Brachiopods are known from throughout the Phanerozoic Eon, and are abundant fossils in both Palaeozoic and Mesozoic shallow-water marine deposits, where they are often very useful in determining stratigraphy and palaeoenvironmental studies. The two inarticulate subphyla, the Linguliformea and the Craniiformea, have been around from the very beginning, persisting from the earliest Cambrian to the present day. During the Ordovician period, some 450 million years ago, the articulate brachiopods began to flourish, and since then, the inarticulate brachiopods have been far less diverse and abundant, except in a few localities. During the Silurian Period, the inarticulate brachiopods went through further extinctions, though the articulate forms continued to flourish.
Some fossilised late Cambrian inarticulate brachiopods from Death Valley, U.S.A.
Their rounded valves can be easily seen at the top of the picture
A few hypotheses have been proposed as to why the brachiopods in
general have declined since the Palaeozoic, when they dominated sea-bed
environments. One is that they were out-competed by bivalves,
with whom they share similar habitats on the benthos. It may have
been that following the end-Permian extinction, brachiopods were much
slower to recover than the bivalves, and so the bivalves took advantage
by radiating into and dominating habitats previously occupied by
brachiopods. Another theory concerns the rise of the predatory starfish,
which became more efficient predators during the Mesozoic.
Brachiopods, the majority of which were unable to burrow down
into the substrate, may have become easy prey for starfish at this
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