LEG FLEXURE AND STANCE IN THE CAMBRIAN TRILOBITE OLENOIDES SERRATUS

Trilobites, although a dominant part of the Paleozoic fossil record thanks to their easily fossilized calcite exoskeleton, rarely have preserved appendages because of their more delicate nature, requiring special circumstances to fossilize. Olenoides serratus from the Burgess Shale is a well-known trilobite, with over 60 specimens with preserved non-biomineralized limbs and was the subject of extensive work by Harry Whittington in the late 1970s to early 1980s. But questions remain about the attachment style of the protopodite to the body wall and exopodite, as well as the flexibility of the limbs. The attachment of the protopodite has been reconstructed as laterally splayed or ventrally hanging, which would have drastic changes on limb range of movement. The lateral spines on podomeres 5 and 4 have been proposed as gill rakes if the leg were extended and angled anteriorly. Recent work has shown the protopodite to be subtriangular in posterior view with an angled attachment to the body wall. An extensive study of O. serratus specimens with preserved appendages prompts revising the appendicular morphology. In this study we measure the angle of flex in appendages of O. serratus to assess the mobility range and test previous assumptions about the use of the lateral spines for gill grooming and stance of the trilobite.

The subtriangular protopodite with angled attachment to the body wall leads to a taller neutral stance than has been shown in reconstructions. The limbs of O. serratus are commonly found in nearly straight positions with slight ventral curves. Measurement of the angle between podomeres shows that the limbs were rarely preserved in flexed positions, although flexure is occasionally found between podomeres 4 and 3 near the distal end of the endopodite. The angle between the protopodite and proximalmost podomere is the smallest of all measured points, with an range from 107 – 160° and an average of 136°. Olenoides serratus displays a broad range of motion in the endopodite, but the lateral spines were only reach the distal half of lamellae on the exopodite.