Jean Le Menn

Nouveaux Échinodermes des schistes et calcairesdu Dévonien inférieur du bassin de Laval (Massif Armoricain, France)

The study of new material brings complementarydata on the crinoid fauna from the Saint-Cenere Formation. 12 species are described; 6 species and a genus are new. The difficulty of determining the systematic position of Idaeumocrinus n. gen. within the cladid inadunates raises the problem of the value of the anal plate characters associated with dorsal cup characters in the definition of suprageneric groups. The identification of isolated columnals of Tiaracrinus soyei Oehlert has been possible owing to the strong correlation that exists between calyx and columnal ornamentation; an important intraspecific variability in the columnal shape and the articular facet has been pointed out. Stabilization and anchorage by mean of stereomic spinose expansions have been discovered on the distal part of stem in Asperocrinus. The survival in the Devonian of the genus Carpocrinus has been established and the one of Neoarchaeocrinus is confirmed. The presence of Diamenocrinus stellatus Jaekel, a common species in the Rhenish massive, corroborates the extension of the Saint-Cenere Formation into the Middle Siegenian. The existence of Edrioasteroidea, represented by the genus Rhenopyrgus, is recorded for the first time in the Armoricain massive.

Growth patterns and evolutionary trends of Devonian crinoid columns

Abstract The general morphology of the crinoid column is briefly reviewed; growth patterns deduced from the study of stem fragments and dissociated columnals are analysed in the two commonest groups occurring in the Devonian strata of France: the Cyclici and the Pentameri. Problems concerning the introduction of new columnals beneath the calyx and the differentiation of cycles in the proximal part of heteromorphic stems are set. Evolutionary trends affecting various morphological elements include size increase, ornamentation development, cirri insertion and distribution, epifacet regression and formation of composite and secondary holdfasts. At present, these trends appear to be limited to small systematic units but proxistelisation, defined as the downward extension of proxistele characters to the other stem regions, seems to be wide-spread in the Pentameri. The difficulty in determining the evolutionary process (neoteny, progenesis…) due to the lack of information on early ontogenetic stages is emphasized.

Crinoid columnal associations and sequence stratigraphy architecture: the Le Faou Formation, Lower Devonian of the Massif armoricain (France)

Crinoid columnals are major macrobenthic components of the Lower Devonian faunas of the Massif armoricain particularly in the Seillou section. Three crinoid columnal associations have been delineated based on R and Q-mode cluster analysis. Distribution of these associations is correlated to the sequence stratigraphic architecture, showing the close relationships between crinoid columnal association dynamics and relative sea-level variations. Comparing the distribution of benthic associations, we note a lesser diversity in crinoid columnal associations than in brachiopod ones. Crinoids (high-level suspension feeders) seem to occupy broader specialised niches than brachiopods (reclining and low-level suspension feeders) and are characterised by a different ecological space utilisation.

Calceocrinids and a new Ramacrinid (Crinoidea, inaduanta)from the Lower Devonian of Bohemia and Armorican Massif

Abstract The calceocrinid crinoids Eohalysiocrinus latus and E. convexus , formerly considered as endemic in theBarrandian and restricted to organodetrital limestones, have been discovered in a shaly layer at the top of the Beg an Arreun Formation (upper Emsian, upper Zlichovian) in the western part of the Armorican massif. A new species, Ramacrinus brevis , found simultaneously in Bohemia and Brittany, is described. It is suggested that the incursions of hercynian faunal elements into the rhenish associations of the Brest area may be in relation with paleogeographical changes allowing communication between the Barrandian and the western part of Europe and migration of both pelagic and benthic organisms.