Article: A new asteroid genus from the Jurassic of England and its functional significance
Daniel B. Blake
The new asteroid (Echinodermata) genus Brachisolaster is based on Solaster moretonis Forbes (Solasteridae), described from Jurassic rocks of Gloucestershire. Brachisolaster (Order Velatida) demonstres that characteristic solasteroid features were defined by the Bathonian (Middle Jurassic); other fossils belonging to different orders further demonstrate the presence of close relatives of the living fauna by this time. Arms are more numerous in Brachisolaster moretonis than in living solasteroids; the appearance is suggestive of that of living Heliaster (Asteriidae). Heliaster feeds largely on molluscs and barnacles, whereas the diet of living solasteroids stresses more active echinoderms. Solasteroids use their fewer but larger arms to subdue and manipulate prey. Brachisolaster is suggested to have had feeding habits more like those of Heliaster than like those of extant solasterids. The interpretation complements an earlier suggestion that Jurassic aster behaviour might have involved more active predation. A solasteroid with fewer arms is known from the Jurassic, therefore to the extent that the suggested functional significance of arm number is accurate, disappearance of species with supernumerary arms reflects a narrowing of the active solasteroid adaptive zone rather than a functional shift. Together, the fossil asteriids and solasterids suggest some narrowing of adaptive zones since the Jurassic.