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The calcichordate hypothesis holds that each separate lineage of chordate (Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group.
The carpoids Cornuta and Mitrata are grouped together in a clade called Calcichordata. Cornutes and mitrates are viewed as sister groups, and mitrates represent stem group chordates. The mitrates (and thus the chordates) are all Dexiothetes, dexiothetism being a synapomorphy for the clade.
In the evolution of the chordates, the ancestors of the chordates underwent a profound remodeling of their bauplan, becoming dexiothetetic. All chordates share a common ancestor which lost its echinoderm stereom calcite skeleton. However, later revisions of the theory had each separate lineage losing its calcitic skeleton independently, as it evolved from its own mitrate ancestor, making the chordates a paraphyletic group.
The central part of the Calcichordate Theory lies in the interpretation of the phylogeny of the two groups of stylophorans, which are termed calcichordates in the theory. Mitrates (and the rest of the calcichordates that evolved from them) are dexiothetic as a synapomorphy, having evolved from a cornute. Mitrates are thought to have formed their tail from the proximal part of the cornute tail, with the distal part atomised, and evolving new appendages. The left hand side in this scheme would be cognate with the Pterobranch left-hand side, with the right hand side a novel feature. This would explain the bizarre embryology of Amphioxus, a basal cephalochordate widely held to be the prime example of a chordate bauplan.
Anatomy according to the Calcichordate view
The appendage of the carpoids is regarded as a tail, with the central canal probably containing a notochord. The large orifice seen is most likely the mouth, with many of the slits along the side assumed to be gill slits. While the Cornuta were interpreted as lying with the flat side ventrally, Jefferies suggested that in Mitrata the flat side was dorsal and the convex side ventral, while the tail was curved underneath to provide forward thrust; many mitrates are preserved with the tail underneath.
The calcichordate theory is not widely accepted as a viable theory on the origins of the chordates. Many cite its overall unparsimonius nature as unnecessary, however there are many specific points that can be raised. All in all, the carpoids are much closer to echinoderms than to chordates.
Loss of Stereom
Stereom calcite is considered to be a synapomorphy of the echinoderms, and there is no evidence for it ever having evolved in any other lineage, and there is no crown group echinoderm that seems to have lost it secondarily. The theory implies either that it must have been lost once or three times, which is considered to be very unlikely.
Timing of appearance
Genetic evidence has shown that Hemichordata is the sister group to Echinodermata.
- Jefferies, R. P. S. (1999). "The Calcichordate Theory". Science 236 (4807): 1476. doi:10.1126/science.236.4807.1476.
- Gee, Henry (1996). "Jefferies' Calcichordate Theory". Before the backbone: views on the origin of the vertebrates. London: Chapman & Hall. ISBN 0-585-25272-6.
- Ruta, M. (1999). "Brief review of the stylophoran debate". Evolution & Development 1 (2): 123–35. doi:10.1046/j.1525-142x.1999.99008.x. PMID 11324028. Retrieved 2010-05-29.
- Taylor, Christopher. "Stylophora: Like a Tee-to-tum They Twirled Him". The Paleos Project. Retrieved 2010-05-29.