TRUNK SEGMENTATION OF EARLY CAMBRIAN EODISCID TRILOBITE NEOCOBBOLDIA CHINLINICA

Detailed studies of the development of trilobite segmentation offer insights into the developmental mechanics by which trilobites constructed their bodies. This information is important because variation in segment form, number and regional allocation is a major contributor to the overall morphological diversity of Trilobita. The early Cambrian eodiscid trilobite Neocobboldia chinlinica shows an interesting pattern of intraspecific variation in the segmentation of the pygidium (posterior shield) in which some, but not all, meraspid (i.e. late juvenile) specimens possessed an additional segment compared to the holaspis (mature) form. During meraspid ontogeny trilobites released segments from the anterior margin of the pygidium into the thorax, as new segments were generated near the rear of the pygidium. Hence segmentation of the meraspid pygidium reflected a dynamic balance between the rates of segment accretion and release. Two hypotheses have been proposed to explain the intraspecific variation observed in N. chinlinica. One is an offset between instars (molts) in which segments were added to the pygidium and those in which segments were released, resulting in alternating pygidial segment numbers between three successive molts. The other scheme is that addition and release of segments were coincident, but different individuals followed one or the other of two alternative ontogenetic pathways; one maintaining constant number of pygidial segments by a coordinated transition in the termination of segment generation and release, while in the other both the start and the termination of the release of thoracic segments was offset by one molt cycle, with the result that the pygidium possessed an additional segment for a series of three consecutive molts. We are employing both a modeling approach based on predicted size frequency distribution and also shape based data to test between these and other possible hypotheses. Preliminary results suggest a presence of a single ontogenetic pathway (contra the second hypothesis), with all individuals having the extra segment at some point during their lifetime, and are inconsistent with the first hypothesis of alternating phases of segment accretion and release. More specimens from the meraspid stage are necessary for a complete understanding of N. chinlinica ontogeny.