For the second time at Catalogue of Organisms, I'm presenting a fossil crinoid family as taxon of the week. However, while the last crinoid family I covered belonged to the subclass Camerata, today's family, the Sostronocrinidae, belongs to the Cladida (figure above from Waters et al., 2003).
The Cladida first put in an appearance in the late Ordovician, and were the most successful of the three major crinoid divisions* of Cladida, Camerata and Disparida. The two further groups of crinoids commonly recognised as subclasses, the Flexibilia and Articulata, are now both known phylogenetically to be within the Cladida (Ausich, 1998). Through the Articulata, the Cladida are also the only one of the three major clades to have survived the Mesozoic. Cladids have three circles of plates involved in the cup (a condition known as "dicyclic"), as opposed to the monocyclic disparids with two circles of plates**.
*At least one small early family of crinoids, the Aethocrinidae, does not fall into any one of these three groups.
**Yes, I know the terms don't quite add up. The outermost (or uppermost, depending on how you're looking at them) radial plates are present in both groups and aren't counted. Dicyclic crinoids have both basal and infrabasal plates, monocyclic crinoids have basal plates only.
The family Sostronocrinidae was recognised only recently, being established by Lane et al. in 2001 (despite its being described as a new family in Waters et al., 2003, the latter paper's authors include the authors of the 2001 paper, and the description is almost word for word identical). Previously, sostronocrinids had been included in the family Scytalocrinidae, but they differ from that family in having twenty arms rather than ten. They have relatively large infrabasal plates that are clearly visible in side view (in many other taxa the infrabasals are smaller and generally hidden by the basals). The arms were pinnulate, and branched once. Members of the family ranged from the late Devonian to the early Permian.
Waters et al. (2003) included four genera in the Sostronocrinidae - Sostronocrinus, Amadeusicrinus, Haeretocrinus* and Tundracrinus. One notable trend over time in the family was the reduction in the number of primibrachials (the plates in the initial section of arm before it branches) and hence a reduction in the length between the base of the arm and the division into branches. The earliest Devonian genera, Sostronocrinus and Amadeusicrinus**, branched on the third or fourth primibrachial. Sostronocrinus survived into the Carboniferous, but species in that time period branched on the second or third primibrachial. By the time Haeretocrinus and Tundracrinus appeared in the Permian, there was only a single primibrachial between the base of the arm and the branches.
*Misspelled as Haertocrinus in both Lane et al. (2001) and Waters et al. (2003).
**Waters et al. (2003) established Amadeusicrinus as a new genus, moving its type species from its original position in the unrelated genus Pachylocrinus. However, it is not clear what, if any, features are supposed to distinguish it from Sostronocrinus.
The implied suggestion in all this is that there is a continuous series of Devonian Sostronocrinus ancestral to Carboniferous Sostronocrinus, itself ancestral to the Permian genera. Unfortunately, workers on fossil crinoids seem to largely eschew cladistic analyses, or even explicit phylogenetic proposals. If I may briefly channel the style of Toby White, maybe there is some hidden crinoid temple somewhere where, like the Eleusinian Mysteries of old, initiates have revealed to them the secrets of knowing the cyathocrinid from the pachylocrinid, or which crinoid group gave rise to what. To the uninitiated, unfortunately, it all looks decidedly unclear.
Ausich, W. I. 1998. Early phylogeny and subclass division of the Crinoidea (phylum Echinodermata). Journal of Paleontology 72(3): 499-510.