Mesozoic stem-group zoroasterid sea stars imply a delayed radiation of the crown group and adaptation to the deep seas
The family Zoroasteridae is a clade of extant sea stars found exclusively in deep marine environments (i.e. bathyal environments deeper than 200 m). In the fossil record, seven species have been compared and/or assigned to the Zoroasteridae, on a time span ranging from the Middle Jurassic to the Miocene. This study describes a new taxon, Viridisaster guerangeri gen. et sp. nov., from the Cenomanian of Le Mans (Sarthe, France) and reappraises two Mesozoic species, the Jurassic Terminaster cancriformis and the Early Cretaceous Protothyraster priscus. To test the relationships of the putative fossil zoroasterids with modern taxa, a phylogenetic analysis was performed combining six of the best-known fossil species and 29 forcipulatacean species, including four Zoroasteridae. Many of the characters that define the extant Zoroasteridae do not appear in the fossil record before the Cenozoic. The family name is maintained for the crown group only. The family Terminasteridae, that accounted for the genera Terminaster and Alkaidia, is found paraphyletic and is rejected here. The order Zorocallida, initially erected for the family Zoroasteridae only, is redefined as the sister clade to the Forcipulatida that includes the crown-group Zoroasteridae and its stem relatives. Mesozoic Zorocallida are small in size compared to the Cenozoic Zorocallida, and they express morphological characters that are typical of juvenile Zoroasteridae; thus, the emergence of modern forms is likely associated with peramorphic evolution. The palaeoecology of the Zoroasteridae and their restriction to deep-sea environments after the Eocene is discussed.