Chordata gnathostomata: TAXONOMY: T. Y. B.Sc. Zoology, Sem. VI; Course Code-USZO601; Unit-II: University of Mumbai 2017-18

TAXONOMY: T.Y. B.Sc. Zoology, Sem. VI; Course Code-USZO601; Unit-II: University of Mumbai 2017-18.
1 By Dr. Sudesh Rathod, VPM’s B. N. Bandodkar College of Science, Thane, Maharashtra, India
GENERAL CLASIFICATION OF CHORDATES
Chordata is an assemblage of heterogeneous animals which differ widely in their characteristics but
have a few common features like they have notochord, dorsal tubular nerve cord, pharyngeal gill
slits and post-anal tail. The notochord is a cartilaginous skeletal rod present at some stage in their
life cycle. Nerve cord is mid-dorsally placed and is hollow containing axial nerve canal. Pharynx is
perforated containing several gill slits primarily were used for filter feeding but later evolved as
respiratory in function in aquatic forms. However, in terrestrial forms they disappear in adults but
are occurred in embryonic stages. The body extends beyond anal terminal to give rise to a
balancing or important part in locomotion called as tail.
DIVISION GNATHOSTOMATA
2.2: Division Gnathostomata (gnathos = jaws; stoma = mouth)
These animals have true jaws and paired appendages. Gnathostomes are further divided into two
superclasses namely Pisces and Tetrapoda. Gnathostomes appeared in mid-silurian period some
450 million years ago in form of Placoderms (bony armoured fish like jawed creatures). According
to Prof. A. S. Romer “perhaps, the greatest of all advances in vertebrate history was the
development of jaws and the consequent revolution in the mode of life of early fishes”.
Their characters are as discussed below:-
Superclass- Pisces
Exclusively aquatic and respire prominently with gills. They paired appendages called fins. Fins are
supported by fin rays. Median fins are present. Exoskeleton made of dermal scales and skin

remains moist. Vertebral column is complete and contains pelvic and pectoral girdles. Gill slits are
reduced in number of pairs from 4 to 11.
Superclass- Tetrapoda
These are aquatic, terrestrial, avian or arboreal. Paired appendages modified as pentadactyle limbs
for locomotion. Gills are replaced by lungs. Skin is dry and cornified.
2.2.1: SUPERCLASS- PISCES - GENERAL CHARACTERS AND CLASSIFICATION UP TO
CLASSES-
A. General characters Superclass- Pisces
Superclass Pisces is classified into three classes on the basis of their phylogenic relationship and
characters namely Placodermi, Chondrichthyes and Osteichthyes. These are fish like aquatic
forms with paired as well as median fins supported by fin rays. Exclusively aquatic and respire
prominently with gills or through skin. Two chambered heart is present. Blood circulation is closed
and single or unidirectional. Exoskeleton made of bony scales of dermal origin and skin remains
moist. These are poikilothermous. Pisces appeared some 450 million years ago in form of
Placoderms. Later they evolved as existing cartilaginous and bony fishes. Primitive jawed fishes
were once believed to be related to each other and therefore placed in a single class Placodermi. But
the spiny sharks are now placed in separate class Acanthodii. Since Ostracoderms and Placoderms
had bony skeleton and were primitive to cartilaginous shark they are placed in the lower class than
Chondrichthyes. However, there is no evidence to support the evolution of modern fishes
(cartilaginous and bony fishes) from the Placoderms.
B. Classification of Superclass- Pisces
I. Class- Placodermi
Placoderms were the earliest of the known fossil vertebrates with jaws. Their bony armour or
shields (modified dermal scales) links them genetically with the Ostracoderms. But Placoderms did
not have heavy armour. Perhaps, they did not require these since they were more specialized with
development of bony jaws and paired fins. These things favoured their survival over Ostracoderms
which lacked both paired fins and jaws. The jaws might have developed from first gill arch
(mandibular arch). Hyoid arch persisted and supported jaws called as aphetohyoidean
Superclass- Pisces
Class- Placodermi Class- Chondrichthyes Class- Osteichthyes

