Fossil Record

(5) Post-Palaeozoic radiation of this group has been attributed to the effects of increased predation pressures.

(4) During the remainder of the Palaeozoic things calmed down from the boom of the Ordovician and bivalve diversity stabilized. During this time certain species developed extensive siphons, thus opening up a whole new range of modes of life. The development of these siphons along with a muscle foot gave bivalves the first opportunity to bury themselves deep within the sediment. This gave a distinct advantage over the previously dominant brachiopods.
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(3) During the Late Ordovician/Early Silurian gills began to perform a filter-feeding function as well as a respiratory one.

(2) There was a rapid radiation in the Early Ordovician resulting in the development of all the major sub-classes by the Middle Ordovician. Dysodont, heterodont and taxodont dentitions were established during this time.

(1) The earliest known bivalves have been reported from the Basal Cambrian.


The end-Permian extinction event saw almost 95% of all species becoming extinct. Prior to this mass extinction, bivalves had slowly risen in diversity, but had failed to dcome to dominance over the brachiopods. Both bivalves and brachiopods were hard hit by the mass extinction, but the bivalves recovered their diversity levels during the Triassic, and the brachiopods did not. Why?

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