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| Ophiactis hex Thuy, Numberger-Thuy, Härer, Kroh, Winkler & Schweigert, 2024 |
Abstract
Asexual reproduction by means of splitting, also called fissiparity, is a common feature in some asterozoan groups, especially in ophiactid brittle stars. Most fissiparous brittle stars show six instead of the usual five rays, live as epibionts on host organisms, and use clonal fragmentation to rapidly colonize secluded habitats and effectively expand the margins of their distribution area. While the biology and ecology of clonal fragmentation are comparatively well understood, virtually nothing is known about the evolution and geological history of that phenomenon. Here, we describe an exceptional fossil of an articulated six-armed brittle star from the Late Jurassic of Germany, showing one body half in the process of regeneration, and assign it to the new species Ophiactis hex sp. nov. Phylogenetic inference shows that the fossil represents the oldest member of the extant family Ophiactidae. Because the Ophiactis hex specimen shows an original six-fold symmetry combined with a morphology typically found in epizoic ophiuroids, in line with recent fissiparous ophiactid relatives, we assume that the regenerating body half is an indication for fissiparity. Ophiactis hex thus shows that fissiparity was established as a means of asexual reproduction in asterozoan echinoderms by the Late Jurassic.
Keywords: Ophiuroidea, late Jurassic, fissiparous, phylogeny, Ophiactidae
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| Ophiactis hex sp. nov., holotype SMNS 70508; from the Nusplingen Lithographic Limestone (Nusplingen Formation), Beckeri Zone, Ulmense Subzone, late Kimmeridgian, Late Jurassic, Nusplingen, Germany. Light photographs of the complete specimen exposing the dorsal side |
Systematic palaeontology
Class Ophiuroidea Gray, 1840
Superorder Ophintegrida O'Hara et al., 2017
Order Amphilepidida O'Hara et al., 2017
Superfamily Ophiactoidea Ljungman, 1867
Family Ophiactidae Matsumoto, 1915
Genus Ophiactis Lütken, 1856
Ophiactis hex sp. nov.
Species diagnosis: Small ophiactid with six arms, dorsal side of disc with a dense cover of granules and spinelets; lateral arm plates with outer surface covered by small tubercles arranged in a faint vertical striation; arm spines large and slightly flattened; distalmost arm segments with at least one hook-shaped arm spine.
Etymology: Species name (used as noun in apposition) referring to Hex, the organic/inorganic/magical super-computer of Terry Pratchett's Unseen University, capable of thinking the unthinkable.
Ben Thuy, Lea D. Numberger-Thuy, Jürgen Härer, Andreas Kroh, Viola Winkler and Günter Schweigert. 2024. Fossil Evidence for the ancient link between clonal fragmentation, Six-fold Symmetry and an epizoic lifestyle in asterozoan echinoderms. Proc. R. Soc. B. 291: 20232832. DOI: 10.1098/rspb.2023.2832
