Pelecypods that live in frigid waters tend to be smaller than those in warmer waters, although some small species live in warm water. Species that range from warm to cool waters live longer, grow slower, and attain larger size in the cooler water. The largest pelecypods are polysyringians; protobranchs and septibranchs rarely attain length of 100 mm. Recent protobranchs are just as small as Ordovician ones. Few Ordovician pelecypods were as large as 100 mm, more Silurian but fewer Carboniferous ones attained that size, and the largest Paleozoic pelecypods are Permian. Increase in size began in the middle Jurassic, with some giants in the Late Creatceous. Average pelecypod size tended to increase through the Cenozoic.

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