Metopaster vs. Recurvaster
Recurvaster (e.g. R. blackmorei) is believed to have derived from from Metopaster parkinsoni during the Santonian, and isolated ossicles of the two taxa can be virtually indistinguishable. Complete specimens, however, can be readily distinguished, by examination of the arm tips; Those of Metopaster have an enlarged ultimate superomarginal, with several corresponding inferomarginals; Those of Recurvaster by contrast have an equal number of tapering superomarginals and inferomarginals.
1). Contrasting arm tips of Metopaster (M. uncatus) and Recurvaster (R. blackmorei); The superomarginals of Metopaster terminate in a single enlarged ossicle, but with several corresponding inferomarginals. This gives a short and blunt arm-tip. Recurvaster has an equal number of superomarginals and inferomarginals, and the arm-tip is elongate and tapering. Image (A) © 2005, The Booth Museum, by kind permission. Image (B) © 2005, The Natural History Museum, by kind permission.