suspensorium. However, jaws were immovable and lacked modern teeth. Pisces appeared some
450 million years ago in form of Placoderms in mid Silurian period, flourished in Devonian and
Carboniferous. Placoderms combined the heavy bony armour of Ostracoderms with powerful jaws
and efficient fins. They were violently predaceous bottom dwellers
e.g. of Placoderm: Coccosteus decipiens
Placoderms became extinct in Permian period of Palaeozoic era. Placoderms although contain bony
skeleton they are considered as primitive to modern cartilaginous fishes therefore they are place
before class Chondrichthyes in classification. Thus bone is now believed as primitive characteristic
as compared to the cartilage. Placoderms survived for a short period only and are often considered
as ‘unsuccessful ancient experiment’, in the evolution of gnathostomes.
II. Class- Chondrichthyes
These are mostly marine and predaceous forms. Body fusiform, covered with minute placoid scales,
tail is heterocercal and pelvic fins contain claspers in males. Endoskeleton is entirely cartilaginous
and vertebrae are complete and separate. They have 5 to 7 pairs of gill slits which are not covered
by operculum. Mouth is ventral, stomach is ‘J’ shaped, cloaca is present. Ear contains three
semicircular canals. Olfactory chambers do not open into pharynx. Brain has large olfactory lobes.
Air bladder and lungs are absent. Have two dorsal fins and tail fin is heterocercal type.
Chondrichthyes are grouped into two categories as- 1. Dogfishes & sharks; 2. Rays and skates & 3.
Rat fishes.
a. Sphyrna (hammer headed shark)
Kingdom: Animalia Cell wall absent, movements are prominent
Phylum: Chordata Notochord, dorsal tubular nerve cord, gill slits,
and tail present
Subphylum Vertebrata Notochord is replaced by vertebral column, brain
box present, gill slits are limited in number
Division Gnathostomata Jaws and paired appendages are present
Superclass Pisces Aquatic; dermal scales, paired fins and gills are
present
Class: Chondrichthyes Cartilaginous endoskeleton, scale placoid or
ganoid type, five pairs of gills, no operculum
Genus: Sphyrna Hammer headed shark

The shape of the head of the hammerhead shark is made up of two projections on either side like
a hammer or shovel in a ‘T’ shape called as cephalofoil, which gives the name to this shark. The
eyes and nostrils of the hammerhead shark are found at the ends of the hammer allowing the
hammerhead shark to have better vision and smell of the surrounding water. The mouth is
ventrally place and narrow, contains serrated teeth. Median fins include two dorsal fins, a caudal
fin (tail fin) and an anal fin. First dorsal fin is larger than the second. Tail fin is heterocercal,
Paired fins include a pair of large pectoral fins originating from last gill and a pair of pelvic fin
surrounding the cloaca. Pelvic fins develop claspers in males. Five pairs of gill slits are placed in
front of the pectoral fins and are not covered by operculum. This shark is gray-brown above with
an off-white belly. These cartilaginous fishes vary in size; the small scalloped bonnet-head (S.
corona) measures only 90 cm (35 inches) long, whereas the great hammerhead (S. mokarran)
grows to over 6.1 metres (20 feet) in length.
Sphyrna (hammer headed shark)
Hammerhead sharks are viviparous: they retain fertilized eggs within the body and give birth to
live young—from 2 to 42. Hammerhead sharks prey on a wide array of fish, cephalopods,
and crustaceans, but specific prey varies between the different species.
Hammerhead sharks are widely distributed in tropical and temperate marine waters near the
coasts and above the continental shelves. They may migrate seasonally, moving equator-ward
during the winter and pole-ward during the summer.
b. Pristis (saw fish)
and tail present

present
Genus: Pristis Sawfish
It is characterized by a long, narrow, flattened rostrum, or nose extension, lined with are
provided with a series of strong tooth-like denticles giving appearance of saw. Although sawfish
look somewhat like sharks, their flattened bodies and wide pectoral fins reveal that they are
actually rays. The head is droso-ventrally flattened with the mouth located underneath having
minute and obtuse teeth in jaws. A pair of nostrils located ahead of the mouth and five pairs of
the gill slits are present behind the mouth. A large pair of pectoral fins remain fused to the lateral
flanks of the posterior head region followed by a pair of pelvic fin near anal region. Pelvic fins
develop claspers in males. Dorsal fins are large, first dorsal fin is opposite to the pelvic fin and
second dorsal fin lie close to caudal fin. Well-developed tail and is terminating in heterocercal
caudal fin. Anal (ventral) fin is absent.
Pristis (Sawfish)
Dorsally, the eyes present behind which a pair of apertures appear called as spiracles. Sawfishes
are able to breath while lying on the ocean floor by drawing water into their gills through these
spiracles. Sawfish is bottom dweller and predaceous which feeds on crabs, fish, lobsters etc.
They are among the largest fish with some species reaching lengths of about 7 m (23 ft).
Sawfishes are relatively slow breeders and the females give birth to live young. They feed on
fish and invertebrates that are detected and captured with the use of their saw.

