 Other animals that would later be classified as <unk> were placed in a group called <unk> , characterized by plate @-@ like skull bones , small limbs , fish @-@ like scales , and branchial arches . Unlike <unk> , they did not have parietal <unk> , small holes in their skulls behind their eye sockets . <unk> , <unk> , <unk> and <unk> were placed in this group and were considered to be the most primitive members of <unk> . Their <unk> vertebrae , notochord , and lack of occipital condyles ( which attached the head to the neck ) were features that were also shared with fishes . Thus , they were considered a link between early fishes and more advanced forms such as <unk> . 
