Potential evolutionary trade‐off between feeding and stability in Cambrian cinctan echinoderms

Imran A. Rahman,Stephan Lautenschlager,James O'Shea,Samuel Zamora,George Sevastopulo

doi:10.1111/pala.12495

Abstract

AbstractReconstructing the function and behaviour of extinct groups of echinoderms is problematic because there are no modern analogues for their aberrant body plans. Cinctans, an enigmatic group of Cambrian echinoderms, exemplify this problem: their asymmetrical body plan differentiates them from all living species. Here, we used computational fluid dynamics to analyse the functional performance of cinctans without assuming an extant comparative model. Three‐dimensional models of six species from across cinctan phylogeny were used in computer simulations of water flow. The results demonstrate that cinctans with strongly flattened bodies produced much less drag than species characterized by dorsal protuberances or swellings, suggesting the former were more stable on the seafloor. However, unlike the flattened forms, cinctans with high‐relief bodies were able to passively direct flow towards the mouth and associated food grooves, indicating that they were capable of more efficient feeding on particles suspended in the water. This study provides evidence of a previously unknown evolutionary trade‐off between feeding and stability in Cambrian cinctan echinoderms.

Highlights

Reconstructing the function and behaviour of extinct groups of echinoderms is problematic because there are no modern analogues for their aberrant body plans
Comparing results for all six cinctan models orientated with the mouth facing downstream, the drag forces were highest for U. quadricornuta, followed by T. bohemicus and Gy. testudiformis, with P. mansillaensis, L. barriosensis and Gr. ambigua producing the lowest drag forces (Fig. 4; Rahman et al 2020, table S3)
The drag force increased as the inlet velocity increased, but the drag coefficient varied less consistently with velocity, increasing as the inlet velocity increased in U. quadricornuta, but decreasing as velocity increased in Gr. ambigua, P. mansillaensis and T. bohemicus

Summary

Introduction

Reconstructing the function and behaviour of extinct groups of echinoderms is problematic because there are no modern analogues for their aberrant body plans. Echinoderms are a case in point; several extinct Cambrian groups lack pentaradial symmetry and perhaps other synapomorphies of living echinoderms Cinctans are one such group of aberrant early echinoderms, known exclusively from the Miaolingian Epoch of the Cambrian The results showed that a position with the mouth facing downstream and the ventral swelling buried produced the least drag and lift, indicating this would have been beneficial for enhancing stability on the seafloor In this position, flow to the mouth and associated marginal groove was minimal in simulations of passive suspension feeding, demonstrating that such a feeding mode would not have been an effective means of acquiring nutrients. The simulations of active suspension feeding showed much stronger flow to the mouth and marginal groove, in agreement with previous interpretations of cinctans as pharyngeal filter feeders

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Discussion

Conclusion