They are found worldwide in distribution found in tropical and subtropical regions in
coastal marine and brackish estuarine waters, as well as freshwater rivers and lakes.
c. Dasyatis (sting ray)
and tail present
present
Genus: Dasyatis Sting ray
Dasyatis (Sting ray)
Body is dorso-ventrally flattened, diamond shaped slightly wider than its length with rounded
outer corners. The anterior margins are almost straight and converge on a pointed, slightly
protruding ahead into a snout. The eyes are smaller than
the spiracles (paired respiratory openings), which are placed closely behind. Mouth is ventrally
placed just behind a pair of nasal openings. There are 28–38 upper tooth rows and 28–43 lower
tooth rows; the teeth are small and blunt, and arranged into flattened surfaces. There are five
papillae (nipple-like structures) across the floor of the mouth. There are also five pairs of gill slits
located in mid-ventral line running parallel to each other. Anterior gills are distant as compared
to the posterior. The posterior margins of the body is lined by large pectoral fins covering almost
entire body. At posterior of the pectoral fins start the pelvic fins surrounding the anus. In male
the pelvic fins develop claspers. Anal (ventral) fin is absent. The tail is slender and whip-like,
measuring approximately half as long as the body. A stinging spine with strong serrations,
measuring up to 35 cm (14 in) long and equipped with a venom gland at its base, is positioned
about a third of the distance along the tail. A second or even third spine may also be present, as

the spines are regularly replaced and new spines grow in before existing ones have been shed.
Older individuals may develop a row of bony knobs along the midline of the back. This species is
a solid gray, brown, reddish, or olive-green above, and whitish below with dark fin margins.
Young rays may have white spots. The common stingray has been reported to reach a width of
1.4 m (4.6 ft) and a length of 2.5 m (8.2 ft), though a width of 45 cm (18 in) is more typical.
This bottom-dwelling species can be found from the shore to a depth of 200 m (660 ft), though it
is not usually found deeper than 60 m (200 ft). It favors sandy or muddy bottoms in calm water,
and is also sometimes encountered near rocky reefs or in estuaries, as it is tolerant of low salinity.
It feeds on a wide variety of bottom dwelling organisms, including crustaceans, cephalopods,
bivalves, polychaete worms and small bony fishes. Like other stingrays are aplacental viviparous
in which the embryos are initially sustained by yolk and then by uterine milk.
d. Hydrologus (spotted rat fish or Chimaera)
and tail present
present
ganoid type
Genus: Hydrologus Spotted ratfish
Hydrologus (Spotted rat fish)
The Chimaeras are a very primitive group of fish, with skeletons composed of cartilage instead of
bone, dating back for more than 300 million years and have changed very little since. They may
actually be the oldest and most enigmatic groups of fishes alive today. Truly an ancient living
fossil!

