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Title: Macroevolutionary patterns in Paleozoic echinoderms with emphasis on stylophorans
Author: Noailles, Fleur
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Stylophora (Comuta and Mitrata) share the same basic anatomical organization, which consists of a (1) delicate single highly flexible tripartite appendage (aulacophore) inserted into a (2) dorsoventrally flattened theca exhibiting various degrees of asymmetry in its outline. Because of this unique anatomy and the controversy surrounding the morpho-functional interpretation of the single appendage, stylophorans have been interpreted either as stem-group chordates, primitive or highly derived echinoderms close to crinoids. Nowadays, their placement within the phylum Echinodermata is still highly debated, as well as their intrinsic relationships. While there is agreement that cornutes and mitrates are closely related to each other, authors have debated whether cornutes are the monophyletic sister-group to mitrates or a paraphyletic array relative to them. This absence of congruence between analyses is mainly due to the fact that there is no agreement on the exoskeletal plate homologies, the choice of characters and their coding, highly influenced by divergent hypotheses of topological similarity. Our present study provides the first detailed cladistic analysis of all the best-known taxa of stylophorans and benefits from recent discoveries of new taxa that shed light on the origin of major stylophoran groups. Our analysis suggests that mitrates are either polyphyletic deriving from paraphyletic symmetrical boot-shaped cornutes, or a natural group deriving from polyphyletic symmetrical cornutes. In each scenario, mitrates and boot-shaped cornutes derive from a basal stock of symmetrical cornutes. The placement of cornute-like mitrates and symmetrical cornutes is particularly issuing in our topologies and we consider them as transitional taxa between mitrates and cornutes. Palaeobiogeographical reconstructions for stylophorans stayed unexplored for a very long time. Patterns of migration and dispersal for mitrocystitidan mitrates are examined in the present study, as well as intrinsic (biological) and extrinsic (palaeoenvironemental) factors and causes of this dispersion. The colonisation of the continent Laurentia by mitrocystitidan mitrates has been realized 8 times, while the Malvinokaffric realm has been colonized only 6 times. Gondwana has been left 4 times for colonization either in Laurentia and/or in the Malvinokaffric Realm. We suggest that the Hirnantian glaciation must have been an important factor for migrations towards shelters located in Laurentia and in the Malvinokaffric realm. The rise of sea level and then its quick fall during this glaciation is concordant with the pattern of dispersion at least for the Ordovician, as suggested by the increase of diversity during this period. The stylophoran dispersion towards Laurentia and the Malvinokaffric Realm is also concordant with the paleo-currents and paleo-upwellings. Stylophorans are typically adapted to a benthic mode of life on soft substrates, by increasing the surface of their body in contact with the seafloor. We characterize here three different strategies for epibenthic stylophoran taxa: (1) the water strider strategy; (2) spine and fan-shaped elements surrounding the theca to increase the body size; (3) hydrodynamic thecas and sediments-gripping devices on the aulacophore. Since the 2000', research interest has been invigorated by new discoveries in Morocco and its recently described, new echinoderm Lagerstatten, The Fezouata biota is one of the richest and most diversified worldwide echinoderm assemblages and a fantastic continuous preservational window open on Cambrian and Paleozoic communities. The echinoderm distribution is not random and is well-divided into four distinct successive assemblages. This pattern is mainly due to an evolutionary process probably combined with a paleoenvironmental process. Recently, abundant remains of a new mitrate (Echinodelmata, Stylophora) were discovered in a brand new echinoderm Lagerstatte from the Early Floian (Lower Ordovician) in the Fezouata Biota. The new taxon, displays a unique combination of characters, mixing features peculiar to anomalocystitid mitrates but also features only seen in the very basal mitrocystitid mitrates. Its basal position is confirmed by the cladistic analysis. Consequently, the new taxon is interpreted as a transitional form between basal mitrocystitidans and anomalocysitid mitrates.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available