The nervous and circulatory systems of a Cretaceous crinoid: preservation, palaeobiology and evolutionary significance

Abstract

Featherstars, comatulid crinoids that shed their stalk during their ontogeny, are the most species‐rich lineage of modern crinoids and the only ones present in shallow water today. Although they are of considerable palaeontological interest as a ‘success story’ of the Mesozoic Marine Revolution, their fossil record is relatively species‐poor and fragmentary. New Spanish fossils of the Cretaceous featherstar Decameros ricordeanus preserve the shape and configuration of nervous and circulatory anatomy in the form of infilled cavities, which we reconstruct from CT scans. The circulatory system of D. ricordeanus was relatively extensive and complex, implying a pattern of coelomic fluid flow that is unique among crinoids, and the peripheral parts of the nervous system include linkages both to the circulatory system and to the surface of the body. A phylogenetic analysis (the first to include both living and fossil featherstars and which includes characters from internal anatomy) recovers D. ricordeanus among the lineage of featherstars that includes Himerometroidea, Tropiometra and ‘Antedonoidea’, among others. D. ricordeanus is larger than almost any modern featherstar, and its elaborate coelomic morphology appears to be a consequence of positive allometry. All featherstars with coelomic diverticula are shown to belong to a single comatulid subclade, and this feature may constitute a synapomorphy of that group. Some preservation of cavities corresponding to soft tissue is probably not exceptional in fossil crinoids, providing an opportunity to study the diversity and evolution of extinct anatomical systems typically only preserved in Lagerstätten.

Dispersals from the West Tethys as the source of the Indo-West Pacific diversity hotspot in comatulid crinoids

“…Fossil placements are roughly similar when using only continuous or discrete characters and when inferring the position of each fossil individually (Supplementary Appendix). Decameros ricordeanus , Decameros wertheimi (both Early Cretaceous), and Solanocrinites depressus (Late Jurassic) form a relatively well-supported clade near Tropiometra , consistent with previous studies (Saulsbury and Zamora 2020). Precise placements for the other three fossils are less certain but all well supported as being in the crown, generally outside the clade containing Heliometra and Cenometra but closer to that clade than to Crinometra .…”

“…We built on a previous analysis of fossil comatulid phylogeny (Saulsbury and Zamora 2020) with added fossil and living taxa and an expanded set of characters. We coded a morphological database of 30 discrete and 24 continuous characters for 24 extant and 7 Jurassic and Cretaceous fossil species, including the Jurassic Paracomatula helvetica as the outgroup.…”

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

“…4); its brachials are known from one putative specimen [ 3 ], and its first pinnule occurs on secundibrachial 2; (v) Hertha mystica von Hagenow (Antedonidae) from Denmark, Germany and Sweden [ 45 ]; its arms are divided at primibrachial 2, and there are synarthries between primibrachials 1 and 2; (vi) Roiometra columbiana A.H. Clark (Antedonidae) is from the Albian of Colombia; it is characterized by a centrodorsal with 10 arms, preserved cirrals, and pinnulars; however, there is no suitable description and illustration for this species ([ 60 ], p. 304); (vii) Decameros ricordeanus (Decameridae) with five arms has no syzygies or synarthries, and the first pinnule occurs on the first brachial. It is noteworthy that Saulsbury and Zamora [ 61 ] recently described a well-preserved representative of this species that preserved the shape and configuration of the nervous and coelomic canals.…”

Predation as an explanation for a latitudinal gradient in arm number among featherstars

“…We devised a new permutation-based approach to investigate the properties of a comparative dataset with respect to the phylogenetic history on which it evolved. We generated a timetree with penalized likelihood (Sanderson, 2002)-implemented with the program treePL (Smith & O'Meara, 2012)-using the molecular phylogeny inferred by Saulsbury and Zamora (2019). Two fossil calibrations were used to scale the tree to units of time (Appendix S1).…”