Introduction Gnathostomata are the jawed vertebrates. (gnathos= "jaw" + (stoma)="mouth". It comprises roughly 60,000 species. (99% of all living vertebrates). Living gnathostomes have teeth, and paired appendages. A horizontal semicircular canal is present in the inner ear. Myelin sheaths is present on the neurons. Adaptive immune system uses V(D) J recombination ( it is the mechanism of somatic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. VDJ recombination is the process by which T cells and B cells randomly assemble different gene segments – known as variable (V), diversity (D) and joining (J) genes – in order to generate unique receptors (known as antigen receptors) that can collectively recognize many different types of molecule. While Agnatha (petromyzon and hagfish) use genetic recombination in the variable lymphocyte receptor gene. It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and basal plates in females—a pattern not seen in any other vertebrate group. It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively – the buccal pump mechanism. Presence of Calcified, bony skull and vertebra are the characteristic features of Gnathostomata (fishes, amphibians, reptiles, birds and mammals). Pelvic fins are situated just in front of the anus. Interventrals and basiventrals present in the backbone. These are the elements of the backbone which lie under the notochord, and match the basidorsals and interdorsals respectively. Gill arches which lie internally to the gills and branchial blood vessels, contrary to the gill arches of all jawless craniates, which are external to the gills and blood vessels. A horizontal semicircular canal in the inner ear. Paired nasal sacs which are independent from the hypophysial tube. There are numerous other characteristics of the soft anatomy and physiology (e.g. myelinated nerve fibres, sperms passing through urinary ducts, etc.), which are unique to the gnathostomes among extant craniates, but cannot by observed in fossils.

1. General characteristics and organization
of early Gnathostomes
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

3. Introduction
 Gnathostomata are the jawed vertebrates. (gnathos= "jaw" + (stoma)="mouth".
 It comprises roughly 60,000 species. (99% of all living vertebrates).
 Living gnathostomes have teeth, and paired appendages.
 A horizontal semicircular canal is present in the inner ear.
 Myelin sheaths is present on the neurons.
 Adaptive immune system uses V(D) J recombination ( it is the mechanism of somatic
recombination that occurs only in developing lymphocytes during the early stages of T and B cell
maturation. VDJ recombination is the process by which T cells and B cells randomly assemble
different gene segments – known as variable (V), diversity (D) and joining (J) genes – in order to
generate unique receptors (known as antigen receptors) that can collectively recognize many
different types of molecule. While Agnatha (petromyzon and hagfish) use genetic recombination
in the variable lymphocyte receptor gene.
 It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair
of both pectoral and pelvic fins.
 In addition to this, some placoderms were shown to have a third pair of paired appendages,
that had been modified to claspers in males and basal plates in females—a pattern not seen in
any other vertebrate group.
 It is believed that the jaws evolved from anterior gill support arches that had acquired a new
role, being modified to pump water over the gills by opening and closing the mouth more
effectively – the buccal pump mechanism.

4. • Presence of Calcified, bony skull and vertebra are the characteristic
features of Gnathostomata (fishes, amphibians, reptiles, birds and
mammals).
•Pelvic fins are situated just in front of the anus.
•Interventrals and basiventrals present in the backbone. These are the
elements of the backbone which lie under the notochord, and match
the basidorsals and interdorsals respectively.
•Gill arches which lie internally to the gills and branchial blood vessels,
contrary to the gill arches of all jawless craniates, which are external to
the gills and blood vessels.
•A horizontal semicircular canal in the inner ear.
• Paired nasal sacs which are independent from the hypophysial tube.
• There are numerous other characteristics of the soft anatomy and
physiology (e.g. myelinated nerve fibres, sperms passing through
urinary ducts, etc.), which are unique to the gnathostomes among
extant craniates, but cannot by observed in fossils.