Ratfish are almost half shark and half ray, with smooth skin, big sparkling green eyes designed to
see in the dark depths, a rabbit-like face, and a small mouth surrounded by large lips. The nose of
the ratfish is studded with electric sensors able to detect the faint electrical signals given off
whenever bottom dwelling prey sensitive enough to detect their heartbeats. Their bodies taper to
an exceptionally long threadlike tail, and together with their rat-like teeth designed for crushing the
shells of their prey, has earned them the common name “ratfish.” These fish have a smooth and
scaleless skin that is a silvery-bronze color, often with sparkling shades of gold, blue, and green. It
has three dorsal fins, a caudal fin, a huge pair of pectoral fin and a pair of pelvic fin. Ventral fin is
absent. The ratfish’s pectoral fins are large and triangular, and extend straight out from the sides of
their bodies like airplane wings. They have a venomous spine located at the leading edge of their
dorsal fin, which is used in defense. They can grow to be up to a meter in length, and females are
much larger than males. Female ratfish lay one or two fertilized eggs into sandy or muddy areas
every two weeks or so. Each sack is twelve 12 cm long, and is basically a leather pouch. The eggs
take a long time to develop and hatch, sometimes staying inside the egg case for up to a year.
Ratfish feeds on crabs, snails, starfish, marine worms, urchins, clams, shrimp, and small fish. The
ratfish is an extraordinary fish found in all the world's oceans, close to the bottom, at depths of
300–500 meters, with a reported maximum depth of 1,663 meters.
III. Class- Osteichthyes
These inhabit in fresh water, brackish water or marine environments. Endoskeleton is mostly made
of bones except some e.g. sturgeon and paddle fish. Scales are of three type ganoid, cycloid or
ctenoid. Spiracles, claspers and cloaca are absent. Have four pairs of gill slits covered with bony
operculum. Mouth is terminal or sub-terminal, anus is present, tail is usually homocercal and
majority have airbladder. Olfactory lobes are reduced while optic lobes are enlarged in brain.
Examples:-
a. Lepidosiren (American lungfish)
and tail present
present
Class: Osteichthyes Bony endoskeleton, scale cycloid , ctenoid or
ganoid type, four pairs of gills, operculum
present
Genus: Lepidosiren American lungfish

Lepidosiren (American lungfish)
Lepidosiren is elongate eel like and grow up to about 125 cm having filamentous but highly mobile
paired appendages (modified fins). The body colour is speckled yellow on black which turns faded
grey or brown in adult. It has weakly developed gills and hence has a modified pair of air bladders
functioning as lungs known as Dipneumona. Skeleton is cartilaginous. They lack with teeth instead
having tooth plates helping to crush the shell fish on which it feeds.
It is found in swamps and slow moving fresh water of the Amazon, Paraguay and Parana River
basins in South America in warm area temperature ranging from 24 to 28 °C. They mostly remain
close to surface need of gulping air frequently in respiration. These fish typically stay very close to
the surface of the water. In times of drought, they can aestivate by burrowing in the mud up to 50
cm and secreting mucous cocoon to avoid desiccation and can live dormant for months together.
The Dipnoi, commonly known as lung fish, are descendants of more primitive lobe fin fish like
Coelacanthus. Dipnoi is represented by three genera, namely Neoceratodus (Australian),
Protopterus (Africa) and Lepidosiren (South America) zoogeographically represent the
discontinuous distribution. Lepidosiren paradoxa is found in the neotropics of South America,
including Argentina, Bolivia, Brazil, Colombia, French Guiana, Paraguay, Peru, and Venezuela.
Lepidosiren exhibits primitive embryogeny with holoblastic cleavage and invagination in
gastrulation. Young ones resemble amphibian tadpoles and have four external gills which
degenerate after seven weeks and start air breathing.
b. Exocoetus (flying fish)
and tail present

present
Class: Osteichthyes Bony endoskeleton, scale cycloid , ctenoid or
ganoid type, four pairs of gills, operculum
present
Genus: Exocoetus Flying fish
Exocoetus (Flying fish)
Exocoetus commonly called as flying fish grows to about 35 to 45 cm. scales are large cycloid, eyes
are large and skeleton is bony. Dorsal fin opposite to the anal. Large wing like pectoral fins. In
some species the pelvic fins are enlarged and thus they are known as four winged flying fish. They
are common in Atlantic water. Some have barbels around the mouth are referred as bearded flying
fish. Dark blue to blackish on back and upper sides, silvery white below; fins mainly pale or slightly
dusky. Adults occur in schools in surface waters, both near and far from the coast. Capable of
leaping out of the water and gliding for long distances above the surface with the help of pectoral
fins before it lands. Carnivorous, feeds mostly on crustaceans and other planktonic animals.
Widespread in tropical and subtropical zones of all oceans including the western Mediterranean Sea
and part of the Caribbean Sea. Parental care is exhibited by females.
2.3: Superclass- Tetrapoda
The four-limbed vertebrates known as tetrapods, namely amphibians, reptiles, birds and
mammals including earlier extinct groups like dinosaurs. Tetrapods are believed to be evolved from
lobe-finned fishes (Sarcopterigii) about 390 million years ago in middle Devonian period.
Tetrapods evolved primarily from aquatic forms later became semiaquatic like modern amphibians.
They spend half of their lives in water, breeding like a fish and remaining part of life on land.
Finally they became completely terrestrial by adopting amniotic sac so as to develop fertilized egg
inside mother’s womb (reptiles, birds and mammals are thus known as amniotes) and avoid
aquatic external fertilization like fish and amphibians. This ability enabled them to survive entirely
out of water. During this transition they underwent with several changes in body plan so as to
move swiftly and breathe on land. Several groups of tetrapods, such as the caecilians, snakes,

