Origin and evolution of ostracoderms

Introduction
Ostracoderms (shell-skinned) are of several groups of extinct, primitive, jawless fishes that were covered in an armour of bony plates.
They appeared in the Cambrian, about 510 million years ago, and became extinct towards the end of the Devonian, about 377 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.
The first fossil fishes that were discovered were ostracoderms.
The Swiss anatomist Louis Agassiz received some fossils of bony armored fish from Scotland in the 1830s.
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.
Characteristics: They use gills exclusively for respiration but not for feeding . Earlier chordates with gills used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. mostly small to medium-sized fishes, protected by a heavy, bony dermal (derived from skin) armor. bottom-dwellers; filter-feeders or grazers. no paired fins, but many with stabilizing paired flaps on either side of head.
(1) Ostracoderms were the first vertebrates.
(2) They were popularly called armoured fishes.
(4) They lived in freshwater.
(5) They were bottom dwellers.
(6) Their body was fish-like and did not exceed 30 cm in size.
(7) Paired fins were absent.
(8) Median and caudal fins were present.
(9) The caudal fin was of heterocercal type.
(10) The head and thorax were covered by heavy armour of bones. It protected ostracoderms from the giant scorpion like arthropods, eurypterids.
(11) Bony skull was well developed.
(12) Mouth was mostly present on the ventral side.
(13) They were having large number of gill slits.
(14) The nervous system had 10 pairs of cranial nerves.
(15) The head had a pair of lateral eyes, and a median pineal eye.
(16) They were filter feeders, feeding like a vacuum cleaner.
(17) The endoskeleton was either bony or cartilaginous.

1. Origin, evolution of Ostracoderms and Affinities. Dr. P.B.Reddy M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM PG DEPARTMENT OF ZOOLOGY GOVERTNAMENT PG COLLEGE, RATLAM.M.P [email protected]

2. Introduction Ostracoderms (shell-skinned) are of several groups of extinct, primitive, jawless fishes that were covered in an armour of bony plates. They appeared in the Cambrian, about 510 million years ago, and became extinct towards the end of the Devonian, about 377 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. The first fossil fishes that were discovered were ostracoderms. The Swiss anatomist  Louis Agassiz  received some fossils of bony armored fish from Scotland in the 1830s. 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.

3. Important Features of Ostracoderms (general organisation): 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 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.

4. Characteristics: They use gills exclusively for respiration but not for feeding . Earlier chordates with gills used them for both respiration and feeding. Ostracoderms had separate pharyngeal gill pouches along the side of the head, which were permanently open with no protective operculum. mostly small to medium-sized fishes, protected by a heavy, bony dermal (derived from skin) armor. bottom-dwellers; filter-feeders or grazers. no paired fins, but many with stabilizing paired flaps on either side of head. (1) Ostracoderms were the first vertebrates. (2) They were popularly called armoured fishes. (4) They lived in freshwater. (5) They were bottom dwellers. (6) Their body was fish-like and did not exceed 30 cm in size. (7) Paired fins were absent. (8) Median and caudal fins were present. (9) The caudal fin was of heterocercal type. (10) The head and thorax were covered by heavy armour of bones. It protected ostracoderms from the giant scorpion like arthropods, eurypterids. (11) Bony skull was well developed. (12) Mouth was mostly present on the ventral side. (13) They were having large number of gill slits. (14) The nervous system had 10 pairs of cranial nerves. (15) The head had a pair of lateral eyes, and a median pineal eye. (16) They were filter feeders, feeding like a vacuum cleaner. (17) The endoskeleton was either bony or cartilaginous.

5. Biological Significance of Ostracoderms: 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 unfavorable 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.

6. Inter-Relationship and Affinities of Ostracoderms: The fossil ostracoderms were specialized 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. It is certain that the group shows many structural similarities with the cyclostomes (the most primitive surviving vertebrates) that 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 unclear. 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.

7. Ostracoderms existed in two major groups, the more primitive heterostracans and the cephalaspids . Later, about 420 million years ago, the jawed fish evolved from one of the ostracoderms. After the appearance of jawed fish, most ostracoderm species underwent a decline, and the last ostracoderms became extinct at the end of the Devonian period Class : ostracodermi is divided into following four orders: • 1.Euphanerida: - Example –Jamoytius. • 2.Pteraspidomorphi :- Example – Pteraspis , Thelodus , Lanarkia and Coelolepsis . • 3.Cephalaspidomorphi :- Example –Cephalaspis, Hemicyclaspis , Ateleaspis . • 4.Anaspida . Example –Birkenia, Pterolepis and Rhyncholepis