Calcichordates

Calcichordates are primitive members of the phylum Chordata with a calcite skeleton of echinoderm type. They occur in rocks of Cambrian and Ordovician age [530 – 300 million years old] and because of their skeletons have traditionally been placed in the phylum Echinodermata. They are however shown to be chordates by many chordate anatomical features. Reconstruction of these ancient animals includes visceral (pharyngeal or gill) slits that would suggest chordate affinities, but the mineralized skeleton was composed of calcite, like echinoderms, not bone as in many chordates. In fact each of the plates covering the body is a single calcite crystal. Their calcite skeletons merely confirm an old view – that echinoderms and chordates are closely related.

The Phylum Chordata includes the well known vertebrates (fishes, amphibians, reptiles, birds and mammals including human beings). The vertebrates and hagfishes together comprise the taxon Craniata. The remaining chordates are the tunicates (Urochordata), lancelets (Cephalochordata).

There are three main groups of Calcichordates – the Soluta, the Cornuta and the Mitrata. The Cornuta are a more primitive form from which, it is believed, the more advanced order Mitrata evolved. It may be that the latest common ancestor of the vertebrates was a primitive Mitrate.

The solutes had a head, a tail and a feeding arm; the cornutes had a head and a tail with gill slits on the left side of the head only; and the mitrates had a head and a tail, with right as well as left gill slits inside the head.

Dendrocystoides scoticus [ventral view]