TESTING THE FUNCTION OF PRODUCTIDE BRACHIOPOD SPINES ON ARENACEOUS SUBSTRATES USING 3D PRINTED MODELS

Brachiopods of the Suborder Productidina can vary in spine morphology from genus to genus; two end-member morphologies are (a) multiple short spines, and (b) few long spines. Despite being a common form, the function of the short and highly dense spine morphology has yet to be determined.

Using accurately weighted spined and aspinose 3D-printed models of the Devonian genus Praewaagenoconcha, the oldest North American productidine possessing many short spines, this study examines the effect of the short spine morphology on shell stability and scour production on a moving substrate (coarse sand). Thirty trials of each model were run in a recirculating flume: fifteen with the model lying fully on top of the substrate (epifaunal), and fifteen with the model’s hingeline flush against the substrate, with the ventral valve pushed into the sand (quasi-infaunal). For each set of fifteen trials, five were run with the hingeline facing upstream, five with the hingeline facing downstream, and five with the hingeline 90° to the current. For all trials, flow velocity was increased by 0.04 m/s every 90 seconds to a maximum velocity of 0.70 m/s. Consistent stability in all positions implies that the morphology could have survived high flow velocities, in environments where current directions often changed.

Models in the quasi-infaunal position resisted transport and overturning better than epifaunally positioned models (19/30 epifaunal specimens were transported vs. 4/30 quasi-infaunal specimens). Furthermore, the quasi-infaunal positioned spinose models resisted transport and scour more effectively than aspinose models, as best observed in the hingeline downstream position, where the aspinose model was reoriented in 4/5 trials at 0.48 m/s, while the spinose model remained stable, even during ripple formation. In other positions, both models remained stable, but the scour around the aspinose model was up to 3x deeper than in the spined model, increasing its risk of destabilization. Given these results, productidines possessing the short-multiple-spine morphology could have lived quasi-infaunally on coarse, arenaceous, mobile substrates, as they would have maintained stability, even at high velocities. Possessing spines may have helped productidines exploit a new niche unavailable to their spineless counterparts.