Ontogeny of the trilobiteParabolina spinulosa(Wahlenberg, 1818) from the upper Cambrian Alum Shales of Sweden

Abstract

AbstractThe ontogeny ofParabolina spinulosais described on the basis of material from Andrarum (Skåne) and Jämtland in Sweden. Protaspides are rare, and indifferently preserved, but well-preserved isolated tagmata and complete specimens from the early meraspid stages onwards were available for study. Whereas the ontogeny is broadly similar to that of the earlierOlenus wahlenbergi, there are subtle differences, notably in a greater spinosity from the earlier stages and a gradual, rather than a sudden, change of surface sculpture in the early holaspid.P. spinulosaat all stages of growth is much more variable than isOlenus. Meraspides have larger articulating halfrings anteriorly and the first thoracic segments are of greater sagittal length than in the adult; these were adaptations which enabledParabolinato undergo cylindrical enrollment from an early stage.Parabolinais considered to have arisen from the earlier genusOlenus, and on the basis of comparative ontogeny many, though not all, features ofParabolinaappear to be of paedomorphic origin. These include the reduction of the preglabellar field, retention of the visual surface, position of the eyes set close to the glabella, retention of a conterminant hypostome, yoking of the librigenae, spinosity of the thorax and pygidium, more numerous glabellar furrows and reduction in number of thoracic segments.Olenus attenuatushas several features intermediate between those of earlierOlenusspecies andParabolina, and the origin of the latter genus is best considered in terms of a mosaic paedomorphocline.