New research from the University of St Andrews has shed light on the evolution of an important yet thus far poorly understood group of marine invertebrate animals - brachiopods.
The work, published in Science Advances, has for the first time, carried out a comprehensive phylogenomic analysis of all five extant brachiopod orders and related lineages. Their results revealed that brachiopods form part of a single evolutionary group called Lophophorata, with brachiopods most closely related to a sister group comprising bryozoans, also known as "moss animals", and phoronids or commonly called "horseshoe worms" that live in tubes and filter food from the water. This means that brachiopods, bryozoans and phoronids share a common worm-like ancestor in which the distinctive lophophore organ evolved, and which gave a rise to a remarkable diversity in body forms and ecological adaptations.
Brachiopods are among the oldest animals on Earth. Their appearance in the fossil record dates back nearly to the origin of the fossil record itself, with first records coming from the earliest Cambrian, approximately 540 to 520 million years ago. They have survived through all known mass extinctions and periods of major environmental and climatic changes and can still be found inhabiting various parts of the worlds' oceans today, having changed little over the course of the last half a billion years. Despite their remarkable evolutionary history, brachiopods remain poorly understood, particularly in terms of their evolutionary origins and sister-group relationships within the animal kingdom.
Brachiopods may look like inconspicuous clams, but they are anything but ordinary. Like bivalve mollusc clams, their bodies are enclosed by two mineralised valves, commonly made of calcium carbonate. The key difference is that the two shell halves of bivalves are mirror images of each other, whereas the two shell halves of brachiopods differ in shape. Inside their shells, brachiopods have a distinctive feeding and respiratory organ called the lophophore, a ciliated crown of tentacles, which bears little resemblance to the soft body of a mollusc. These distinctive features make brachiopods easy to distinguish, but have also made their evolutionary relationships with other animals difficult to establish.
Co Author Dr Hana Jurikova from the School of Earth and Environmental Science at the University of St Andrews, said: "This research not only helps to illuminate the enigma of what brachiopods actually are and place them into the evolutionary tree of life, but also sheds light on the evolution of the shell in brachiopods. A really striking finding is that brachiopod shell mineralogy changed at least once during their evolutionary history. This means that brachiopods seem to have originally evolved a calcitic shell, and later also evolved an organophosphatic shell. This major change in mineralogy from calcium carbonate to organophosphatic shell material composed primarily of calcium phosphate occurring within one animal group is thus far unheard of and points to the striking flexibility and adaptability of these fascinating organisms and "living fossils" .
Category Research
