FORM OF THE TRILOBITE DIGESTIVE-SYSTEM - ALIMENTARY STRUCTURES IN PTEROCEPHALIA

Three types of alimentary canals (=midgut) occur in the Annelida and n on-trilobite Arthropoda: 1) a sagittal tube with metamerically paired diverticula related to the number of somites; 2) a tube that is constr icted slightly between somites; and 3) a simple tubular gut that may t aper slightly backwards to the anus. At least two of these three types (1 and 3) occur in the Trilobita. Pterocephalia and Olenoides share t he first type with the probable sister taxon to the Trilobita, Naraoia (Nectaspida), and this is probably the plesiomorphic condition for th e class. Varying feeding habits may well have made this character homo plastic within each of these groups. The preservation of parts of the alimentary tract in specimens of Upper Cambrian Pterocephalia n. sp. ( McKay Group, British Columbia) was probably a function of taphonomic a nd/or very early diagenetic changes that resulted from the type of foo d prefer-red by that trilobite. Other trilobites from the same beds do not have their soft parts preserved. The alimentary structures are pr eserved in a different fashion from, apparently unattached to, and an order of magnitude larger than genal caeca that occur in this taxon. T hus, genal caeca are regarded as imprints of circulatory rather than a limentary structures. Energy dispersive analysis of a fragment of pres erved alimentary tract of Pterocephalia n. sp. showed the presence of Ca, Si, Al, Fe, P, K, Na, and Cl. These alimentary tracts are composed of a complex mixture of minerals that probably includes clays, detrit al quartz, carbonates, phosphates, and oxides or hydroxides. The struc ture of these dark fillings is microcrystalline. The presence of detri tal minerals as part of this mixture would suggest that this trilobite was a deposit feeder.