 Early members of the ceratopsian group , such as Psittacosaurus , were small bipedal animals . Later members , including ceratopsids like Centrosaurus and Triceratops , became very large <unk> and developed elaborate facial horns and frills extending over the neck . While these frills might have served to protect the vulnerable neck from predators , they may also have been used for display , <unk> , the attachment of large neck and chewing muscles or some combination of the above . <unk> ranged in size from 1 meter ( 3 ft ) and 23 kilograms ( 50 lb ) to over 9 meters ( 30 ft ) and 5 @,@ 400 kg ( 12 @,@ 000 lb ) . 
