Origin and phylogeny of velatid asteroids (Echinodermata, Neoasteroidea)—new evidence from the Jurassic

Asteroids of the Jurassic—Recent group of the Velatida evolved during the Late Triassic or Early Jurassic from the family Tremasteridae, previously placed, as a subfamily, in the Asterinidae. Tremasterids and velatids are characterized by the possession of interradial ducts that are lined with chevron ossicles, which are used to brood young. Here, the clade Chevronida is introduced for this group. Fossil tremasterids are represented by the genus Mesotremaster from the Bajocian and Oxfordian of Switzerland. A series of morphologically intermediate forms link tremasterids to the Jurassic—extant family Korethrasteridae, including the Early Jurassic (Hettangian) Protremaster, and Thuyaster fontenoillensis gen. et sp. nov. Korethrasterids, in turn, gave rise to the present day “cushion stars” or “slime stars” (Pterasteridae), highly derived asteroids, which brood young beneath a canopy, are protected by copious mucus secretion, and have a complex respiration system. It is concluded that basal Pterasteridae were present in the Bajocian (Middle Jurassic; Longwyaster delsatei gen. et sp. nov.), and that forms very similar to extant species were present by the lower Oxfordian [Hansaster (gen. nov.) trimbachensis, Propteraster amourensis gen. et sp. nov.]. The suggestion, based upon molecular studies, that pterasterids are basal to the neoasteroids, has been tested using morphology and the fossil record and is found to be incorrect.