hollows left by dickinsonia specimens in seafloor mats. dickinsonia are species in the ediacara biota

age ancient animal biota bone classification cretaceious darwin dickinsonia dinosaur ediacara era evolution exoskeleton exploration extinct fauna fossil fossilized geological geology history hollow holocene imprint jurassic mat natural nature organism paleontology past pliocene prehistoric radiometric remains science seafloor shell skeleton specimen trace trilobite wildlife worm zoo zoology