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Wiktionary
diplodocid

n. (context zoology English) Any member of the Diplodocidae.

Wikipedia
Diplodocid

Diplodocids, or members of the family Diplodocidae ("double beams"), are a group of sauropod dinosaurs. The family includes some of the longest creatures ever to walk the Earth, including Diplodocus, Supersaurus, some of which may have reached lengths of up to .