Stephanocrinidae

Stephanocrinus angulatus, from here.

Belongs within: Crinoidea.

The Stephanocrinidae are a group of crinoids known from the Middle Ordovician to the Upper Silurian, characterised by unique sausage-shaped cover plates and biserial arms (Moore et al. 1978).

Characters (from Moore et al. 1978): Crinozoans with three circlets of regularly arranged thecal plates, without pores and infolds of any type, with five biserially arranged arms. Basal circlet composed of three plates, the azygous basal located in anterior right (AB) interradial position; with five radials and five large interradial plates on ventral part of theca; radials and interradial plates prolonged ventrally into high coronal processes. Small primaxil plate attached rigidly to adorally constricted end of each radial plate, adoral part of which rests internally on adjacent interradial plates. Each primaxil surmounted by two plates, and these by two more, and so on for at least nine sets of plates. Bottom pair of plates each appearing to give rise to a biserial arm, second pair so arranged that plate on left supports a bottom plate of second armlet, and plate on right supports top plate of second arm on that side, bottom plate receiving support from first plate below. Thus, from this point upward, armlets are arranged alternately and are not paired. Each armlet coiled planispirally adorally, with three or four revolutions to each armlet and single food groove on adoral surface. Food grooves on left side lead to a food groove on left side of main plates of arm on that side, and food grooves on right side lead to a food groove on right side of main plates. Thus, primaxils have two food grooves on their adoral surface. Arms appear to have flexible sutures above primaxils. Food grooves extend adorally along margins of interradial plates, meeting along suture between interradial plates about one-third of distance from mouth, forming single food groove in each ambulacrum that leads to mouth. Two large, sausage-shaped cover plates above food grooves on each ambulacrum abutting against primaxil and slightly overlapping adoral portion of primaxil. Ambulacra floored by adjacent interradial plates which meet along midline of each ambulacrum. Oral opening covered by five large oral plates which are interradial in position, each abutting against interradial and adjacent cover plates. Single large anal opening on aboral side of anal interradial plate at its junction with adjacent radial limbs, with four subtriangular cover plates.

<==Stephanocrinidae [Coronata, Stephanoblastidae]
|–Mespilocystites Barrande 1887 [=Mespilocystis Bather 1889] ML78
|    `–*M. bohemicus Barrande 1887 [=*Mespilocystis bohemicus] ML78
|–Stephanoblastus Jaekel 1918 ML78
|    `–*S. mirus (Barrande 1887) [=Rhombifera mira] ML78
|–Paracystis Sjöberg 1915 ML78
|    `–*P. ostrogothica Sjöberg 1915 ML78
|–Tormoblastus Jaekel 1927 ML78
|    `–*T. bodae Jaekel 1927 ML78
`–Stephanocrinus Conrad 1842 ML78
|–*S. angulatus Conrad 1842 ML78
|–S. gemmiformis ML78
`–S. osgoodensis ML78

*Type species of generic name indicated

REFERENCES

[ML78] Moore, R. C., N. G. Lane, H. L. Strimple, J. Sprinkle & R. O. Fay. 1978. Inadunata. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 2 pp. T520–T759. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

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