A methodological scheme to analyse the early Palaeozoic biodiversification with the example of echinoderms

The early Palaeozoic biodiversification is the most significant radiation of marine ecosystems of Earthâs history, starting with the appearance of invertebrate organisms near the PrecambrianâCambrian boundary and followed by a significant diversification during the Great Ordovician Biodiversification Event (GOBE). The apparently sudden appearance of major phyla of metazoans during the earliest Cambrian (i.e., the âCambrian Explosionâ, c.a. 540 Ma) has been considered by many as corresponding to an âexplosiveâ process that took place during a very short time interval. Similarly, the GOBE has been considered as a short spectacular global event triggering a massive biodiversification during the early Middle Ordovician (c.a. 470 Ma). However, it appears that both âeventsâ have been more intensively studied in a few locations, creating multiple gaps, and thus a bias, in the biodiversity datasets. For example, the âFurongian gapâ (late Cambrian) is clearly an artefact in the Paleobiology Database (PBDB), which includes mostly data from North America and Western Europe. These geographic areas were more intensively sampled and recorded in the PBDB than others (e.g., China), separating the âCambrian Explosionâ and the GOBE artificially, whereas the Geobiology Database (GBDB), which is focused on data from eastern Asia, records a more gradual increase of the global diversity during the early Palaeozoic. These diversity curves are indeed not truly global but reflect patchy data from different palaeocontinental margins. Moreover, the evolution of global biodiversity is mostly estimated only in two dimensions (taxonomic richness versus time) and spatial distribution is rarely assessed. The organisms might have occupied the Earthâs surface heterogeneously because of constraints on their ecological niches, generating âdiversity hotspotsâ that were recorded in some databases and not in others. We want to test whether global âexplosionsâ of diversity ever occurred, or instead a single, but very complex, long-term evolutionary process took place over space and time, starting in the late Precambrian and lasting throughout most of the early Palaeozoic, with changing âdiversity hotspotsâ at different palaeogeographical locations. To do so, we use echinoderms as a model and we propose a protocol to (1) assess the validity of diversity curves based on data currently available online, and (2) analyse the spatio-temporal evolution of their diversity (i.e., generic richness) in the Cambrian and the Ordovician. We first compare echinoderms diversity curves between the PBDB, the GBDB and an original database built from a comprehensive synthesis of the literature, with a temporal resolution at the scale of the stratigraphic stage. Then, we gather the three datasets to build a synthetic database that includes a revised taxonomy. To avoid fake spatio-temporal diversity peaks and hotspots, we define comparable stratigraphic units in terms of temporal range and gather sampled sections that refer to the same locality (e.g., the Montagne Noire). We will present the comparison of the diversity curves between the three databases and the details of the whole methodological protocol

Echinoderm Lagerstätten from high-latitude Gondwana – filling the gap in the Ordovician diversification of the phylum

The phylum Echinodermata is a major component of marine ecosystems since the first appearance of echinoderms in the Cambrian. Like other metazoan phyla, echinoderms underwent two major evolutionary pulses in early Palaeozoic times: the Cambrian Explosion and the Ordovician Radiation. In the Ordovician, the exploitation of new ecological niches triggered an exponential diversification of the phylum, leading to its highest morphological disparity and class-level diversity (about 18 classes compared to only five extant classes) of the whole Phanerozoic. However, the precise spatio-temporal patterns of the Ordovician Radiation of echinoderms remain poorly known, owing to geographical and taphonomic biases. Over two centuries, the sampling effort was mostly focused on faunas from Europe and North America, providing a wealth of information on crinoid- and/or blastozoan (âcystoidâ)-dominated low-latitude assemblages from Avalonia, Baltica and Laurentia. Moreover, echinoderm Lagerstätten, bearing fully articulated skeletal elements, provide valuable snapshots of past communities but are rare in the rock record because echinoderm endoskeletons disarticulate into many calcite plates shortly after death. Recent discoveries of several new Ordovician Lagerstätten from high-latitude Gondwana, i.e., the Czech Republic (Barrandian area), France (Armorican Massif, Montagne Noire) and Morocco (Anti-Atlas, Meseta), provide a unique opportunity to fill the existing knowledge gap regarding the Ordovician diversification of the phylum. Echinoderm assemblages from high-latitude areas are dominated by asterozoans, blastozoans and stylophorans, with very few crinoids, unlike those from low-latitude regions. By comparing and combining these two sets of information, it is possible to coherently frame the early diver­sification of echinoderms in space and time

New occurrence of the Ordovician eocrinoid Cardiocystites: palaeogeographical and palaeoecological implications.

