Starfish - Taxonomy and Evolutionary History

Taxonomy and Evolutionary History

Asteroidea is a large and speciose class within the phylum Echinodermata. Like other classes in that group, members are characterised by having radial symmetry as adults, usually five-fold symmetry. In contrast, during their early developmental stages, the larvae have bilateral symmetry. Other characteristics of adults are the possession of a water vascular system and having calcareous skeletons consisting of flat plates connected by a mesh of mutable collagen fibres. Asteroids are characterised by a central disc with a number of radiating arms, typically five. The ossicles that form the hard element of the skeletal structure extend from the disc onto the arms in a continuous arrangement which gives the arms a broad base. This is in contrast to the ophiuroids in which the disc is clearly separated from the long, slender arms.

Asteroids are poorly represented in the fossil record. This may be because the hard skeletal parts separate as the animal decays or because the soft tissues collapse into distorted, unrecognisable remains. Another reason may be that most asteroids live on hard substrates where conditions are not favourable for fossilisation. The first known asteroids date back to the Ordovician. There were two major extinction events during the late Devonian and late Permian but, although many species died out at these times, others survived. These diversified rapidly within a time frame of sixty million years during the Early Jurassic and the early part of the Middle Jurassic.