
A diverse fauna of trilobites, graptolites, inarticulate brachiopods, sponges, and a worm is described from the Tremadoc rocks cropping out in the Llangynog district, SW of Carmarthen. It permits identification of the Clonograptus tenellus Zone and older Tremadoc strata; younger Tremadoc strata are apparently absent. The fauna shows closest affinities with the Shineton Shales of Shropshire, while some of the trilobites also indicate a close relationship with those from the Tremadoc of NW Argentina.
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