whales, sea lions have lost some or all of their limbs through further speciation and evolution;
some have only concealed vestigial bones as a remnant of what were their limbs in the distant past.
Many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of
the group. Modern examples of aquatic tetrapods that evolved back to aquatic life include
mammalian species such as whales, porpoises, seals, walruses etc.
Following are some salient features of Superclass Tetrapoda—
 four limbs (or descended from ancestors with four limbs)
 various adaptations of the skeleton and muscles that enable proper support and movement on
land
 adaptations to the cranial bones that allows the head to remain stable while the animal moves
 a layer of dead cells that reduces evaporation and water loss across the surface of the body
 well-developed muscular tongue
 the parathyroid gland that in part controls calcium levels in the blood
 a gland that lubricates the eyes (Harderian gland)
 an olfactory organ (vomeronasal organ) that enables the detection of pheromones and plays a
role in taste and smell
 an absence of internal gills
Superclass Tetrapoda is further classified into four classes as Amphibia, Reptilia, Aves and
Mammalia.
2.3.1: Class- Amphibia:
I. General overview
 Amphibians are cold blooded vertebrates which can hue on land and in water.
 Amphibians show four limbs with which they can swim in water and jump or walk on the
land, (But in apoda limbs are absent.). Limbs have 2 to 5 digits without claws.
 In Amphibians exoskeleton is absent. But in apoda animals small cycloid scales are present.
 In Amphibians the adult animals lungs are present. Gills are absent. But In some urodelans
the gills are present. Amphibian’s skin is moist, slimy and respiratory in function.
 The Amphibian skull is dicondylic. Amphibian procoelous vertebrae, lacks ribs.
 In Amphibians the body divisible into head and trunk tail, neck is absent. Anurans lack tail.
 Amphibian’s digestive system is well developed. A well-developed liver Is present
 External ear is absent. Middle and inner ears are present; the middle ear columella auris is
present.
 Amphibians Heart is 3 chambered with 2 auricles and 1 ventricle. The blood contains
nucleated R.B.Cs. Blood circulation is double. Blood vascular system contains hepatic and
renal portal systems.
 Amphibians Kidneys are mesonephric. Urinary bladder is present. It stores urine.

 Central nervous system is well developed. The brain occupies completely the cranial cavity.
The brain is divided into fore, mid and hind brains. Brain continuous as spinal cord. 10 pairs
of cranial nerves will arise.
 Sexes are separate. Male and female can be identified - Sexual dimorphism.
 Fertilization is external; eggs are telolecithal, cleavage is holoblastic and unequal. In
Amphibians the life history a larva stage (tadpole) may be present. Extensive parental care is
exhibited by amphibians.
 Parental care in amphibians is very elaborative. This is the phenomenon of care taking of the
eggs while incubation and hatching. It also includes the protection and feeding of young ones.
Anurans of tropics show greater diversity in parental care. It occurs in two types – (1)
protection through building and protecting the foam/mud/tree nests (Rana, Hyla), nurseries
(salamanders) and shelters (salamanders) protecting territories. (2) Direct caring by parents
such as coiling around the eggs (Ichthyophis), transferring tadpoles to water (Phyllobates,
Arthroleptis), gastric incubation (Arthroleptis), developing brood pouches (Pipa) and
vivparity (salamander).
II. Amphibia: Classification, characteristics and salient features up to orders:-
Class Amphibia is divided into three orders namely Apoda, Urodela and Anura. Their
characteristics and salient features are as discussed below—
Salient features of Order- Apoda or Gymnophiona (Limbless amphibians)
1. These animals are limbless. Hence they me called Apoda.
2. The body is long and snake like. Hence it is called Gymnophiona. The body is divisible into
head and hunk. Tail is absent
3. On the head two eyes are present. They are covered by skin and tiny cycloid scales. Hence they
are blind (Cecaelians) and hence called as blind worms.
4. Lungs are asymmetrical.
5. The skin shows minute cycloid scales. in the male apoda animals copulatory organs present.
Fertilization is internal.
e.g. Ichthyophis (blind worm)
and tail present
Superclass Tetrapoda Terrestrial; paired limbs and lungs are present
Class: Amphibia Aquatic as well as terrestrial, high parental care,
skin without exoskeleton and moist.
Order Apoda or Gymnophiona Legs are absent, tiny cycloid scales on skin
Genus: Ichthyophis Caecilian or blind worm

