The pre‐radial history of echinoderms

Abstract

Gene sequence data now identify a robust phylogeny of deuterostomes and provide a framework within which the evolution of echinoderms can be interpreted. The topology of the molecular tree makes a number of important predictions about the morphological characters of the earliest echinoderm at its split from hemichordates: it possessed gill slits (but not a notochord), had a bilaterally symmetrical body plan in the adult and, less certainly, underwent torsion during development. Carpoids, a highly contentious group of extinct deuterostomes with a plated calcite skeleton that have variously been interpreted as stem-and crown-group chordates, stem-group echinoderms or stem-and crown-group echinoderms, display many of these basal characters and provide critical evidence for how the latest common ancestor of hemichordates and echinoderms was transformed into a pentaradiate crown-group echinoderm.Cinctans have a large atrial opening in addition to mouth and anus, and are interpreted as pharyngeal basket feeders. The paired grooves associated with the mouth indicate the presence of a hydrovascular system, but not necessarily one built along the echinoderm plan (that is, derived from just the left hydrocoel). Stylophorans have a bilateral body plan that is externally masked by torsion and possess gills, either unpaired and external, or paired and internal, opening into an atrial cavity. Their bilateral appendage is a locomotory organ, not an ambulacrum, and there is no evidence that stylophorans ever possessed a welldeveloped hydrovascular system homologous to the water vascular system of echinoderms and the tentacles of pterobranch hemichordates. Solutes are the most crownward, having a true echinodermal ambulacral system with a single hydropore and no pharyngeal gill openings.

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

“…However, echinoderms have a mineralized skeleton with a very distinctive morphology termed stereom that is underpinned by a unique family of genes 42 . It is the first synapomorphy of echinoderms that can be recognized 17 and the evolution of a mesoskeleton composed of stereom must thus predate diversification of the clade into bilateral, asymmetrical, spiral-radiate and pentaradiate body plans. Because of its distinctive microstructure, even isolated plates of echinoderms are easily recognizable in petrological thin sections.…”

“…570 Ma, implying some 50 my of missing echinoderm fossil record. Furthermore, some argue from comparative morphological grounds that peculiar asymmetric to bilaterally symmetric clades of echinoderm represent the primitive echinoderm body plan, even though they appear in the fossil record somewhat later than radiate forms [16][17][18][19] .…”

The Evolutionary Emergence of Vertebrates From Among Their Spineless Relatives

“…4 The interrelationships of living and fossil echinoderms within the context of Deuterostomia. Primarily based upon Smith (2005Smith ( , 2008 and Bourlat et al (2008) indicate a position higher in a tree are absent because they were never present or merely because they rotted away (Donoghue and Purnell 2009). Thus, various fossil organisms, such as the yunnanozoans (including Haikouella), have been interpreted as primitive deuterostomes (Shu et al 2003b), primitive ambulacrarians (Shu et al 2004), or as primitive chordates (Shu et al 2004) because they seem to possess those few necessary characteristics, such as gill slits.…”

Deciphering deuterostome phylogeny: molecular, morphological, and palaeontological perspectives

“…The hemichordate gill slits and bars are located dorsally, opposite to the ventral mouth, while in the cephalochordates and vertebrates, the gill slits and bars are located ventrally (Swalla 2007). Gill openings are also dorsal in stemgroup echinoderms (Smith 2005). Recent results have shown that the dorsal side of a juvenile hemichordate is determined by a stripe of expression of bone morphogenetic protein (BMP) gene expressed dorsally, and its ventral side by an expression stripe of BMP antagonists, including chordin , just as has been reported for the arthropods (Sasai & De Robertis 1997).…”