Article: The solute Dendrocystoides scoticus from the Upper Ordovician of Scotland and the ancestry of chordates and echinoderms
A study of the solute Dendrocystoides scoticus (Bather), from the Upper Ordovician Ashgill Series, near Girvan, Scotland produces much new anatomical information. The positions of head, tail, the tube feet in the feeding arm, hydropore, gonopore, gonad, pharynx, stomach, anus, brain, left trigeminal ganglion, gill slit (at posterior left in the head), notochord, and tail muscles are deduced. In their basic structure, solutes resemble cornutes, especially the primitive cornute Ceratocystis, but differ in retaining a water vascular system, and in other ways. They can be compared in detail with the inferred latest common ancestor of chordates and echinoderms, visualized as resembling the hemichordate Cephalodiscus lying on its right side. The tail of solutes is almost certainly homologous with that of cornutes, and therefore with the tail of chordates in general. It is also probably homologous with the locomotory stalk of Cephalodiscus. If so, the tail ( = stalk) was lost early in the evolution of the Echinodermata, while the water vascular system was lost in the early evolution of the Chordata. The Soluta, if characterized as retaining both a tail and a water vascular system, would therefore have included the latest common ancestor of chordates and echinoderms. Dendrocystoides scoticus probably belonged to the stem group of the Chordata, on the basis of several advanced features shared with Cornuta including, especially, the posterior left position of the gill slit. The 'carpoids' placed in the group Cincta, on the other hand, probably belonged to the echinoderm stem group but were primitive for echinoderms in possessing a gill slit, located in an anterior, presumably primitive, position like the left gill slit of Cephalodiscus. If the tail of solutes is homologous with the tail of cornutes, then the latter organ, contrary to the ' aulacophore' interpretation of Ubaghs, cannot be a feeding arm.