5. Classification of Gnathostomata:
The classification of gnathostomes is a
major field of controversy among
authors. The present classificatory
scheme of Gnathostoma adopted here
is that of J.Z. Young (1981).
Fishes:
Living fishes with jaws mostly fall into
two well marked classes, the
cartilaginous fishes (Chondrichthyes)
and the bony fishes (Osteichthyes)
including the familiar ray- finned fishes
and the lung fishes.
These two groups arose in the late
Devonian period of geological time
scale. Before that time various other
types of fish dominated the waters.

6. Class — Placodermii: (plate-skinned)
 All the fishes of this group are extinct.
 Their epidermis were covered by heavy bony plates.
 Instead of gill slits they possessed spiracles for respiration.
 Were lived from the late Silurian to the end of the Devonian Period.
 Their head and thorax were covered by articulated armoured plates and
the rest of the body was scaled or naked, depending on the species.
 Placoderms were among the first jawed fish; their jaws likely evolved
from the first of their gill arches.

7. Class — Chondrichthyes: It includes jawed fishes having a cartilaginous skeleton. The class includes a diverse
group of fishes including sharks, rays, skates and chimaeras. They are mostly marine fishes.
1. Chondrichthyes characteristics
 They are mostly marine fishes and cold blooded. They contain a pair of powerful jaws.
 The mouth is present ventrally.
 They contain cartilaginous endoskeleton, the deposits of calcium salts provide strength to it.
 The notochord is present throughout life.
 Most of them contain a heterocercal tail. Vertebrae extends into it.
 The skin is covered by minute tooth-like structures called placoid scales.
 Their teeth are modified placoid scales and are not attached to jawbones. They are embedded in the
tissue. Old teeth fall and are continuously replaced by the new teeth formed behind it.
 They contain 5-7 pairs of gills. They lack air bladders so they swim actively to avoid sinking.
 They are predatory fishes, they feed on other fishes, crustaceans and molluscs.
 The heart is two-chambered, contains one auricle and one ventricle.
 They contain a brain and a spinal cord, which is protected by vertebrae.
 Sense organs are well developed. They have the ability to detect their prey electrically. Sharks contain
electroreceptors on their head, which helps them in navigation.
 It also has sensory cells in the lateral line organ, which detect all the kinds of vibration, motion, water
pressure surrounding them.
 Some of them possess electric organs or poison sting, which are used for defence as well as predation.
 The digestive system comprises a mouth, pharynx, stomach, intestine (straight) and cloaca present on the
ventral side. Cloaca has a dual function in females and also acts as a reproductive organ apart from
excretion.
 Male and females are separate and have internal fertilization.
 Skates and some sharks are oviparous, most of the sharks are ovoviviparous and a few are viviparous.
 Adult males bear claspers on their pelvic fins. These are used to transfer sperms to the cloaca of a female.

8. Elasmobranchii:
 Swim bladders are absent. However elasmobranchs maintain buoyancy with large livers that
are full of oil. This stored oil may also function as a nutrient when food is scarce. Deep sea sharks
are usually targeted for their oil, because the livers of these species can weigh up to 20% of their
total weight.
 Five to seven pairs of gill clefts opening individually to the exterior.
 Skin is covered by plocoid scales.
The teeth are in several series.
 The upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper.
 Jaw suspensions are of amphistyly, orbitostyly, hyostyly, and euhyostyly.
 The eyes have a tapetum lucidum (is a biologic reflector system that enhancing visual
sensitivity at low light levels) .
 Claspers are present in the inner margin of each pelvic fin in the male fish.
 These fish are widely distributed in tropical and temperate waters.

