GEOLOGY | FOSSILS | key fossil groups
A paraphyletic grouping of several distinctive clades of swimming vertebrates., fish are cold-blooded creatures that inhabit all types of liquid water bodies, from lacustrine, riverine, and marine. They are vertebrates, with a backbone made of cartilage or bone and a brain case protecting the brain. They swim using fins and take oxygen from the water by means of gills (although some also have lungs).
Since the late Cambrian, fish have evolved into thousands of species and today they form over half of all living vertebrates.
The jawless fishes, also known as as ostracoderms, were generally armoured and were most abundant in the Devonian having appeared in the fossil record during the early Cambrian.
They had a semicircular head shield with eyes and nostrils on the dorsal side of the shield and a mouth on the ventral side.
They evolved into evolved into many genera, including Loganellia and Jamoytius. Some, for example, Cephalaspis, were protected by thick scales or by a bony head shield. None had bones within their bodies, although they had a notochord.
One group, the conodonts, were for decades known only from teeth found in Palaeozoic rocks, but very rare fossils of their soft, eel-like body have now been found.
Jawless fish ate tiny organisms and food particles by sifting sediments on the sea floor. Some may have been parasitic.
Modern jawless fish include lampreys and hagfish.
Image: Pterichthyodes milleri (Devonian). National Museum of Scotland
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