Cyclocrinitids

Abstract

The cyclocrinitids are a small group of middle Ordovician to early Silurian macrofossils. They are usually regarded as an extinct tribe of dasycladacean algae, since they are morphologically very similar to the living dasycladacean Bornetella. Cyclocrinitids had a central main axis bearing numerous lateral branches, which radiated outwards in all directions. The branches expanded at their tips to form thick lateral heads, which were generally hexagonal in outline and usually 1–3 mm wide. The lateral heads were tightly packed together to form a faceted outer cortex, which was spherical to claviform in shape and typically 1–4 cm in size. The thallus was encrusted by calcium carbonate, particularly around the lateral heads. Cyclocrinitid preservation is quite variable, and this has often confused taxonomists; new descriptions of the European and Asian forms are needed. Cyclocrinitids are important associates of many benthic invertebrate palaeocommunities. They are often abundant locally and may be significant rock-builders; several rock units have been informally named after them. Cyclocrinitids were restricted to shallow, quiet waters at low latitudes. Eco-phenotypic thallus-size variation within Silurian species has been used to indicate relative palaeodepths. Ordovician cyclocrinitids have much potential value as palaeoenvironmental indicators.