Salamon MA et al. , Ausichicrinites zelenskyyi gen. et sp. nov., a ...

ECB-ART-51006

R Soc Open Sci null;97:220345. doi: 10.1098/rsos.220345.

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Ausichicrinites zelenskyyi gen. et sp. nov., a first nearly complete feather star (Crinoidea) from the Upper Jurassic of Africa.

Salamon MA , Jain S , Brachaniec T , Duda P , Płachno BJ , Gorzelak P .

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Fossil comatulids, referred to as feather stars, are mostly known from highly disarticulated specimens. A single isolated element (centrodorsal) has been the basis for taxonomic description of a vast majority of fossil comatulids. Here, we report a nearly complete, and thus extremely rare, comatulid from the Upper Jurassic (Tithonian) of the Blue Nile Basin in central western Ethiopia that provides a unique insight into the morphology of comatulid arms and cirri. It is assigned to Ausichicrinites zelenskyyi gen. et sp. nov. and is the first Jurassic comatulid from the African continent. The new taxon shows some similarities with representatives of the Mesozoic Solanocrinitidae but also has close resemblance with the modern family Zygometridae, exclusively known from the Holocene of western Pacific and eastern Indian Oceans. This morphologic similarity is considered to be due to convergence. The first example of pinnule regeneration in a fossil feather star is reported, which reinforces the hypothesis about the importance of predation in the evolution of these crinoids.

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	Figure 1. . Geological and locality map. (a) Geological map of Ethiopia showing the three sedimentary basins, Ogaden, Blue Nile and Mekele (redrawn and slightly modified after [23]). (b) Major Jurassic localities mentioned in the text from Ethiopia, Somalia and Yemen. The shaded part in (b) (within Ethiopia) is enlarged in (c). (c) Locality map of the measured section at Mugher and the reference section at Dejen for the Blue Nile Basin (mentioned in the text). (b) and (c) are redrawn and modified after Jain [20].
	Figure 2. . Stratigraphy of the measured section at Mugher (Blue Nile Basin; figure 1c for the location of the section) marking the stratigraphic position of the crinoid sample (black arrow). Additionally, the black pentagon symbol marks the position of sample 2043b that has yielded upper Tithonian calcareous nannofossils (redrawn and modified after [21]). The depositional setting and inferred relative sea-level curve for the Mugher section is redrawn and slightly modified after Jain [20] and Jain & Singh [21].
	Figure 3. . Slab showing several comatulids in upside down position (black triangulars) surrounded by putative traces of arm movements (cf. [33]). Dejen, Kurar section, Ethiopia (figure 1c), lower Kimmeridgian. Scale bar equals 10 mm.
	Figure 4. . Ausichicrinites zelenskyyi gen. et sp. nov. from the upper part of the Antalo Limestone Formation (38°22'49.1″ E; 9°28'41.8″ N; 2114 m elevation), 21 m above the upper Tithonian calcareous nannofossil-yielding sample 2043b, Ethiopia (see also figure 2 for its stratigraphic position). Scale bar equals 10 mm (a,c,e,f,g) and 1 mm (b,d,h,i). (a,c). Specimen with centrodorsal, arms and cirri ((a) non-whitened, (c) whitened) with magnifications (b,d) of IBr2 articulation (note a ‘dotted’ suture line (red arrows) from the outer surface of the articulation (b) and a fine ridge (red arrows) on the partly exposed facet (d)). (e) Lateral view showing a centrodorsal (non-whitened). (f,g) Tomographic images of slices of the fossil comatulids showing cryptosyzygial articulation at IBr2 (red arrows). (h) Proximal pluricirral (lateral view) and isolated cirri (facet view, blue arrow). (i) Regenerating pinnules consisting of one to three pinnular plates (blue arrows).

References [+] :

Gahn, Evolutionary history of regeneration in crinoids (Echinodermata). 2010, Pubmed, Echinobase