Ostracoderms were early jawless vertebrates that lived from the Cambrian to the late Devonian period. They were covered in bony plates and resembled modern hagfish and lampreys. While some evidence suggests they lived in freshwater, their habitat is still debated. Later in the Devonian, jawed fish evolved from ostracoderms and outcompeted them, contributing to their extinction by the end of the period. Ostracoderms were divided into two main groups and played an important role in the early evolution of vertebrates.

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
reddysirr@gmail.com

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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