Notes on silicified screwstones (Lower Carboniferous, Mississippian) from River Maas gravels in the Netherlands

抄録

Crinoid pluricolumnals and natural moulds are amongst the least loved of fossils, yet may provide important information on crinoid form. Herein, we document two crinoid columnal taxa in open no-menclature based on moulds, namely platycrinitid gen. et sp. indet. and Pentagonocyclicus? (col.) sp. Platycrinitids can rarely be classified to below family level from columnals alone; they presumably evolved neotenously from juveniles with synarthrial articulations. Pentagonocyclicus? (col.) sp. is rec-ognised by ‘lumping’ together pluricolumnals of unimportantly differing morphologies, the sum of which might be found in the xenomorphic column of a single species. The pluricolumnal is robust, homoeomorphic to heteromorphic, N2221222; column circular or rounded pentagonal in section; columnals low; articulation radial symplectial; no areola; lumen rounded pentagonal; broad spatium and narrow axial canal.