Ostracoderms are- Armoured fish or bony fish and first vertebrates that appeared on this earth.They are jawless vertebrates. Jaymotius is the primitive and typical example of ostracodermii. Ostracoderms originated about 500 million years ago in late cambrian period of palaeozoic era. They lived up to devonian period for about 200 millon years and they gradually became extinct. They were freshwater bottom dwelling forms. • Body was fish like and did not exceed 30 cms in size. • Paired fins were absent. Median and caudal fins were present. • Caudal fin was heterocercal type. The head and thorax were covered by heavy armour of bones. Heavy armoured Ostracoderms protected from the giant scorpion like eurypterids.Bony skull was well developed. Mouth was mostly present on the ventral side. They were having large number of gill slits • Nervous system has 10 pair of cranial nerves.The head had a pair of lateral eyes and a median peneal eye. • They were filter feeders and feed like vacuum cleaner. • The endoskeleton was either bony or cartilaginous.A single median nostril was located • anterior to penial eye. The mouth was anteroventral, small and without jaws or teeth. The gill-slits were round and all had similar gill-pouches. Sensory fields on head were probably a part of the lateral line system.The flattened body and feeble fins suggest that they were probably bottom dwellers and filter feeders like most of the present day lower chordates. Very little is known about ostracoderm internal anatomy. The endoskeleton was moderately ossified. They had no axial endoskeleton or vertebrae. An internal ear with two semicircular canals was present. Biological Significance of Ostracoderms: The Ostracoderms were the primitive vertebrates recorded in the geological history.The ostracoderms are specially interesting because they represent the oldest known vertebrate fossils in the late Cambrian and Ordovician rocks dating back to nearly 500 million years. They are the remote ancestors of all the vertebrates including man. 2. The microscopic studies of their fossilized bony tissues reveals a great complexity of structure. This implies that these vertebrates were far advanced and had undergone a considerable period of evolution before becoming fossilized. 3. The lack of earlier vertebrate fossils shows that they had perhaps evolved in a habitat (freshwater?) which was inimical to fossilisation. It is also likely that the earliest ancestors lacked hard skeletal materials as bones. 4. The ostracoderms had developed heavy bony armour perhaps for survival against the attacks of contemporary giant arachnid predators, the eurypterids. After the disappearance of giant arachnids (eurypterids) the descendants of ostracoderms, the cyclostomes, also lost the unvented heavy armour which was a hindrance in rapid progression. 5. The cartilage of cyclostomes and sharks and skates (Chondrichthyes) was previously considered a precursor to bone and more primitive. Since the ostracoderms had bony skeletons, the bone is now considered more primitive and the cartilage is interpreted as a degenerate condition.

• published: 19 Aug 2020
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