X-RAY MICROSCOPY OF ARMORED WORMS: THE PLATE ASSEMBLY AND PROTECTIVE SYSTEM OF EARLY DEVONIAN ARTICULATED MACHAERIDIANS FROM AUSTRALIA

Machaeridians are a unique group of Paleozoic armored annelids that produce a calcitic dorsal scleritome comprised of two to four longitudinal series of interlocking shell plates. The disarticulated plates form a common constituent of marine fossil assemblages ranging from the Early Ordovician to Permian Periods, but fully-articulated specimens are rare. Among the articulated specimens documented to date, most have been preserved as internal or external molds, recrystallized calcite, and even pyrite replacement. These modes of preservation offer low taphonomic resolution, and preclude examination of the interlocking mechanisms of the two opposing series of plates. Herein, we document the first fully and partially articulated silicified lepidocoleids from the early Devonian (Pragian) Garra Limestone of New South Wales, Australia. Unlike lower resolution preservational modes, silicification is particularly amenable to X-ray micro-computed tomography (µCT) and provides a unique opportunity to investigate biomechanical features involved in the articulation of individual plates within the scleritome. µCT was employed to produce tomographic models of both straight and enrolled articulated lepidocoleids for virtual dissection of the plate assembly. Individual plates and composite scleritomes where assessed in terms of their three-dimensional and cross-sectional geometry, spatial distribution of thickness, amount of plate-to-plate overlap, and dorsal hinge articulation. In sum, we (1) establish key differences in plate morphology between lepidocoleids with alternate vs opposing articulation methods; (2) describe a novel articulation method not previously observed in machaeridians; (3) determine that a combination of plate geometry and plate-to-plate overlap helps to maintain a relative consistency in scleritome thickness. This research provides novel insights into the complex protective armour of the machaeridians that have no modern analogue and provides a basis for comparative morphological studies with other multi-element scleritomes.