Conodonts ranged in length from a centimeter to the 40 cm "Promissum". Their large eyes had a lateral position, which makes a predatory role unlikely. The preserved musculature hints that some conodonts ("Promissum" at least) were efficient cruisers but incapable of bursts of speed. In 2012 researchers classified the conodonts in the phylum Chordata on the basis of their fins with fin rays, chevron-shaped muscles and notochord. Some researchers see them as vertebrates similar in appearance to modern hagfish and lampreys, though phylogenetic analysis suggests that they are more derived than either of these groups. Ostracoderms "(shell-skinned)" are armoured jawless fishes of the Paleozoic. The term does not often appear in classifications today because it is paraphyletic or polyphyletic, and has no phylogenetic meaning. However, the term is still used informally to group together the armoured jawless fishes. The ostracoderm armour consisted of 3–5 mm polygonal plates that shielded the head and gills, and then overlapped further down the body like scales. The eyes were particularly shielded. Earlier chordates used their gills for both respiration and feeding, whereas ostracoderms used their gills for respiration only. They had up to eight separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. Unlike invertebrates that use ciliated motion to move food, ostracoderms used their muscular pharynx to create a suction that pulled small and slow moving prey into their mouths. The first fossil fishes that were discovered were ostracoderms. The Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s. He had a hard time classifying them as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeons but later realizing that they had no movable jaws, classified them in 1844 into a new group "ostracoderms". Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids. Later, about 420 million years ago, the jawed fish evolved from one of the ostracoderms. After the appearance of jawed fish, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period.