Fig. 1

Morphology of extant pterasterids. a–d Partially dissected individual of Pteraster corynetes Fisher, 1911. a and c Abactinal view, to show reticular pattern of abactinal ossicles when the canopy is removed. Compare with drawing in Fig. 4i. b, d Actinal view, to show interradial groove floored by chevron plates, compare Fig. 4f. e, f Pteraster pulvillus Sladen, 1889. Actinal view, to show structure of interradial groove beneath canopy, floored with chevron plates. Note how the abactinal spines support the canopy e. g, h, Cartoon to show complex adambulacral morphology in Remaster g and Pteraster h, modified after Gale (2011a, text-fig. 12). Actinolateral spine muscles shown in green; interadambulacral muscles in blue. Key: adadm, adamb–adamb muscle; adexa, adamb extension articulation; adexm, adamb extension muscle; ads, adamb spine; adsm, adambulacral spine membrane; alm, actinolateral membrane; als, actinolateral spine; ap, aperture; aps, apertural spine. Scale bars in mm