10 pagesInternational audienceFlattened eocrinoids are very rare in the fossil record, notably because of their fragility. Recent investigations in the Anti−Atlas (Morocco) have provided one of the oldest known specimens of Cardiocystites from the Upper Ordovician (early–middle Sandbian). This discovery increases the number of eocrinoid genera known in Morocco. This new material is the fifth published specimen of the genus Cardiocystites. It is well preserved, thus allowing morphological details, such as the location of the anal pyramid and the plane of thecal flattening, to be observed. Palaeoecological reconstruction can be deduced or confirmed from these new details. The respiration of Cardiocystites now seems due to the combination of both epidermal gaseous exchange and cloacal pumping. Stem length and synostosial articulation indicate that the stem might have been used as a mooring line allowing the theca to float in the currents. The flattening of the Cardiocystites theca seems to be an adaptation to high energy hydrodynamic conditions and cold waters. Occurrences of Cardiocystites bohemicus in Morocco, in the early–middle Sandbian, and in Bohemia, in the early Katian, indicate that the genus probably originated in the west Gondwanan margin. Migration could explain the occurrence of Cardiocystites in this area and also in Avalonia in the late Sandbian. The global sea−level rise and the presence of cool water circulation from west Gondwana to Avalonia and Laurentia in the early Sandbian favour such a hypothesis

New specimens of Lingulocystis Thoral, 1935 (Eocrinoidea, Blastozoa) from the Arenig (Lower Ordovician) of Montagne noire (southern France): intraspecific morphological variability, stratigraphic, and palaeoecological implications.

16 pagesThe genus Lingulocystis Thoral, 1935 is a preponderant component of the echinoderm fauna of the Lower Ordovician of Montagne noire (France), as suggested by its abundant remains. Its morphology is characterized by a flattened theca, composed of a frame enclosing numerous small polygonal central plates. The new specimens have been determined partly as Lingulocystis elongata Thoral, 1935 and partly as Lingulocystis aff. deani Ubaghs, 1994. This study is the first report of Lingulocystis in the Foulon Formation. New stratigraphic ranges have been defined for both species: L. elongata from the early Tremadoc (Saint- Chinian Formation) to the middle Arenig (lower part of the Landeyran Formation), L. aff. deani from the early Arenig (Cluse de l'Orb Formation and Foulon Formation), and L. deani from the middle Arenig (upper part of the Landeyran Formation). The new specimens of L. elongata highlight its intraspecific variability, mainly concerning the size and arrangement of the central plates, the size of the theca, and the type of stem columnal articulations. Sedimentological studies in Montagne noire currently use the presence of Lingulocystis remains as taphonomic and palaeoecological indicators. The presence of two types of columnal articulations enhances the palaeoecological use. The development of the synostosial articulation seems to be correlated with a proximal platform palaeoenvironment, whereas development of the symplexial type seems to be related to a deeper palaeoenvironment or relatively high hydrodynamic conditions

Unravelling extrinsic and intrinsic factors of the early Palaeozoic diversification of blastozoan echinoderms.

19 pagesInternational audienceThe Subphylum Blastozoa represents more than one third of the early Palaeozoic echinoderm fauna. A comprehensive database including all records of blastozoans was built to provide quantitative analyses of palaeogeography and diversity patterns and processes, for the 10 classes currently included in this subphylum during the early Palaeozoic. The global pattern of taxonomic diversity shows two peaks during the Cambrian Series 3 and the Late Ordovician intervals. In Cambrian times, the high taxonomic diversity seems to be related with a high turnover rate and a high endemicity of blastozoan genera, whereas in Ordovician times, the rise in diversity is associated with a low endemicity and a low turnover rate. The gradual increase of Ordovician diversity can be explained by the progressive geographic extension of blastozoan genera onto most palaeocontinental margins, associated to the long-term gradual sea level rise. This global pattern is subdivided into local trends to compare the relative influence of regional distributional patterns. Blastozoans occurred mainly on Gondwanan and Laurentian margins during the Cambrian. In both Ordovician and Silurian times, blastozoans colonised different provinces, characterised by varied latitudinal position and environmental conditions: the Laurentian province was mainly located in the equatorial latitudes (temperate to warm waters, mainly carbonate platforms), and both the northwestern peri- Gondwanan and Asian provinces, in the intermediate to high latitudes (temperate to cold water, mainly siliciclastic environments). After the Hirnantian mass extinction, blastozoans finally recovered chiefly on Laurentian margins, where climate and environmental conditions were probably more favourable. The two diversification events recorded for blastozoans in the early Palaeozoic concerned two distinct evolutionary faunas (Cambrian Evolutionary Fauna vs. Palaeozoic Evolutionary Fauna). They were driven by distinct processes, even if the Ordovician diversification had indeed started during the middle Furongian with a medium origination event. Difference in processes might be related to the evolution of global environmental/ climatic conditions coupled with the position of continental masses

Paleogeographic control on the diversity fluctuations of Blastozoans (Echinodermata, early paleozoic)

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Evolution of ecospace occupation of early paleozoic echinoderms

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The great Ordovician diversification of echinoderms: deciphering acomplex global signal

