CLASPER-LIKE APPENDAGES IN THE MID-CAMBRIAN TRILOBITE OLENOIDES SERRATUS FROM THE BURGESS SHALE

Trilobites are a dominant group of Paleozoic euarthropods that despite boasting an impressive diversity of over 20,000 described species, details of their appendicular morphology are only known from 31 taxa. Trilobites with preserved appendages feature a pair of uniramous antennae followed by a largely homonomous series of biramous appendages, typically with three underneath the cephalon and one per trunk tergite. Whereas the trunk biramous appendages in most trilobites share a similar functional morphology, the only clear evidence of differentiation to date consists of the presence of progressively smaller posterior appendage pairs, and slight alterations for food mastication. Olenoides serratus is a well-known corynexochid trilobite from the Burgess Shale (Wuliuan, Miaolingian; ~508 Ma, Canada) with preserved appendages known from over 70 specimens. However, O. serratus is unique among trilobites by having preserved ­­caudal cerci, thus showing a higher degree of limb tagmosis than other representatives. Here, we demonstrate the first evidence of strong morphological differentiation of the trunk biramous appendages in O. serratus based on undescribed material. An exceptionally preserved specimen shows differentiated appendages on the seventh thoracic and first pygidial segments consisting of greatly reduced endopods, composed of seven podomeres with unique morphology, and a large sub-circular protopodite, yet the exopod appears to be unaltered. These morphologically specialized appendages demonstrate an unparalleled degree of limb tagmosis in trilobites, and carry broader implications for trilobite reproductive biology. The reduced endopods are functionally comparable with the clasper appendages seen in many adult male euarthropods, such as the pedipalp in the Atlantic horseshoe crab Limulus polyphemus, which suggests that male trilobites most likely used them to grasp onto females prior to or during mating.