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

Abstract

The 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.