Articulation and exuviation in Cambrian trilobites

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Abstract
Articulation of the exoskeleton is reviewed in trilobites where the inner portion of the thoracic pleura was horizontal. Devices enabling articulation in the thorax of the olenelloid Holmia , and of species of Paradoxides are described. The thoracic pleura in these taxa did not have the inner portion horizontal, but the body could be enrolled. Arching of the body, and opening of the cephalic sutures, are considered to have been essential preliminary manoeuvres in exuviation, allowing egress anteriorly of the newly moulted animal. Inversion of the free cheeks and rostral-hypostomal plate beneath the thorax in exuvial configurations of Paradoxides and Ogygopsis may be explained by such manoeuvres. There is no compelling evidence for an opening between cephalon and thorax in these specimens, and such an opening does not appear to have been necessary in freeing the vital soft parts and facilitating emergence. Exuviae consisted of the mineralized exoskeleton, held together by articulatory membranes and the unmineralized cuticle that had covered the ventral side of the body and the appendages. Consideration needs to be given to how such exuviae came to be buried and partially preserved, to the effects of progressive decay of unmineralized cuticle before burial, and of transport before or during burial, before claiming that a particular specimen is an exuvial configuration. The criteria for recognizing such a specimen are problematical, but it is considered that the symmetrical inversions described here may be such configurations.

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