Bystrowicrinus westheadi

Bystrowicrinus westheadi is a newly described species (2013) based on remarkably large crinoid column (stem) fragments from the Lower Carboniferous (Mississippian) deposits at Salthill Quarry, Clitheroe, Lancashire, UK. These columns, long known about but previously not officially described or named, are unusual for their incredibly large size and distinctive pentastellate axial canal. The species name honours Stanley Westhead (1910–1986), a noted collector of fossil crinoids from Clitheroe whose contributions to the understanding of local crinoid fauna are still recognised today.

The columns of Bystrowicrinus westheadi are particularly noteworthy for their diameter, often reaching 3–6+cm, making them among the largest crinoid stems ever recorded. Their gross morphology includes a waisted shape in some pluricolumnals, where there is a sudden increase in diameter distally. This is unlike the gradual tapering beneath the crown seen in other crinoid stems and suggests a unique mesistele-dististele transition in Bystrowicrinus westheadi (that is, the transition between the thicker lower stem around the ‘root’ attachments (dististele), and the more flexible and narrow middle part of the stem (mesistele). This abrupt expansion may have served a stabilising function for the crinoid, facilitating attachment to the substrate by allowing room for the growth of robust, unbranched radices (roots). While the dististele appears inflexible, the distal mesistele shows flexibility at symplectial articulations.

The pentastellate shape of the axial canal is another unusual feature, especially given the column’s large size relative to its relatively small lumen. This structure would have allowed for limited soft tissue presence in the axial canal, predominantly serving nervous functions similar to extant crinoids, with nutrient absorption occurring through the ectoderm rather than much nutrient transport through the internal canal. Comparisons to Silurian crinoids showing radiating intracolumnal canals for nutrient transport, highlight that Bystrowicrinus westheadi likely lacked such extensive internal networks. However, the pentastellate canal and its extensions across the articular facets suggest a functional analog for slow nutrient and gas transport, potentially facilitating root development by branching to near each radix attachment site where the radial canals would occur, providing nutrients to the many radix holdfasts.

Despite the incomplete nature of the fossils, the considerable size of these crinoid stems implies they played a role in both anchoring the organism with their weight, and perhaps offering some form of protection. The absence of significant zoobiont infestation, common in other specimens from this site, may indicate a more defensive or protective function for the large columns of Bystrowicrinus westheadi.

Stanley Westhead, after whom this species is named, was an amateur geologist and prolific fossil collector in the Clitheroe area. His collection, now housed in the Natural History Museum, London, contains many rare and important crinoid specimens. While Westhead published little himself, his expertise in the local fossil echinoderm fauna was well respected, with several species described from his collection. His contributions to the field, especially regarding the crinoids of the Clitheroe area, continue to influence paleontological research today.

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.