A probable case of heterochrony in the solutan Dendrocystites Barrande, 1887 (Echinodermata: Blastozoa) from the Upper Ordovician of the Prague Basin (Czech Republic) and a revision of the family Dendrocystitidae Bassler, 1938

International audienceThe morphology of the Late Ordovician solutan Dendrocystites is reevaluated based on more than 300 specimens from the Letna and Zahorany 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 Letna and Zahorany 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, amajor 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

Les niveaux à échinodermes de Bou Nemrou, un gisement à préservation exceptionnelle de l'Ordovicien supérieur du Tafilalt occidental (Maroc)

International audienceLe site de Bou Nemrou, situé dans le Tafilalt occidental (Anti-Atlas oriental, Maroc) compte parmi les rares gisements à préservation exceptionnelle (Lagerstätten) connus actuellement dans l'0rdovicen supérieur à avoir livré des restes d'organismes peu ou pas minéralisés, associés à une faune marine benthique abondante et diversifiée. Ce même gisement a également livré plusieurs niveaux trèsè riches en échinodermes (starfish beds), dont la préservation est tout à fait remarquable. Elle résulterait de l'enfouissement rapide, in situ et de leur vivant, de véritables "prairies" à échinodermes par des dépôts de tempête. Les starfish bes de Bou Nemrou sont dominés par les éocrinoïdes et les stylophores, auxquels sont associés des crinoïdes, des cyclocystoïdes, des édriastéroïdes et des ophiures. Il s'agit d'un assemblage caractéristique des eaux froides de la Province méditerranéenne. Ces niveaux peuvent témoigner de la colonisation opportuniste du fond marin par des populations denses d'échinodermes suite à des pertubations environnementale

Palaeoecological aspects of the diversification of echinoderms in the lower Ordovician of Central Anti-Atlas, Morocco

Echinoderms are one of the major components of benthic faunas in the Lower Ordovician sequence near Zagora, central Anti-Atlas, Morocco. The Fezouata Shale (Tremadocian–late Floian) has yielded numerous, exquisitely preserved echinoderm assemblages, ranging through several stratigraphic levels and palaeoenvironmental conditions. These associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high-latitude, siliciclastic-dominated western Gondwana sediments, where rapid in situ burials facilitated excellent faunal census conditions. Lower shoreface deposits of the Fezouata Shale provide the most complete record of successive echinoderm faunas. In late Tremadocian times, these relatively shallow shelf deposits show the progressive replacement of low-diversity, opportunistic, Cambrian-like, dwarfed communities dominated by cornute stylophorans, in unhospitable, dysoxic environmental conditions by higher diversity benthic assemblages, more typical of the Great Ordovician Biodiversification Event and dominated by blastozoans, on well-oxygenated sea-floors. The turnover of the Ordovician radiation was apparently slightly delayed in more proximal settings. Eocrinoid meadows persisted in shallower environmental conditions up to the middle Floian. In the late Floian, they were replaced by diploporite-dominated communities, typical of later Ordovician high-latitude peri-Gondwanan faunas. From a palaeobiogeographic point of view, low-diversity assemblages display relatively strong affinities with cosmopolitan late Cambrian echinoderm faunas, whereas high-diversity communities are dominated by peri-Gondwanan taxa