Trilobite appendage structure — Eoredlichia reconsidered

Abstract

New interpretations are available from recently published appendages of Eoredlichia intermedia (Early Cambrian Chengjiang fauna), the most primitive trilobite for which appendages are known. The basis (formerly identified as the coxa) is preserved in one appendage. An allegedly long proximal podomere in the endopod is actually subdivided into two and, at least in posterior appendages, each of these podomeres bears a strong, angular endite. The large, subquadrate basis, angular endite on the first podomere of the endopod, and attachment of the exopod along the whole length of the basis are features shared with naraoiids, for which Naraoia longicaudata Zhang & Hou is exemplar. The long line of attachment of the exopod to the basis works as a hinge joint in E. intermedia and is a feature shared by Olenoides serratus. This joint allows rotation of the basis and endopod around the hinge line, permitting extensive leg movement without forcing the large exopod fan out of position. The shape of the basis in O. serratus is reinterpreted here, and a new reconstruction of the exopod shows detailed similarity to E. intermedia. The limbs of Eoredlichia show that appendage structure in primitive trilobites is little altered from shared ancestry with naraoiids.

Keywords:

• limb structure
• trilobites
• naraoiids
• Chengjiang fauna