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| Lingula anatina — a primitive brachiopod |
Brachiopods are marine invertebrates with a stalk and two shells connected along a hinge. They are often confused with bivalves such as clams.
Bivalves have shells on the sides of their bodies. Brachiopods have shells on the top and bottom. As a result, the plane of symmetry in a bivalve runs along the hinge while it runs perpendicular to the hinge in brachiopods.
Lingula forms are regarded as the most primitive brachiopods and represent the first certain appearance of brachiopods in the fossil records dating back 530 million years.
Their shells do not have any locking mechanisms. Instead, they rely on complex musculature to move their shells. They are the first known examples of animal biomineralisation — a process whereby living organisms stiffen or harden tissues with minerals. Their shells are composed of calcium phosphate and collagen fibres, characters shared only by evolutionarily distant vertebrates.
Lingulid brachiopods had changed so little in appearance since the Silurian, 443-419 million years ago, they are referred to as living fossils — a term bestowed upon them by Charles Darwin himself.
Photo: Wilson44691 - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8624418
