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Sci Rep
2018 May 31;81:8493. doi: 10.1038/s41598-018-26877-5.
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Unravelling the origin of the basket stars and their allies (Echinodermata, Ophiuroidea, Euryalida).
Thuy B
,
Stöhr S
.
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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.
Figure 1. Phylogenetic tree of the updated matrix of Thuy and Stöhr15 including the extinct species Aspiduriella scutellata, A. Streichani and Melusinaster alissawhitegluzae gen. et sp. nov. (marked in bold) but excluding Ophiosparte gigas, inferred using MrBayes. Numbers at nodes indicate posterior probabilities. Extinct species are marked by a cross.
Figure 3. Aspiduriella scutellata from the Middle Triassic of Wollmershausen (a,b), Bettenfeld, Bavaria (c), and Schillingstadt (d,e), southwestern Germany. (a) GZG.INV.90012, articulated disc preserving proximal to median arm segments in dorsal view. (b) Detail of GZG.INV.90012 showing mouth plating. (c) GZG.INV.90013 isolated vertebra in dorso-distal view. (d) GZG.INV.90014 isolated proximal lateral arm plate in external (d1) and internal (d2) views and with detail of spine articulations (d3). (e) GZG.INV.90015 isolated oral plate in adradial view. Abbreviations: adMF: adradial muscle fossa; AOS: adoral shield; APa: apical oral papilla; di: distal; do: dorsal; G: granule; LPa: lateral oral papilla; LR: lip-shaped ridge; MO: muscle opening; NO: nerve opening; OS: oral shield; TS: tentacle scale; ZC: zygocondyle; 1VAP: first ventral arm plate. Scale bars equal 1âmm (a,b) and 0.2âmm (câe).
Figure 4. Aspiduriella streichani from the Middle Triassic of Rüdersdorf, Germany. (a) GZG.INV.90016, articulated skeleton preserving two arms and proximal segments of a third, exposing the dorsal side. (b) GZG.INV.90017, detail of oral plating of an articulated skeleton. (c) GZG.INV.90018, detail of proximal arm segments of an articulated skeleton in ventro-lateral view. (d) GZG.INV.90018, isolated vertebra in distal view. (e) GZG.INV.90019, isolated lateral arm plate in external (e1) and internal (e2) views. (f) GZG.INV.90018, proximal lateral arm plates of an articulated skeleton showing detail of spine articulations in lateral view. Abbreviations: AOS: adoral shield; APa: apical papilla; di: distal; do: dorsal; IR: ridge on the inner side of the lateral arm plate; LPa: lateral oral papilla; LR: lip-shaped ridge; MO: muscle opening; NO: nerve opening; SP: spur; TS: tentacle scale; VR: vertical ridge proximally bordering muscle opening; 1VAP: first ventral arm plate. Scale bars equal 1âmm (a) and 0.2âmm (bâf).
Figure 5. Melusinaster alissawhitegluzae gen. et sp. nov., from the Lower Jurassic (Torulosum Subzone, Aalensis Zone, uppermost Toarcian) of Quedlinburg, Germany. (a) MnhnL OPH028 (holotype), dissociated proximal lateral arm plate in external (a1) and internal (a2) views. (b) MnhnL OPH029 (paratype), dissociated median lateral arm plate in external (b1) and internal (b2) views. (c) MnhnL OPH030 (paratype), dissociated distal lateral arm plate in external (c1) and internal (c2) views. (d) MnhnL OPH031 (paratype), dissociated proximal vertebra in distal (d1) and dorsal (d2) views. Melusinaster arcusinimicus gen. et sp. nov., from the Middle Jurassic (Humphriesianum Zone, lower Bajocian) of Rumelange, Luxembourg. (e) MnhnL OPH033 (holotype), dissociated proximal lateral arm plate in external (e1) and internal (e2) views. (f) MnhnL OPH034 (paratype), dissociated proximal lateral arm plate in external (f1) and internal (f2) views. (g) MnhnL OPH035 (paratype), dissociated median lateral arm plate in external (g1) and internal (g2) views. (h) MnhnL OPH036 (paratype), dissociated proximal vertebra in distal (h1) and lateral (h2) views. Abbreviations: di: distal; do: dorsal; IR: ridge on the inner side of the lateral arm plate; MO: muscle opening; NO: nerve opening; pr: proximal; ZC: zygocondyle. Scale bars equal 0.1âmm.
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