The Diplodocids (or Diplodocidae) were a group (family) of huge, quadrupedal, long-necked, whip-tailed, plant-eating dinosaurs (sauropods ). Diplodocids had a small head with had peg-like teeth and a squarish snout - the two nostrils opened at the top of the head (with one opening). Some Diplodocids included Apatosaurus , Diplodocus , Seismosaurus ...