9. Subclass — Holocephali:
General characters:
1. The earliest fossils are of teeth and come from the Devonian period.
 This group includes the rat fishes in the genus Chimaera, and the elephant fishes in the genus
Callorhinchus.
 Pectoral fins are large.
 The erectile spine in front of the dorsal fin is sometimes venomous.
 Tail is long, thin and slender.
 Live close to the seabed feeding on benthic invertebrates.
 They lack a stomach, food moving directly into the intestine.
 Small mouth aperture is guarded by jaws and lips.
 Plate like teeth firmly attached with the jaws.
 Holostylic upper jaw, i.e. rigidly attached to the skull.
 Gill slit partially covered by operculum.
 Cloaca absent, i.e., anus and urinogenital aperture separate.
 Skin is naked in adults.
 In addition to pelvic clasper males possess another clasper on the head.
 Spiracle absent.
Examples:
Chimaera (Rat fish)(Fig. 1.27D), Callorhynchus, Harriotta.

10. Osteichthyes
 Osteichthyes is a class of jawed fishes (Gnathostomata) having a bony
endoskeleton. It is the largest class of vertebrates and includes a diverse
group of marine and freshwater bony fishes.
 The vast majority of fish are osteichthyes, which is an extremely diverse
and abundant group consisting of 45 orders, with over 435 families and
28,000 species.It is the largest class of vertebrates in existence today.
Osteichthyes is subdivided into three subclasses:
1. Acanthodii: (Acanthodii, or spiny sharks). They are a class of extinct
fishes, sharing features with both bony and cartilaginous fishes. They
resembled sharks, but their epidermis was covered with tiny rhomboid
platelets. They have a cartilaginous skeleton, but their fins had a wide,
bony base. Ex: Acanthodes bronni.
2. Actinopterygii- ray-finned fish: Their fins are webs of skin supported by
bony or horny spines ("rays"), hence called ray finned fish. Fin rays attach
directly to the proximal or basal skeletal elements of pelvic and pectoral
girdles. They are ubiquitous throughout freshwater and marine
environments from the deep sea. Ex: Sea horse.
3. Sarcopterygii (Crossopterygii)- lobed finned fish: Sarcopterygii sometimes
considered synonymous with Crossopterygii. Fins are fleshy and lobed. Fins
are borne on a fleshy, lobe like, scaly stalk extending from the body.
Pectoral and pelvic fins have articulations resembling those of tetrapod
limbs. Most species of lobe-finned fishes are extinct. The living non-
tetrapod sarcopterygians include two species of coelacanths and six
species of lungfish.

11. Dipnoi: Distribution, Morphology and Affinities
Structure of Dipnoi:
 Dipnoi (Gr. di-two, pnoe-breathing) is a small order of fresh water bony fishes.
 They respire by gills and lungs. Dipnoi evolved during Devonian period.
 They are characterized by short jaws, crushing plate from teeth, internal nares, reduced exo-
and endo- skeleton, and diphycercal tail.
 The air bladder i.e., so called ‘lungs’ are one or two. They are functional with related changes
in the circulatory system and in the heart.
Distribution of Dipnoi:
Fossil evidence support to the view that once dipnoans enjoyed cosmopolitan (worldwide)
distribution. Two fossil forms are Dipterus from Devonian period and Ceratodus from Triassic
period. But the modern lung fishes show discontinuous distribution.
 The three surviving genera of lung fishes are Neoceratodus (=Epiceratodus) Protopterus and
Lepidosiren.
 All they are inhabitants of river. But Protopterus lives in large lakes too. They all breath air.
 Neoceratodus is found only in the Burnett and Mary rivers of Queens-land in Australia, so
commonly called as ‘Burnett Salmon’ or Australian lungfish.
 Neoceratodus is the only living genus of the family Ceratodontidae, the other being extinct
Ceratodus.
Protopterus lives in large lakes and rivers of tropical Africa. It is commonly called as Nile
lungfish’ or African lung fish.
Lepidosiren is found in river Amazon and Paraguay basin in South America. It is commonly
called as ‘Amazon lungfish’ or South American lungfish.

