Brachiopod evolution

Brachiopods originated in the early Cambrian; all three subphyla are known from this time and exist today. In the Cambrian, the Linguliformea and Craniiformea outnumbered the Rhynchonelliformea, although since this time the latter have dominated. In the early Ordovician the rhynchonelli-forms underwent a massive radiation, possibly in response to the opening up of new habitats associated with continental break-up. They continued to flourish until the end of the Ordovician, when glaciation resulted in a significant decrease in diversity, although there were no major extinctions in the brachiopods. The radiation of rhynchonelliforms continued through the Silurian but the diversity of linguliforms and craniiforms was generally reduced.

A general brachiopod radiation occurred in the Devonian and they reached their maximum diversity in this period. However, by late Devonian times there was an overall decrease in diversity and abundance with significant orders of rhyn-chonelliforms becoming extinct. During the Carboniferous and Permian, rhynchonelliforms rediversified. This was a time when some of the most unusual and spectacular forms of brachiopods existed - brachiopods with a coral-like form and the semi-infaunal brachiopods with spiny ventral valves. However, brachiopods declined slowly through the Upper Palaeozoic and the end-Permian extinction, the largest mass extinction in Earth's history, removed the dominant Palaeozoic brachiopod groups.

The Mesozoic saw a general replacement of brachiopods by bivalves. The well-adapted siphon of the bivalves enabled them to exploit an infaunal mode of life, and to occupy environments inaccessible to brachiopods. Whether competition with the bivalves or the rise in brachiopod predators resulted in their decline, they never recovered their Palaeozoic status and only a few brachiopods made the transition to the modern fauna. Those that survive today tend to live in fairly hostile environments. Lingula lives on intertidal mud flats and most other living brachiopods are found in deep water.

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