INSIGHTS INTO THE AFFINITY OF THE ENIGMATIC SMALL SHELLY FOSSIL STOIBOSTROMBUS FROM THE CAMBRIAN OF SOUTH AUSTRALIA

While several small shelly fossils (SSF) reveal distinct and comparable morphologies derived from a single scleritome, select problematica pose a greater challenge as they likely represent unique or individual morphs within a more complex exoskeleton. One such example is the enigmatic Cambrian cone- or spine-shaped fossils of Stoibostrombus Conway Morris & Bengtson. 1990. This fossil continues to elude phylogenetic placement, with the most recent suggestion aligning these sclerites with the dermal plates of palaeoscolecid worms. Despite the recovery of articulated sclerites, the arrangement of these spines in the context of the gross morphology of the scleritome remains unclear.

Isolated specimens from the Ajax Limestone and Mernmerna Formation (Cambrian Series 2, Stage 3) of the Ikara-Flinders Ranges, South Australia were CT-scanned to reveal the three-dimensional morphology of these sclerites for the first time. Prior work employing SEM imaging has clearly revealed the characteristic fine-scale pustulose ornament on the exterior surface and uniform shallow pits on the internal surface. Virtual reconstructions show that the internal pits are approximately uniform in depth, but not always in diameter or shape. Moreover, the interior of the cone exhibits a sharp taper towards the apex before continuing as a narrow tube to the opening. While no smoking gun, these 3D features do bear some resemblance to the longitudinal scalids of priapulid worms such as Priapulus caudatus. While still falling within an ecdysozoan group, the gross morphology of these Stoibostrombus spines are likely closely comparable to priapulid-like scalidophorans. Comprehensive morphometric analysis of published and new Stoibostrombus material in comparison to the dimensions and attributes of modern priapulid scalids will further test this hypothesis.