Early Palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms

Abstract

Stylophorans (cornutes, mitrates) represent one of the most diverse classes of Cambro-Ordovician echinoderms. They were free-living, benthic, non-radiate forms, closely related to asterozoans and crinoids. Taphonomic, sedimentological, and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology. Such a combined approach suggests that the rarity of stylophorans in proximal environments (above storm-wave base) was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test. Conversely, stylophorans were relatively abundant in deeper settings (below storm-wave base), where they frequently occur in low-diversity dense beds (several tens to hundreds of individuals per square metre). Such assemblages were either largely dominated by stylophorans only (Middle Cambrian–Darriwilian), or by the association of both stylophorans and ophiuroids (latest Darriwilian–Silurian). Comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well-oxygenated, oligotrophic waters with low predatory pressure, and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans. Seven main stylophoran biofacies are identified and described. The associated environments and tropical distribution of primitive, heavily-plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings (earliest Middle Cambrian–Late Cambrian). However, from this primitive stock, stylophorans radiated several times towards deeper and/or colder environments, possibly because of the lower predation pressure in such settings (“cothurnocystid” cornutes in the Middle Cambrian, mitrates in the Late Cambrian, hanusiids in the Tremadocian). As a consequence, the fact that most Cambro-Ordovician cornutes and mitrates were “psychrospheric” (i.e. cold-water adapted) probably explains their relatively wide palaeobiogeographic distribution. In high palaeolatitudes, Cambro-Ordovician stylophorans were abundant and diverse in shallow to deep environments, but they were generally restricted to deep settings in low palaeolatitudes. The great Caradocian transgression favoured the extension of stylophorans from circum-polar regions (e.g. western margin of Gondwana) towards external settings at lower palaeolatitudes (Avalonia, Baltica, and eastern margin of Laurentia), where one group of mitrates underwent a major radiation (anomalocystitids), and extended towards shallower (and warmer) palaeoenvironments. In contrast, all Silurian stylophoran assemblages known so far are rare, poorly diversified, and restricted to shallow, warm to temperate environments in tropical regions. This distribution partly results from (1) the probable eradication of psychrospheric forms (e.g. cornutes) because of the prolonged anoxia of deep environments in the late Hirnantian–early Silurian of West Gondwana, (2) the radiation of some mitrates (anomalocystitids) in shallow tropical settings in the late Ordovician, and (3) of strong taphonomic and/or sampling biases (numerous ghost lineages).