Palaeos Palaeos Homalozoa
Deuterostomia Cincta

Cincta

Gyrocystis The Cincta are known only from the Middle Cambrian.  They possessed a round, flattened theca with a plated stele, giving an overall shape something like a frying pan.  The theca is mostly covered by small tesselated plates with a single row of large marginals.  There are three openings at the front and dorsal of the theca -- a large central orifice with operculum, a smaller orifice to the right and a pyramidal orifice to the left.  Under the reconstruction favoured here, the right orifice is the mouth and the left the anus, implying a U-shaped gut.  The central orifice is the gill opening. A small opening behind the mouth represents the hydropore, perhaps combined with the gonopore.  Dominguez-Alonso, 1999.  Other authors would interpret the pyramidal opening as the gonopore, and the central and left openings would be the mouth and anus, with differing interpretations as to which was which.  Ubaghs (1967a); Parsley (1999). A biramous ambulacrum is lodged in grooves recumbent on the anterior side of the theca. The two branches differ in size, and the right ambulacrum is reduced or missing in at least one genus, Gyrocystis.  Unlike Stylophora and Soluta, the tail is simple in structure and not divisible into sections.  Compared to that of other homalozoans, it appears to have been fairly rigid, though a small amount of "waggling" may have been possible.  A lot of debate has occurred over whether the "tails" of the homalozoan classes are homologous, as has already been touched on in Soluta.  Those who would derive them from more typical echinoderm ancestors would usually say not, regarding them as independent adaptations of radial arms.  Supporters of the reconstructions used here are more likely to regard them as homologous on the basis of similarities in position and structure.  Most authors do agree that the "tails" are probably not homologous with pelmatozoan stems, as they differ in structure and being lateral to the theca rather than ventral.  An exception is Parsley (1999) who, using the pyramid-as-gonopore reconstruction, argued that the Cincta were derived Blastozoa and identified the cinctan stele with the blastozoan column.

TrochocystitesIn the current reconstruction, the double ambulacrum and its postion recumbent on the body rather than restricted to the arm argue for a more crownward postion for Cincta than Soluta.  Jefferies (1997).

Cincta probably lay on the sediment with the theca directed downstream, where back-eddies from water flowing over the theca would provide optimum conditions around the ambulacra for filter feeding.  Parsley (1999).  The stele would have been used to dig into the sediment and anchor the animal in place. if the central orifice of the theca is correctly interpreted as a gill opening, water being forcibly expelled from the gill could have aided in pushing the stele back into the substrate.  Jefferies (1997).  

Christopher Taylor  CT050908.  


Images: Trochocystites from Prof. Cowen's History of Life, Fourth Edition site.  Gyrocystis from Проблемы эволюции.  Not shown is a large version of this specimen which can be seen at www.paleontologia-hispana.com (Gyrocystis).  



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