Ichthyophis (Blind worm)
These are limbless amphibians usually live in moist soil and lead a borrowing life. 2. It is elongated
and like animal measures up to 30 cm in length. The Ceylon caecilian grows to about 23 to 40 cm
(9.1 to 15.7 in) long and resembles a large earthworm. The skin is formed into over 300 transverse
folds which give the caecilian the appearance of being segmented. Skin is covered by tiny cycloid
scales along with slime glands & squirt glands to produce irritating fluid and is commonly known
as coecilla. The head has a rounded snout and a pair of extensible tentacles near the mouth, rather
closer to the eyes than to the nostrils. Skull compact, roofed with bony vertebrae. Limbs and
girdles are absent. Tail is short or vestigial. Eye is small. The colour of this caecilian is steely blue
above and pale yellow underneath, with a yellow band running along either side of the body. Male
copulates with female by a protrusible cloaca and thus fertilization is internal. The female shows
parental care by carefully coiling her body around the eggs till they hatch.
Salient features of Order- Urodela or Caudata (Tailed amphibians)
1. This order includes Salamanders and newts. These urodelans are more in North America.
Hence North America is called Head Quarters of Urodela
2. The body is divisible into head, trunk and tail. Only In these amphibians tail is present. Hence
these are called Urodela.
3. The fore limbs and hind limbs are equal.
4. The body’s not covered by scales.
5. In some adults the gills are presents
6. Some forms show neoteny and paedogenesis. In North India only one species is available,
Tilototriton venvcosa. In South India urodelan animals are absent.
e.g. Amphiuma (Congo eel)
and tail present
skin naked and moist
Order Urodella or Caudata Legs and tail are present
Genus: Amphiuma Congo eel

Amphiumas are eel-like salamanders with four weak and vestigial limbs having 2 or 3 digits and
long tail growing to about 1 M in length. It is popularly known as Congo eel, sometimes mistaken
for conger eel. They lack eyelids and a tongue. They have sharp prominent marginal teeth on the
upper jaw and dentaries, as well as a second row of teeth on the vomers aligned parallel to those on
the maxillae. Adults have no external gills but the paired lungs and a pair of gill slits.
All 3 species of amphiumas are aquatic inhabiting murky waters of swamps, ditches, streams,
marshes, lakes and calcareous swamps although A. means has been found active on land of N.
American during rainy nights. They are distributed in Southeastern and central United States,
including the southern half of the Mississippi River valley and along the coastal plain to Virginia,
through the Gulf States to the shore of the Gulf of Mexico. They are sympatric with many species of
crayfish and often seek shelter in abandoned crayfish burrows.
Salient features of Order- Anura or Salientia (Tailless amphibians)
1. This order Anura includes frogs and toads.
2. They can live in water and on land.
3. The fore limbs are small, the hind limbs are long.
4. In the adult stage tail is absent
5. They show a pair of eyes. tympanic membranes.
6. Their Life history includes a larval stage called tad-pole larva.
e.g. Hyla (Tree frog)
and tail present
skin without exoskeleton and moist.
Order Anura or Salientia Legs are present, tail is absent
Genus: Hyla Tree frog

