Shell-skin fishes made gills for breathing alone
Ostracoderms—“shell-skins”—informally gather Paleozoic armored jawless fishes. Their quiet revolution: gills devoted only to respiration, not dual-purpose filter feeding as in earlier chordates. Louis Agassiz stared at Scottish armored fossils in the 1830s that matched nothing alive.
Agassiz first compared his Scottish specimens with armoured fishes still living, such as catfish and sturgeon, before noticing they had no movable jaws. In 1844 he set them apart as a new group, coining the name from the Greek words for shell and skin. Modern classifications rarely use the label formally, because it is paraphyletic: it leaves out the jawed fishes and perhaps the lampreys and hagfishes too, if anaspids turn out to be closer to them. It survives as a convenient shorthand, and older schemes placed the group within the jawless Agnatha beside the living cyclostomes.
The gill pouches lay in a row along each side of the head, permanently open and without any protective cover. Instead of moving food with beating cilia, as many invertebrates do, these fish used a muscular throat to create suction and draw small, slow prey into the mouth. Their heads appear built from tiny tooth-like elements.
Two main groups stand out. Heterostracans were the more primitive; cephalaspids had side stabilisers that gave finer control while swimming. Researchers long believed only pteraspidomorphs and thelodonts had paired nostrils, with the rest limited to a single central opening, but galeaspids turned out to hide two nasal organs behind their one external hole.
Jawed fishes such as placoderms, acanthodians and sharks appeared around 420 million years ago, and most ostracoderms then declined, with the last dying out at the close of the Devonian. Newer research suggests the jawed newcomers were less to blame than once thought, since both lived side by side for roughly 30 million years with no obvious fall in ostracoderm numbers.
Source: Ostracoderm
