Middle Ordovician- Upper Ordovician
Pycnocrinus is an extinct genus of stationary upper-level epifaunal suspension feeders
Identification in Hand Sample:
- Rays bifurcate only once on calyx
- Ten arms that branch to twenty arms
- 5 or 6 pinnules in each ray
- Numerous secundibrachs
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- Pycnocrinus is distinguished by rays that bifurcate only once on the calyx, producing ten arms that then branch immediately to twenty arms. In Glyptocrinus, the rays bifurcate twice on the calyx, producing twenty arms that exit from the calyx.
- Eckert (1987) restricted Pycnocrinus to glyptocrinids with uniserial arms that only branch at the secundibrach level, whereas Brower and Veinus (1978, p. 416-423) included species with multibranched arms having the proximal branch on secundibrach 5 or 6 or higher plates. I disagree with Eckert because at least some of the multi-branched crinoids are probably related to pycnocrinids.
Treatise on Invertebrate Paleontology, Part T, Vol. 2(2) (1978):
- Secundibrachs numerous; fixed pinnules as many as 5 or 6 in each ray, 1st one borne by fixed secundibrachs 2 (outer) and 2nd one by fixed secundibrachs 4 (inner). Arms 10, branching once or twice, uniserial.