Aethocrinus
There is some debate over the oldest known crinoid; some
feel that the Middle Cambrian Echmatocrinus from the Burgess
Shale is the oldest crinoid, while others dispute the Echmatocrinus
affinity to the crinoids and consider Aethocrinus from the lower
Ordivician as the oldest crinoid. Simms (1993) treats Aethocrinus
as the oldest crinoid group and suggests that they are the sister group
to all other crinoids. Simms bases this on evidence suggesting that
the thecal plate organization of four circlets of plates is primitive,
that Aethocrinus appears very early in the fossil record, and
that Aethocrinus has other primitive characteristics.