International audienceThe most dramatic increase in echinoderm diversity and disparity took place during the Ordovician, with a peak of about 15 classes instead of e.g. only 5 in the latest Cambrian (Furongian) and in post-Palaeozoic seas. The global analysis of about 2,000 Ordovician echinoderm taxa recovered from over 300 areas worldwide suggests that their diversity remained relatively low from the Tremadocian to the Dapingian, before rising from the Darriwilian to the Katian, and finally collapsing in the Hirnantian. Taxonomically, this pattern largely depicts the major diversification of crinoids, which represent from 30 to 70 percent of the whole echinoderm diversity, and become, during the Ordovician, one of the major components of marine ecosystems. However, the analysis of the geographic origin of included taxa indicates a strong historical and/or sampling bias towards Laurentian faunas (e.g. North America, Scotland; 20 to 50 percent of the data), themselves largely dominated -faunistically- by crinoids. Zooming in on the situation in other well-represented areas in the database (e.g. Avalonia, Baltica, high-latitude Mediterranean Province) provides distinct regional temporal patterns of biodiversity, and especially on non crinoid-dominated faunal compositions. However, Ordovician echinoderm faunas remain very poorly known in many other areas in the world (e.g. Australia, Central and Southeastern Asia, China, South America, Siberia). In recent years, significant sampling efforts in some areas (e.g. Morocco) have significantly modified our knowledge of some regional faunal assemblages and contributed to fill in databases, which however, still remain largely dominated by data from a single palaeocontinent

First report of sphaeronitid blastozoans (Echinodermata) in the Middle Ordovician of the Taurides, Turkey

International audienceArticulated echinoderm remains are described for the first time in the Ordovician of Turkey. They occur massively, forming a relatively thick "cystoid bed" within the detrital limestone succession of the Sobova Formation (latest Dapingian-earliest Darriwilian) in the western Taurus Mountains. The "cystoid bed" encompasses a monospecific echinoderm assemblage of densely packed, 3-dimensionally preserved thecae. The presence of numerous suborganised plates with diplopore respiratory structures suggests probable affinities with sphaeronitid blastozoans. Comparable sphaeronitid dense beds are well known in the early Darriwilian of Baltica, and in the mid Darriwilian of the Middle East and Asian terranes

Evolution of rift-related cover-basement decoupling revealed by brecciation processes in the eastern Pyrenees

International audienceBreccias associated with tectonic, fluid and sedimentary evolution of rifted margins can provide information on a variety of processes reflecting the modes of extension. In this paper, we analyse the numerous breccias exposed in the Agly Massif that was part of the European side of the Cretaceous rift now inverted in the eastern Pyrenees. Using a combination of petrologic and sedimentologic analyses, field-based structural study, and multivariate analysis of clast shape and diversity, binding lithology and size, and breccia fabrics, we distinguish 5 types of breccias reflecting depositional, tectonic, and salt-related processes. The integration of these processes in the tectonic history of the eastern Pyrenees confirms the attribution of these breccias to the Cretaceous rifting. We emphasize the major role played by the evaporitic Triassic particularly during the first stages of rifting as a major decoupling level at the basement/cover interface. Salt tectonics and shearing assisted by the circulation of fluids are reflected by hydrofracturing at the base of the Mesozoic cover. As this weak mechanical layer is later extracted as extension increases, a brittle detachment system developed along the cover-basement interface to exhume of deep crust and mantle. The relationships between brecciation and Cretaceous extension in the Pyrenees argue for a mixed mode of rifting associated with ductile and brittle deformation during the formation of the hyper-extended rift domain.Les brèches associées à l’évolution tectonique, sédimentaire et fluide des systèmes de rifts sont des marqueurs d’une variété de processus qui reflètent les modes d’extension. Dans ce papier, nous présentons une analyse des différentes brèches du Massif de l’Agly qui appartient à la partie européenne du rift crétacé inversé dans les Pyrénées Orientales. Cette étude pluridisciplinaire combine des données sédimentologiques, pétrologiques, structurales ainsi qu’une analyse statistique intégrant les formes et diversité des clastes, les lithologies et granulométrie du liant et la fabrique des brèches. Nous distinguons 5 types de brèches qui reflètent des processus sédimentaires et tectoniques, et la remobilisation des niveaux évaporitiques. L’intégration de ces processus dans l’histoire tectonique des Pyrénées Orientales confirme la formation de ces différentes brèches durant l’épisode de rifting crétacé. Nous mettons ainsi en évidence le rôle majeur des évaporites du Trias qui jouent le rôle de niveau de décollement aux interfaces socle/couverture durant les phases précoces du rifting. La tectonique salifère et le cisaillement sur ces interfaces socle/couverture, assistés par les circulations fluides sont exprimés par les nombreuses évidences de fracturation hydraulique à la base de la couverture mésozoïque. Avec l’augmentation de l’extension, ce niveau faible mécaniquement est extrait, entraînant la formation d’un système de détachement à comportement fragile. L’évolution de ce détachement préférentiellement localisé au niveau des interfaces socle/couverture, conduit à l’exhumation de la croûte profonde et du manteau. Les relations entre les processus de formation des brèches et l’extension crétacée mettent en évidence un mode de rifting dominé par un régime mixte à la fois cassante et ductile au cours de la formation des domaines hyper-étirées du rift pyrénéen