12. Morphology of Dipnoi:
1. Neoceratodus forsteri:
 The body of Dipnoi is elongated and compressed measuring about 90 to 150
cms.
 The body surface including the head and paired fins is covered with thin, bony,
overlapping cycloid scales which are not regarded as denticles.
 Paired fins are lobe like paddles, enabling the fish to crawl over the bottom. But
these are weak to support the fish outside water.
 The fish has a diphycercal tail with symmetrical tail fin. Tail fin rays are un-
jointed and fibrous. External nostrils lie enclosed within the upper lip. Internal
nostrils (choanae) are present.
 Branchial arches are four in number and bi-segmented.
 The first four branchial arches carry holobranches. Gills are covered with bony
opercula.
A lateral line is present.
 Lower jaw is with a small toothless dentary on each side. Dental plates are oval
crescentic or triangular terminating in a smooth or feebly denticulated biting
margins.
 Neoceratodus is inactive and sluggish in habit, usually lying motionless on the
bottom. It is carnivorous and feeds on fresh water crustaceans, worms and
molluscs.

13. 2. Protopterus has four species.
 The body is elongated, cylindrical and more or less eel-like.
 It grows to a length of about 200 cms.
 Body is covered over by over-lapping cycloid scales.
 The pectoral and pelvic fins are represented as limbs.
 They are long, thin and filamentous and help in walking along the bottom.
 The caudal fin and its supporting fleshy lobe taper to a posterior point i.e. it is isocercal or
protocercal. The mouth is small.
 The dorsal anal and tail fins are continuous and are supported by partially calcified fibre-
like rays called camptotricha. Inside the mouth margin internal nostrils are present.
 The fleshy lips extend back on the sides of the head.
 The eyes are small.
 Six branchial arches and five gill slits are covered by operculum.
 The opercular opening is limited to a slit just in front of the pectoral fin.
 Gills are weakly developed.
 The lateral line system is well developed.
 The cloacal opening lies ventrally at the root of the tail and close to it are two abdominal
pores.
 Protopterus is a carnivores and voracious feeder.
 It feeds on worms, crustaceans, insects, frogs and many other small animals.
 During un-favourable seasons it undergoes summer sleep or aestivation and burrow into
the soil to a depth of about 60 cms for atleast 6 months in waterproofed “cocoon” made of
clay and mucus.

14. 3. Lepidosiren paradoxa:
 Body is elongated, cylindrical and more or less eel-like.
 It grows to the length of about 125 cms.
 The skin covering the body contains very small cycloid scales.
 Gill slits are 5 pairs covered with operculum.
 Gills are weakly developed, respiration is supplemented with two lungs
(Dipneumona).
 Paired fins are thin and filamentous. Dorsal and caudal fins are united.
 Pelvic fin have vascular filamentous in male during breeding season.
 Eyes are moderate and well developed like Neoceratodus.
 External (cutaneous) gills are absent in adult but present in larva.
 Like Protopterus, they survive drought by secreting mucous cocoons in mud.
 During aestivation the air bladder is used as lungs.
 Lepidosiren is not exclusively carnivorous, food mainly comprises fresh snails
and mass of algae.

15. Affinities of Dipnoi:
Presence of the lungs in Dipnoi lead to the view that they are the ancestors of
amphibia. Other-words, they were considered as the connecting link between
Pisces and Amphibia. This view is no more supported. Present view is to treat them
as a specialized or degenerate descendants of the more primitive lobe-finned fishes
to which they closely resemble.
The affinities of Dipnoi can be studied under following heads:
1. Affinities with fishes:
Lung fishes are undoubtedly true fishes.
(a) Affinities with fishes in general:
1. Spindle-shaped, eel-like body.
2. Body covered with scales (Cycloid).
3. Presence of paired fins.
4. Diphycercal caudal fins.
5. Persistent notochord.
6. Skull with little ossification.
7. Paired gill-slits.
8. Branchial respiration.
9. Lateral line sense organs.

