Taxonomy

Morphology

Pentacrinites fossilis like most echinoderms, is composed of numerous calcite plates which are arranged into different organs.

Pentacrinites fossilis is divided into 3 sections:

  • arms
  • calyx (body or theca)
  • stem

The stem raises the calyx and arms away from the drift wood and is composed of a stack of numerous 5-sided star-shaped plates. The stem also bears flexible, fingerlike appendages called cirri that are used to attach the individual to the driftwood and other crinoids.

The calyx is formed of two bands of five plates that form a small cup-shaped body. From these the bases of the arms arise. The top of the calyx is covered by numerous small polygonal plates and the mouth and anus are found on this surface. Five arms originate in the calyx but these frequently divide, similar to tree branches, so that there may be over fifty arms in total.

The arms are formed of stacks of U-shaped calcite plates and bear numerous projections called pinnae (like the pinnae/”leaves” of a fern frond). It is on these pinnae that tube feet, covered in mucus, extend into the water and catch plankton. The arms are only weakly moveable. The arms are ‘U’ shaped and have a grove running down their centre. These groves run the length of the arm and onto the calyx and convey food to the mouth.

Diagnostic description

  • Stalked crinoids with long stems bearing cirri on specialized columnals called nodals, although when missing an oval scar remains
  • Columnals are pentaradiate with marginal crenulae, are pentalobate to pentagonal in cross section with sharp interradii. Persistent alternation and indefinite intercalation of columnals lacking tubuli. The columnal radial latera display a keel and/or conical projects
  • The areolae between the symplectial crenulae are narrow, elliptical or parallel-sided. Crenulae are numerous and small
  • Cirri ossicles display strongly compressed, angular, rhomboidal sections
  • The theca (cup or body) is small, possess a weakly armoured tegmen and bears long, slender branching arms
  • Radial plates in broad contact and are frequently with spinose tubercles around margins
  • Arms branch endotomously beyond IIIBr. Aboral ligament fossa of proximal brachial distinct and semicircular
  • Syzygies absent
  • Proximal pinnules not incorporated in tegmen
  • Seven secundibranchs, syzygies absent, spinose pinnules and columnals commonly displaying tuberculate latera

Evolution

Pentacrinites fossilis is thought to have evolved from the isocrinids a group of free living crinoids which lived on the sea floor.

Their evolution into a pseudo-planktonic lifestyle enabled them to take advantage of feeding areas unavailable to other crinoids. However, they appear to have become extinct at the end of the middle Jurassic and have left no direct living descendents. Other isocrinids however still populate the world’s oceans usually at a water depth of exceeding 150 metres.