Crinoidea (unidentified)
Echinodermata is a phylum of marine invertebrates that includes well-known groups like starfish, sea urchins, brittle stars, sea cucumbers, and crinoids. Echinoderms are characterised by their radial symmetry, typically arranged in fives, and their unique water vascular system, which aids in locomotion and feeding. This phylum is exclusively marine, and its members are often found on the sea floor, from shallow waters to the deep ocean. Echinoderms exhibit pentameral symmetry as adults, though their larvae are bilaterally symmetrical, reflecting their evolutionary relationship with other deuterostomes, including chordates.
Within this phylum, Class Crinoidea includes marine animals commonly referred to as sea lilies and feather stars. Crinoids are distinguished by their cup-shaped body (the calyx), a set of radiating arms, and a long stalk (in some species) that anchors them to the seabed. The arms are typically branched and covered with feathery extensions that aid in filter feeding, capturing small particles from the water. Though modern crinoids tend to be less prominent in marine ecosystems, they were once much more abundant and diverse, particularly during the Palaeozoic era.
Crinoids first appeared in the Ordovician period, about 480 million years ago, and quickly diversified. They were especially abundant during the Palaeozoic, with their greatest diversity occurring during the Carboniferous period, when extensive shallow seas created ideal conditions for large crinoid populations. Fossil crinoids are especially common in limestone deposits from this time, with entire beds of rock often composed almost entirely of disarticulated crinoid fragments, particularly their stems. These fossils are widespread in regions like the UK, where crinoid-rich limestone formations are frequently found.
Crinoids come in two main forms: stalked crinoids, or sea lilies, which attach to the sea floor via a flexible stalk, and unstalked crinoids, or feather stars, which are mobile and can swim or crawl along the substrate using their arms. In the fossil record, stalked crinoids were much more abundant, with long, segmented stalks that could grow several meters in length. The stalks are composed of individual ossicles, small calcareous plates that are commonly found as fossils, especially in Carboniferous limestone beds. The most well-known fossil remains of crinoids are these stem ossicles, which are often referred to as "Indian beads" due to their cylindrical shape.
Crinoids reached their peak during the Palaeozoic, forming extensive colonies in shallow seas, often in association with coral reefs. Their filter-feeding mechanism allowed them to occupy a specialised ecological niche, and they played an important role in marine ecosystems as suspension feeders. However, crinoids were significantly affected by the Permian-Triassic mass extinction about 252 million years ago, which wiped out many marine species. Although crinoids survived this event, their diversity and abundance were greatly reduced.
In the Mesozoic era, crinoids experienced a resurgence, though not to the same levels of diversity as in the Palaeozoic. Feather stars (unstalked crinoids) became more prominent during the Jurassic and Cretaceous periods, adapting to more mobile lifestyles compared to their sessile ancestors. Today, feather stars are found in a variety of marine environments, from shallow reefs to deep-sea habitats, while stalked crinoids are largely restricted to deep water.
Crinoids are unique among echinoderms in that they are suspension feeders, using their feathery arms to catch plankton and other small particles from the water. Their arms are lined with cilia that move captured food towards their central mouth, which is located on the upper surface of the calyx. This feeding strategy differs from other echinoderms, such as sea urchins, which graze on algae, or starfish, which are typically predatory.
Despite their decline in modern oceans, crinoids remain important in the fossil record due to their abundant and well-preserved remains, particularly in Palaeozoic and Mesozoic sedimentary rocks. The characteristic segmented stalks and calyx plates of crinoids make them highly recognisable fossils, and they provide key insights into the structure and biodiversity of ancient marine ecosystems. In particular, Carboniferous limestone deposits, such as those found in the UK, are often rich in crinoid remains, offering palaeontologists a detailed record of these once-dominant marine invertebrates.
