Chapter 13 Cambrian echinoderm diversity and palaeobiogeography

Abstract

The distribution of all known Cambrian echinoderm taxa, encompassing both articulated specimens and taxonomically diagnostic isolated ossicles, is documented for the first time. The database described by 2011 comprises 188 species recorded from 65 formations from around the world. Formations that have yielded articulated echinoderms are unequally distributed in space and time. Only Laurentia and West Gondwana provide reasonably complete records at the resolution of Stage. The review of the biogeographical distributions of the eight major echinoderm clades shows that faunas from Laurentia and Northeast Gondwana (China and Korea) are distinct from those of West Gondwana and Southeast Gondwana (Australia); other regions are too poorly sampled to make firm palaeobiogeographical statements. Analysis of alpha diversity (species per formation) shows that diversity rose initially to Cambrian Stage 5, declined into Guzhangian and Paibian before returning to Stage 5 levels by the end of the Cambrian. This pattern is replicated in Laurentia and West Gondwana. We show that taxonomically diagnostic ossicles found in isolation typically occur significantly earlier than the first articulated specimens of the same taxa and provide important information on the first occurrence and palaeobiogeographical distribution of key taxa, and of the phylum as a whole.Supplementary material:Articulated Cambrian echinoderms and Isolated plates of Cambrian echinoderms are provided at:http://www.geolsoc.org.uk/SUP18668

The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid

“…Because of its distinctive microstructure, even isolated plates of echinoderms are easily recognizable in petrological thin sections. It is therefore noteworthy that the earliest ossicles showing stereom appear more or less contemporaneously in Siberia, Laurentia and Gondwana, within the Delgadella anabara Zone of Cambrian Series 2, Stage 3 (Atdabanian), at between 520 and 525 Ma 9,43 . Furthermore, no recognizable stereom plates have yet been discovered among the slightly earlier small shelly Tommotian faunas.…”

“…Their fossil record starts in the early Cambrian (Series 2, Stage 3) with isolated plates. Articulated specimens first appear slightly later, in same stage, but they remain rare until the mid Cambrian (Series 3, late Stage 5) 9 . Initially, individual faunas typically contain just one dominant species, and have come almost exclusively from the palaeocontinent Laurentia.…”

A bradoriid and brachiopod dominated shelly fauna from the Furongian (Cambrian) of Västergötland, Sweden

“…They are more similar to the plates from the basal attachment disc of Totiglobus, which bear parallel to radiating ridges along their inner and outer surfaces (Bell and Sprinkle 1978;Clausen and Peel 2012). Together with the ambulacral plate (see below), the edrioasteroid plates described herein are among the oldest occurrences of edrioasteroids, demonstrating that the latter were already widespread in Cambrian Stage 4 (Clausen and Peel 2012;Zamora et al 2013). …”

“…Most authors agree that blastozoans ("brachiole bearing echinoderms" ) are probably derived (e.g., David et al 2000;Paul and Smith 1984) from edrioasteroids (without feeding appendages). The oldest known edrioasteroids are also reported from the upper part of Cambrian Stage 3-lower Stage 4 (see below; Zamora et al 2013). Therefore, the earliest brachioliferous echinoderms from Siberia demonstrate a very fast but still poorly illustrated initial diversification of echinoderms during Stage 3.…”

“…The described plates from the middle Atdabanian Stage of Siberia are among the earliest record of echinoderm skeletal elements (Clausen and Peel 2012;Kouchinsky et al 2012;Zamora et al 2013). Together with an elongated fragment of a brachiolar plate from the upper Atdabanian Stage of Siberia (Rozhnov et al 1992: pl.…”