Miniaturization during a Silurian environmental crisis generated the modern brittle star body plan

Ben Thuy,David F Wright,Johan Lindgren,Mats E Eriksson,Manfred Kutscher,Lea D Numberger-Thuy

doi:10.1038/s42003-021-02971-9

Abstract

Pivotal anatomical innovations often seem to appear by chance when viewed through the lens of the fossil record. As a consequence, specific driving forces behind the origination of major organismal clades generally remain speculative. Here, we present a rare exception to this axiom by constraining the appearance of a diverse animal group (the living Ophiuroidea) to a single speciation event rather than hypothetical ancestors. Fossils belonging to a new pair of temporally consecutive species of brittle stars (Ophiopetagno paicei gen. et sp. nov. and Muldaster haakei gen. et sp. nov.) from the Silurian (444–419 Mya) of Sweden reveal a process of miniaturization that temporally coincides with a global extinction and environmental perturbation known as the Mulde Event. The reduction in size from O. paicei to M. haakei forced a structural simplification of the ophiuroid skeleton through ontogenetic retention of juvenile traits, thereby generating the modern brittle star bauplan.

Highlights

Pivotal anatomical innovations often seem to appear by chance when viewed through the lens of the fossil record
We present an exceptional case where the origin of a major modern animal clade can be directly pinpointed to a specific pair of temporally consecutive species that occurred during a well-constrained time interval coinciding with major environmental and faunal reorganizations
Modern brittle stars all belong to the subclass Myophiuroidea[9], the sole surviving clade of ophiuroids that radiated during the Early Paleozoic, between 480 and 420 Mya

Summary

Introduction

Pivotal anatomical innovations often seem to appear by chance when viewed through the lens of the fossil record. Locality and horizon: Trench exposure 300 m south of Klintehamn on Gotland, Sweden; locality called Svarvare; Gannarve or Svarvare member (extinction phase of the Mulde Event), Ozarkodina bohemica longa conodont Zone, Fröjel Formation, Slite Group, Whitwell Stage, Wenlock Series, 428.5 ± 0.7 Mya. Description: Lateral arm plates (Fig. 1; Supplementary Fig. 3) small, elongate, with dorsal edge slightly concave due to weak constriction.

Results

Conclusion