No abstract

The palaeobiogeographical distribution of the six major clades of Ordovician echinoderms (asterozoans, blastozoans, crinoids, echinozoans, edrioasteroids and stylophorans) is analysed based on a comprehensive and up-to-date database compiling 3701 occurrences (1938 species recorded from 331 localities) of both complete specimens and isolated ossicles. Although historically biased towards a limited number of regions (Europe, North America, Russia), the resulting dataset makes it possible to identify six main palaeobiogeographical provinces for Ordovician echinoderms: Laurentia, Baltica, West Gondwana, East Gondwana, Avalonia and Siberia. At a global scale, the high endemicity of echinoderms during the Early to Middle Ordovician coincides with the time of maximum dispersal of continental masses. Late Ordovician faunas tend to become more cosmopolitan, possibly as a consequence of changing palaeogeography and/or relatively higher sea-levels in the Sandbian–Katian interval. Regional biodiversity patterns of Ordovician echinoderms confirm that their major diversification during the Ordovician is not a single, universal evolutionary event, but rather results from the complex addition of contrasted local evolutionary trends.

The morphology of the Late Ordovician solutan Dendrocystites is reevaluated based on more than 300 specimens from the Letná and Zahořany formations (Prague Basin, Czech Republic). This genus is reported for the first time from the Bohdalec Formation, and its presence is confirmed in the Vinice Formation. The morphology of all specimens of the stratigraphically older species D. barrandei (Sandbian) is identical to that of small to medium-size individuals of D. sedgwicki (Katian). Distinctive characters of D. sedgwicki occur only in the largest specimens, and are all size-related (more asymmetrical thecal outlines, stronger ornamentation, rosetting pattern of thecal plates, proliferation of platelets in the proxistele). Consequently, the transition from D. barrandei to D. sedgwicki is interpreted as the result of heterochronic processes, with the largest individuals of D. sedgwicki displaying hyperadult morphologies (hypermorphosis). Dendrocystites is locally abundant in both the Letná and Zahořany formations, but extremely rare in the deeper deposits of the Vinice and Bohdalec formations. This pattern coincides closely with first order fluctuations of the sea-level in the Prague Basin. The life orientation and implied feeding strategy of Dendrocystites and other solutans are both critically discussed. Several independent lines of evidence suggest that solutans were more likely detritus-feeders. Finally, it is proposed that two morphologically distinct patterns of dististele organization were elaborated independently from the polyplated, undifferentiated stalk-like appendage of Coleicarpus (plesiomorphic condition). Consequently, a major subdivision of the class Soluta into two main clades (Dendrocystitida ord. nov. and Syringocrinida ord. nov.) is proposed. The monophyly of each order is supported by apomorphies based primarily on the organization of the dististele and the morphology of the periproct. "Dendrocystites" rossicus is reinterpreted as belonging to an unknown genus of syringocrinids, whereas Heckericystis kuckersiana may represent a third species of Dendrocystites. •

Stylophorans are a Palaeozoic group of non-pentamerous echinoderms, morphologically well-adapted to a benthic mode of life on soft sediment seafloors. By developing a thin and wide theca, they successively increased the surface area in contact with the substrate resulting in an even distribution of the body mass resting on the ground, and efficiently preventing the body from sinking into non-indurated sediments (snowshoe strategy). In stylophorans, the body surface is dramatically increased in result of the expansion of two integumentary areas on the lower thecal surface. While infracentral areas are reduced in primitive forms to the detriment of massive marginals; in boot-shape cornutes, infracentral areas are larger, polyplated and framed by very delicate marginals. In heart-shaped cornutes, the reduction of both infracentral areas is compensated by the development of spiny elements on the theca outline. In forms where the degree of bilateral symmetry is high, the left infracentral area is larger than the right area, resulting in an elongated thecal shape. In structural geology, changes of shape of a rock submitted to a strain can be categorized by translating the obtained deformation into a strain ellipsoid plotted on a Flinn diagram, while in biology changes of shapes of organisms are traditionally measured through morphometric analysis. By applying Flinn's principle of strain ellipsoid to biological objects, the present study aims to characterize different life adaptations across stylophorans, observing changes of shape of both infracentral areas interpreted as two ellipsoids. Once plotted on a Flinn diagram, three significantly separated clusters are observed when focusing on the left area. This concerns forms with reduced infracentral areas, highly and weakly asymmetrical forms. According to these results, three new morphological adaptations are described (water strider, flat fish and stream-lined body). These newly described adaptations enabled snowshoe strategist stylophorans to remain stable on top of the seafloor. □ Echinodermata, Flinn diagram, Palaeozoic, snowshoe strategy, strain ellipsoid, Stylophora.

No abstract