The following evidence strongly suggests that Proisocrinus ruberrimus belongs within the order Isocrinida:
- The existence of cirri in the proximal stalk.
- The pentalobate outline of the columnals with 5 lanceolate ligamentry fields surrounded by crenulae.
Diagnostic description of Proisocrinus ruberrimus
(Adapted from Oji and Kitazawa, 2008)
- Arm division occurs 2-3 times, with 15-20 arms in total.
- There are short cirri on the nodals in the proximal part of the stalk as in all isocrinid crinoids.
- The proximal stalk is sub-rounded to pentagonal in outline and consists of short columnals.
- Nodals are slightly thicker than internodals. There are gradual insertions of internodals toward the distal stalk and the number of internodals per nodi taxis increases from 1 to more than 30.
- The cirri become gradually shortened and no cirri are observed in the intermediate to distal part of the stalk.
- In the intermediate to distal stalk the distinction of nodals from internodals becomes unclear leaving no trace of the cirrus scar with an almost uniform thickness of columnals.
- There is a gradual increase in stalk diameter distally, from 5-7mm to 9-10mm, and the outline of columnals also changes distally from sub-rounded pentagonal to perfectly circular. The articulation between the columnals changes from pentastellate to circular pattern with peripheral crenulation.
- The basals are high and contiguous.
- The radials are directed outward forming a wide conical cup.
- The primbrachials are united by ligmentry articulations with a flat outer surface presumably by synostosis. There are also similar ligamentry articulations in the secundibrachials and tertibrachials.
The columnal from the intermediate stalk looks very much like that of Millericrinida. However the surface articulations are very different, and so any close resemblance to some Jurassic millericrinids is likely superficial and a result of convergent evolution.