In this article we will discuss about:- 1. Occurrence of Ostracoderms 2. Important Features of Ostracoderms 3. Biological Significance 4. Inter-Relationship and Affinities.
Occurrence of Ostracoderms:
The earliest known vertebrates to appear in fossil record were jawless primitive fish-like animals collectively known as the ostracoderms. These were placed under the class Ostracodermi. The class Ostracodermi is represented by the fossil vertebrates of late Cambrian and middle Ordovician periods.
Although the fossils are frequent in the sea, there are ample reasons to believe that their progenitors were freshwater forms living near estuaries. The ostracoderms resembled the present day cyclostomes (lampreys and hagfishes) in many respects and together with them constitute a special group of jawless vertebrates, the Agnatha.
Ostracoderms were first observed as fragmented fossils occurring in rocks of late Cambrian and middle Ordovician periods approximately 500 million years ago. They were quite abundant during the upper Silurian and Devonian periods. Most of fossils of Ostracodermi were preserved in the bottom sediments of freshwater streams. However, the opinion is sharply divided as to whether their habitat was freshwater or marine.
Important Features of Ostracoderms:
The ostracoderms were primitive vertebrates, small to medium in size. Their body form was fish-like, usually flattened dorsoventrally, with a huge head and gill region, a tapering but muscular trunk and some sort of tail fin. They had no jaws and no pectoral or pelvic fins but had only median fins. These earliest vertebrates were bony and heavily armoured. The bony plates and the scales of the ostracoderms were well preserved.
The head was covered in a solid shield made of broad bony dermal plates, while the rest of the body surrounded by a series of smaller plates often called dermal scales. This has led to their names “ostracoderms” “armoured fishes” or bony skinned animals (Gr., ostrakon = shell; derma = skin).
It has been suggested that the heavy endoskeleton served as a protection against the giant scorpion-like arthropods, the eurypterids which were the dominant predators of Cambrian, Ordovician and Silurian periods. Later, when these enemies disappeared, the jawed descendants of ostracoderms also lost their heavy armour which only hindered rapid progress.
The ostracoderm head had a pair of large lateral eyes and a median penial eye on the top. 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 biological significance of ostracoderms have been discussed as follows:
1. 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 fossilised 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 fossilised.
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 univanted heavy armour which was a hinderance 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.
Inter-Relationship and Affinities of Ostracoderms:
The fossil ostracoderms, as we find them, were specialised and products of a long evolutionary past. The ostracoderms had probably evolved from unarmoured ancestral forms such as Jamoytius and became diverged to various modes of life. Now the question arises whether the ostracoderms should be placed in the direct line of descent of vertebrates.
It is certain that the group shows many structural similarities with the cyclostomes (the most primitive surviving vertebrates) are the degenerated descendants of some forms of ostracoderms. There are strong evidences in favour of this view.
But the relationship of ostracoderms with gnathostomes is still obscure. The gnathostomes (jawed vertebrates) evolved simultaneously with the ostracoderms during the Devonian period and no fossil link is so far discovered depicting the line of transformation.
The ostracoderms could not compete with the jawed fish that evolved in such diversity during Devonian and became extinct. Before extinction the ostracoderms gave rise to the first bony fishes, the placoderms, and the cartilaginous chondrichthyes.