Bivalvia - Evolutionary History

Evolutionary History

The Cambrian explosion took place 543 to 525 million years ago. In this geologically brief period all the major animal phyla appeared and these included the first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time and left their fossilized remains behind in the rocks. The fossils of both were formed when the sediment in which they were buried hardened into rock. Often it is the impression made by the valves that remains as a fossil rather than the valves themselves. During the Early Ordovician there was a great increase in the diversity of bivalve species and the dysodont, heterodont and taxodont dentitions evolved. By the early Silurian, the gills were becoming adapted for filter feeding and during the Devonian and Carboniferous periods siphons first appeared which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment.

By the middle of the Paleozoic, around 400 million years ago, the brachiopods were among the most abundant filter feeders in the ocean and over 12,000 fossil species are recognized. By the Permian-Triassic extinction event 250 million years ago, bivalves were undergoing a huge radiation of diversity. The bivalves were hard hit by this event but re-established themselves and thrived during the Triassic period that followed. In contrast, the brachiopods lost 95% of their species diversity. Scientists have speculated that the ability of some bivalves to burrow and thus avoid predators was a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative buoyancy in soft sediments by developing spines on the shell, gaining the ability to swim, and in a few cases, adopting predatory habits.

It was for a long time thought that bivalves were better adapted to aquatic life than brachiopods were, out-competing and relegating them to minor niches in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. An alternative view is that the prominence of modern bivalves over brachiopods might merely be due to chance disparities in their response to extinction events.