Agostocrinus and Acolocrinus, Two New Ordovician Crinoids with Peculiar Ray and Respiratory Structures

TL;DR: The 2-1-2 pentaradial symmetry evolved into the true pentarADiality as a result of shifting the timing of tentacle branching to earlier stages of ontogenesis and even spreading of the five tentacle primordia over the ambulacral ring.

TL;DR: The enigmatic blastozoan echinoderm Ampheristocystis concentricus, based on four specimens from the upper part of the Osgood Member, Salamonie Dolomite, southeastern Indiana, is most likely an eocrinoid, but it is presently unclear whether the life habits of this genus were more like that of the common Osgood diploporites or stemmed contemporaries such as Caryocrinites and Eucalyptocrinites.

TL;DR: A review of progressive knowledge gained on the three subphyla Homalozoa, Blastozoa and Crinozoa during the last decade is given in this article.