The earliest known fishes were agnathous (jawless) forms with small terminal or subterminal mouths. The majority of known agnathans were armoured with dermal bony plates, but a few possessed only a little armour and it is possible that many unossified and at present unknown forms existed. The known fishes fall into rather sharply defined taxa, the relations of which are not altogether clear. The earliest group, the heterostracans, appears in the Middle Ordovician and like the other two important groups of Palaeozoic agnathans, the osteostracans and anaspids, which do not occur till the Upper Silurian, is not found later than the Upper Devonian. Possibly related to the heterostracans are the little-known denticle-covered fishes, the thelodonts. The heterostracans and osteostracans have the head and front part of the body covered by bony shields, whereas the head of anaspids is only covered by small scales. None of the agnathans have pelvic fins, but some of the osteostracans have pectorals and the anaspids have long lateral fins. The anaspids and heterostracans are peculiar in having hypocercal tails, in which the lower lobe is the larger. The fossil record of lampreys is restricted to one occurrence of a strikingly `modern’ form in the Upper Carboniferous, which shows almost all the specializations of the living species. The famous little fish Palaeospondylus, sometimes considered to be an ancestral myxinoid, is of uncertain affinities, and the hagfishes have no fossil record.
KeywordsFossil Record Middle Devonian Lower Devonian Benthic Fish Olfactory Organ
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