Ostracoderm

An innovation of ostracoderms was the use of gills not for feeding, but exclusively for respiration. Earlier chordates with gill precursors used them for both respiration and feeding. Ostracoderms had 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.

Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s,. He had difficulty classifying them, as they did not resemble any living creature. He compared them at first with extant armored fish such as catfish and sturgeon, but later realized that they lacked movable jaws. He classified them in the newly named genus Cephalaspis. In a later 1844 study he recognised that their anatomy was fundamentally very different from sturgeons and catfish and that they were not closely related. In 1889, Edward Drinker Cope erected the grouping Ostracodermi to house them meaning 'shell-skinned' (from Greek ὄστρακον óstrakon + δέρμα dérma).

Ostracoderm headshields are partially composed of enameloid.

It was long assumed that pteraspidomorphs and thelodonts were the only ostracoderms with paired nostrils, while the other groups have just a single median nostril. It has since been revealed that even if galeaspidans have just one external opening, it has two internal nasal organs.

Following their first appearance in the Ordovician-Silurian, ostracoderms reached their apex of diversity during the Early Devonian, subsequently declining afterwards during the Middle and Late Devonian, with the last species becoming extinct in the end-Devonian mass extinction. The decline of the ostracoderms corresponds with the emergence and diversification of jawed vertebrates, though the idea that the decline of ostracoderms was drived by direct competition with jawed fish has been questioned.

The Subclass Ostracodermi has been placed in the division Agnatha along with the extant Subclass Cyclostomata, which includes lampreys and hagfishes.

Major groups