16. 2. Affinities and Dissimilarities with Amphibians:
(a) Affinities:
1. Semi aquatic habitat.
2. Internal nostrils
3. Vomerine teeth.
4. Autostylic jaw suspensorium.
5. Multicellular cutaneous glands.
6. Cutaneous gill in larva.
7. Pulmonary respiration.
8. Dermal scales as in Apoda.
9. Auricle and sinus venosus partially divided.
10. Ventral aorta short or absent
11. Presence of anterior abdominal vein, posterior vena cava, pulmonary artery and veins.
12. Thin walled pericardium.
13. Long and narrow cerebral hemispheres.
14. Similar structure of egg and development
(b) Dissimilarities:
1. Paired lobate-fins
2. Maxillae and premaxillae are absent.
3. Peculiar crushing tooth plates.
4. Few anterior vertebrae fused with skull.
5. Cartilagenous skull.
6. Lungs lie dorsal to gut.
7. Urinary bladder from dorsal wall of cloaca.

17. 3. Primitive Characters:
1. Unconstricted notochord.
2. Presence of cloaca.
3. Spiral valves in intestine.
4. Valves in the conus.
4. Specialized characters:
1. Premaxillae and maxillae absent
2. Anterior dorsal fin reduced or absent
3. Dental plate in jaws.
4. Cartilagenous cranium without ossification.
5. Reduction in number of dermal bones of skull.
6. Thin cycloid scales by modification of thick cosmoid scale.
7. Fusion of anterior vertebrae with skull.
8. Functional dorsally placed air bladder.
5. Conclusion:
The above affinities indicate that dipnoans are not most advanced Pisces from which
amphibians could evolve. They are degenerate descendants of Crossopterygii. According to
Jarvik (1968) dipnoans are more specialized than crossopterygian. According to latest view,
both dipnoans and amphibians have originated from some crossopterygian like ancestor.
There must have been a common ancestor for Dipnoi, Crossopterygii and Labyrinthodont
amphibia. So most probably, dipnoans are not the “fathers of the amphibia”, but “uncles of
the amphibian”.

18. Sarcopterygii or Crossopterygii (class Osteichthyes) : Sarcopterygii sometimes considered
synonymous with Crossopterygii. Subclass of bony fish comprising both fossil and living lobe-
finned or tassel-finned fish, including the Coelacanthiformes and Rhipidistia.
 The Rhipidistia did become extinct, although not before they gave rise, in the Devonian, to the
amphibians.
 The Crossopterygii are characterized by the fact that all fins (except the tail-fin) are based on
movable stalks or lobes. The tail fin is either heterocercal or diphycercal.
 Early lobe-finned fishes are bony fish with fleshy, lobed, paired fins, which are joined to the
body by a single bone.
 The fins of lobe-finned fishes differ from those of all other fish in that each is borne on a fleshy,
lobe like, scaly stalk extending from the body.
 The morphology give indications of the transition from water to terrestrial life.
 Pectoral and pelvic fins have articulations resembling those of tetrapod limbs.
 The first tetrapod land vertebrates, basal amphibian organisms, possessed legs derived from
these fins. Sarcopterygians also possess two dorsal fins with separate bases.
 The braincase of sarcopterygians primitively has a hinge line, but this is lost in tetrapods and
lungfish. Many early sarcopterygians have a symmetrical tail.
 All sarcopterygians possess teeth covered with true enamel.
 Most species of lobe-finned fishes are extinct. The largest known lobe-finned fish was
Rhizodus hibberti from the Carboniferous period of Scotland which may have exceeded 7 meters
in length. Among the two groups of extant (living) species, the coelacanths and the lungfishes,
the largest species is the West Indian Ocean coelacanth, reaching 2 m (6 ft 7 in) in length and
weighing up 110 kg (240 lb). The largest lungfish is the African lungfish which can reach 2 m
(6.6 ft) in length and weigh up to 50 kg (110 lb).