Unravelling the origin of the basket stars and their allies (Echinodermata, Ophiuroidea, Euryalida)

Ben Thuy,Sabine Stöhr

doi:10.1038/s41598-018-26877-5

Ben Thuy, Sabine Stöhr

Open Access

https://doi.org/10.1038/s41598-018-26877-5

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Abstract

Euryalids, which include the spectacular basket stars, form a morphologically aberrant group of brittle stars. Surprisingly, the most recent molecular work found them to be sister to ophiurid brittle stars, thus challenging the traditional dichotomy between euryalids and non-euryalids, and leaving an enormous ghost lineage of more than 100 million years between the oldest unambiguous euryalid fossils and their predicted divergence from ophiurids during the Triassic. Here we examine both previously known and newly collected fossils to explore the evolutionary history of euryalids. A morphology-based phylogenetic estimate confirms the Triassic Aspiduriella as a basal member of the euryalid clade that superficially resembles members of the living ophiurid sister clades. Furthermore, we use lateral arm plates and vertebrae to identify two new Jurassic ophiuroids, Melusinaster alissawhitegluzae and Melusinaster arcusinimicus, as early euryalids that are morphologically intermediate between Aspiduriella and extant euryalids. Our phylogenetic analysis is the first to combine data from completely preserved skeletons and from microfossils in order to bridge morphological and stratigraphical gaps between the sampled taxa. It fills a major gap in the fossil record of euryalids and sets a robust phylogenetic framework to understand the morphological transition from ophiurid-like ancestors to the typical modern euryalids better.

Highlights

As clear-cut as this distinction seemed, it caused considerable controversy and perplexity with respect to its evolutionary origin
We provide detailed descriptions of both previously known and newly collected fossils, in particular belonging to the Triassic group of Aspiduriella and to a new Jurassic genus, and put these in a phylogenetic context to explore the evolutionary history of euryalids
We explore the evolutionary history of euryalids based on a reassessment of previously known fossil ophiuroids and newly collected material

Summary

Introduction

As clear-cut as this distinction seemed, it caused considerable controversy and perplexity with respect to its evolutionary origin. The most recent advances in ophiuroid systematics and phylogeny[10,14,15,16] have fundamentally changed perspectives by unveiling unexpectedly close ties between euryalid and ophiurid brittle stars, forming a clade that is sister to all remaining modern ophiuroids These new insights have still implied an excessively long missing fossil record between the predicted euryalid divergence during the Triassic and the oldest unambiguous euryalid fossils from the uppermost Cretaceous, but for the first time, it has become clear that the assumed early euryalids would have had ophiurid-like morphologies[14]. We provide detailed descriptions of both previously known and newly collected fossils, in particular belonging to the Triassic group of Aspiduriella and to a new Jurassic genus, and put these in a phylogenetic context to explore the evolutionary history of euryalids

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