Articulata
All living crinoids are Articulata, originating in the Triassic,
they are the only crinoids
that originated after the Paleozoic. Most Articulates are monocyclic and have
lost a circlet of plates. Traditional interpretations assumed that the
infrabasals were the missing plates, but Simms shows that any of the circlets
could have been lost. Based on the changes in orientation between the arms
and the stem Simms demonstrates that the lost circlet is likely the radials,
not the infrabasals. This interpretation suggests that the Articulates,
along with the Cladida, flexibilia, and cyathocrinia all derived from the
Aethocrinus group by loss of a circlet of thecal plates.