Articulata (unidentified)
Brachiopoda, or lamp shells, is a phylum of marine invertebrates that superficially resemble bivalve molluscs but belong to a distinct lineage. These animals are characterised by two shells (valves), which enclose the soft body, but unlike bivalves, the shells of brachiopods are dorsal and ventral rather than lateral. Brachiopods use a specialised feeding structure called a lophophore, a crown of ciliated tentacles that filter food particles from the water. This lophophore links brachiopods to other lophophorate phyla, including Bryozoa and Phoronida, which also possess this feeding apparatus. However, brachiopods are unique in their morphology and the articulation of their shells.
Brachiopods are divided into two main classes: Articulata and Inarticulata. The Articulata class comprises brachiopods that have hinged valves, which are held together by a complex arrangement of teeth and sockets, allowing them to open and close efficiently. In contrast, Inarticulata lack this precise hinge mechanism, relying instead on muscles to hold their shells together. Articulate brachiopods have dominated the fossil record since the Ordovician period, around 485 million years ago, and are characterised by their hard, calcified shells, which preserve well in the fossil record.
During the Palaeozoic era, brachiopods, especially articulate forms, were among the most abundant and diverse marine organisms. They were a dominant component of marine benthic communities, particularly in shallow, warm seas, where they contributed to reef structures and seabed ecosystems. However, despite their former dominance, brachiopods were significantly outcompeted by bivalves following the Permian-Triassic mass extinction approximately 252 million years ago. The more adaptable bivalves, with their lateral shells, versatile feeding strategies, and ability to thrive in a wider range of habitats, gradually replaced brachiopods as the dominant shelled organisms in marine environments. Today, while bivalves such as clams and mussels remain abundant and widespread, brachiopods have become relatively rare, with only a few hundred extant species compared to tens of thousands of bivalve species.
Despite their decline in modern ecosystems, brachiopods remain highly significant in palaeontology due to their rich fossil record. The articulated forms, with their sturdy, calcified shells, are particularly well-preserved and are often found in abundance in Palaeozoic rock formations. Fossil brachiopods are key index fossils, used by geologists to date rock strata. In the Silurian and Devonian periods, brachiopods formed large assemblages, rivaling corals and other reef-building organisms. Notable groups within the class Articulata, such as the Rhynchonellida and Terebratulida, are especially well-represented in both the fossil record and among living species. These articulate brachiopods display a wide range of shell morphologies, from smooth, globular forms to highly ribbed and ornamented types, reflecting their adaptation to various environmental conditions.
The lophophore of brachiopods is another important feature that links them evolutionarily to other lophophorate animals, such as bryozoans and phoronids, which also use this structure for suspension feeding. The U-shaped gut and method of feeding by filtering plankton from water are similar across these phyla. However, in contrast to brachiopods, bryozoans are colonial animals, and phoronids are worm-like and unsegmented, highlighting the morphological diversity within the lophophorates.
Though brachiopods and bivalves appear superficially similar, their internal structures, feeding mechanisms, and evolutionary histories are vastly different. Bivalves, belonging to the phylum Mollusca, use gills not only for respiration but also for filtering food, whereas brachiopods rely entirely on their lophophores for feeding. The hinge mechanism in articulate brachiopods is highly specialised, with teeth and sockets that allow the valves to open and close smoothly. This articulation is a key feature that helps differentiate Articulata from Inarticulata, and from bivalves, whose shells hinge in a different orientation.
In Britain and many other regions, fossil brachiopods are a common find in Palaeozoic rocks, especially in Carboniferous limestone and Silurian deposits. Their abundance in these ancient marine sediments has made them valuable for studying palaeoecology and evolutionary patterns in marine life. Though their numbers dwindled in the Mesozoic era and beyond, the articulate brachiopods that survived continue to occupy niche habitats in modern oceans, particularly in cold and deep waters, where competition from bivalves is less intense.
