Thuy B et al. (2019), Brittle stars looking like starfish: the first ...

ECB-ART-48796

PeerJ 2019 Jan 01;7:e8008. doi: 10.7717/peerj.8008.

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Brittle stars looking like starfish: the first fossil record of the Astrophiuridae and a remarkable case of convergent evolution.

Thuy B , Gale A , Numberger-Thuy L .

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The genus Astrophiura, which ranks among the most extraordinary of modern brittle stars, is the type genus of the recently resurrected family Astrophiuridae within the order Ophiurida. On account of its absurdly enlarged and strongly modified lateral arm plates, Astrophiura bears a closer resemblance to a pentagonal starfish than to a typical ophiuroid. Although molecular evidence suggests an ancient origin of the Astrophiuridae, dating back at least to the Early Jurassic, not a single fossil astrophiurid has been reported so far. Here, we describe dissociated lateral arm plates from the Campanian of Cringleford near Norwich, UK, and the Maastrichtian of Rügen, Germany (both Upper Cretaceous) with unambiguous astrophiurid affinities and assign these to a new species, Astrophiura markbeneckei. This represents the first fossil record of the family. In addition, the Rügen material included lateral arm plates that superficially resemble those of A. markbeneckei sp. nov. but differ in having spine articulations that are typical of the ophionereidoid family Amphilimnidae. We assign these plates to a new genus and species, Astrosombra rammsteinensis, an extinct amphilimnid with morphological modifications similar to those of Astrophiura, and thus representing a remarkable case of parallel evolution amongst brittle stars looking like starfish.

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	Figure 1. Map showing position of sampled localities at Cringleford near Norwich, Norfolk, United Kingdom (1) and the Isle of Rügen, northeast Germany (2).
	Figure 2. Astrophiura permira, Recent, off Natal, South Africa.Complete skeleton in dorsal (A) and ventral (B) aspects; detail of radius in ventral view (C); modified proximal lateral arm plate in ventral (D) and dorsal (E) views, with anatomical position within articulated animal (C) indicated by dashed line; detail of spine articulations (left) in external view (F); regular lateral arm plate in external (G) and internal (H) views, with anatomical position within articulated animal (C) indicated by dashed line; proximal vertebra in ventral (I) and distal (J) views.Abbreviations: AS: arm spine; di: distal; do: dorsal; ext: external; int: internal; pr: proximal; TN: tentacle notch; ve: ventral. Scale bars equal 1 mm (A−C) and 0.1 mm (D−J).
	Figure 3. Astrophiura markbeneckei sp. nov., dissociated lateral arm plates from the upper lower Maastrichtian of Rügen, Germany (A–I) and from the upper Campanian of Cringleford near Norwich, Norfolk, United Kingdom (J, K).MnhnL OPH078 (holotype) in dorsal (A) and ventral (B) views, and details of spine articulations (C) and of inner side (D). MnhnL OPH079 (paratype) in dorsal (E) and ventral (F) views, and detail of spine artriculations (G). MnhnL OPH080 (paratype) in dorsal (H) and ventral (I) views. MnhnL OPH082 in ventral (J) and dorsal (K) views. Abbreviations: di: distal; do: dorsal; ext: external; int: internal; MO: muscle opening; NO: nerve opening; pr: proximal; TN: tentacle notch; ve: ventral. Scale bars equal 0.5 mm (A,B, E, F, H−K) and 0.2 mm (C, D, G).
	Figure 4. Astrosombra rammsteinensis gen. et sp. nov., dissociated lateral arm plates from the upper lower Maastrichtian of Rügen, Germany.MnhnL OPH083 (holotype) in dorsal (A) and ventral (B) views, with details of distal (originally ventral) (C) and proximal (originally dorsal) (D) arm spine articulations, and with detail of inner side (E). MnhnL OPH084 (paratype) in dorsal (F) and ventral (G) views, with detail of spine articulations (H). MnhnL OPH085 (paratype) in dorsal (I) and ventral (J) views, with detail of inner side (K). Dissociated proximal lateral arm plate of Amphilimna olivacea, Recent, in external (L) and internal (M) views, modified from Thuy et al. (2014). Dissociated proximal lateral arm plate of Amphilimna intersepultosetme, Maastrichtian of South Carolina, in external (N) and internal (O) views and with detail of spine articulations (P), modified from Thuy, Numberger-Thuy & Jagt (2018). Abbreviations: di: distal; do: dorsal; ext: external; int: internal; K: knob for vertebral articulation; MO: muscle opening; NO: nerve opening; P: perforation; pr: proximal; TN: tentacle notch; ve: ventral. Scale bars equal 0.5 mm (A, B, F, G, I, J, L−O) and 0.2 mm (C−E, H, K, P).

References [+] :

O'Hara, Restructuring higher taxonomy using broad-scale phylogenomics: The living Ophiuroidea. 2017, Pubmed