Plate translocation in spatangoid echinoids: its morphological, functional and phylogenetic significance

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Abstract
Analysis of relative changes in plate position during ontogeny was made on a number of Cenozoic spatangoid echinoids: species ofBreynia, Lovenia, ProtenasterandEchinocardium. Contrary to the generally held view that adjacent ambulacral and interambulacral columns in echinoids always remain in a fixed relative position during ontogeny, many spatangoids show great fluidity in plate growth, with adjacent columns ‘sliding past’ one another during ontogeny. Furthermore, in many genera plates from one column may undergo strong lateral growth and bisect pairs of plates in adjacent rows. This phenomenon of relative plate movement, both meridional and equatorial, is herein termed ‘plate translocation’. It is considered to occur by localized resorption and redeposition in a narrow zone along adjacent plate boundaries. Plate translocation has been of considerable phylogenetic significance to spatangoids and, combined with an increase in differential allometries between plates, has been one of the most important factors in the evolution of the Spatangoida. Furthermore, the initiation of plate translocation in the apical system in certain Jurassic echinoids may have been the trigger for the migration of the periproct out of the apical system, and a major factor in the evolution of irregular echinoids. Heterochrony in plate growth has been critical in controlling the course of evolution in many lineages of spatangoid echinoids.

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