Hyla (Tree frog)
Hyla arborea is 5 to 8 cm long and green in colour. Terminal bones of digits are swollen basically
into glandular adhesive discs which enable them to climb trees. An extra cartilage between two last
phalanges gives them a better grip. Large vocal sacs help in making a very loud voice which can be
heard from a long distance. During the mating season, tree frogs make loud croak-like calls to one
another in order to attract a mate. The female tree frog lays her eggs on a leaf above the water,
which develop into tadpoles in just a few days when they fall into the water below. The
metamorphic process from tadpole to adult tree frog can take anywhere from a few weeks to
several months. Hyla faber shows parental care by making enclosures in shallows water on the
border of the pond for protection. Genus Hyla includes arboreal frogs adapted for life in trees. It is
a large genus containing 350 species spread world over. True tree frogs are found inhabiting the
forests and jungles in the warmer regions all over the world.
2.3.2: Neoteny in Amphibia E.g. Axolotl (larva of Ambystoma salamander).
Axolotl Ambystoma
The larva of Ambystoma (tiger salamander) is known as Axolotl. It has three pairs of external gills
and a tail having a caudal fin. It exhibits the phenomenon of neoteny. Due to certain factors Axolotl
can’t undergo metamorphosis due to physiologically unable to produce TSH. Axolotls became
permanently neotonized when an ancestor was born with a defect in its hormonal system.
Metamorphosis in salamanders is triggered by a hormonal pathway involving the thyroid- axolotls
lost the ability to produce one of the first hormones in this sequence, thyroid stimulating hormone
(TSH), which triggers the thyroid to release thyroxine (T4) to start metamorphosis.

Neoteny is governed by some intrinsic and extrinsic factors such as—
1. Extrinsic factors: Insufficient food, salinity and lower temperature or iodine deficiency in
surrounding water may hinder metamorphosis in Axolotl.
2. Intrinsic factors: low responsive ness of tissue to thyroxine, genetic mechanism. Etkin and
his coworkers (1968) also exhibited that the elevated levels of prolactin inhibits the process
of metamorphosis.
These factors may affect metamorphosis in the Axolotl and it does not change into an adult. But
in the larval form it becomes sexually mature to start sexual reproduction. It was found that
injections of iodine as well as “thyroid extract” could trigger metamorphosis in the axolotl.
Iodine is used by the thyroid to manufacture thyroxine. Later scientists directly used thyroxine
to get the axolotl to metamorphose. The process is delicate even under laboratory conditions,
with many individuals not fully metamorphosing and/or dying. Axolotl were considered as
separate genus until 1865. Axolotl is found in mountain regions of Mexico.
FURTHER READING
Anamniotes and Amniotes
Further the tetrapods are represented by amniotes, example of those who have amniotic sac or
amnion which enabled them to reproduce without water and avoid external fertilization as seen
in fishes and amphibians. External fertilization had several constraints as they were easily
exposed to predation and environmental hazards. That is the reason why aquatic forms breed
more frequently and lay huge number of eggs (high fecundity) to ensure fertilization and
survival of the young ones. On the contrary, for amniotes, internal fertilization and carrying the
embryo in womb ensured high survival rate and also enabled to move away from water along
with saving lots of time and energy for other activities. On other hand, moving out from water,
not only enabled amniotes to escape from increased competition in aquatic environment but also
to explore unoccupied terrestrial habitat.
To adapt with the terrestrial environment tetrapods had to undergo several changes in their
body plan. Their gills were replaced by lungs; skin became dried and cornified; their limbs
The amniotic egg generally consists of four membranes: the amnion, the allantois, the chorion, and the yolk
sac. The amnion encloses the embryo in a fluid that serves as a cushion and provides an aqueous environment
in which it can grow. The allantois is a sac that holds metabolic wastes. The chorion encloses the entire
contents of the egg and together with the allantois helps the embryo breath by providing oxygen and disposing
of carbon dioxide. The yolk sac, in some amniotes, holds a nutrient-rich fluid (called the yolk) that the embryo
consumes as it grows (in placental mammals and marsupials, the yolk sac only stores nutrients temporarily and
contains no yolk).

became elongated and supported by extra muscles to locomote in terrestrial habitats. Their
vision, audition and olfaction improved. In egg laying amniotes parental care improved with
decreasing fecundity.
Key Characteristics of Amniotes
 amniotic egg
 thick, waterproof skin
 strong jaws
 more advanced respiratory system
 high-pressure cardiovascular system
 excretion processes that reduce water loss
 a large brain modified sensory organs
 larvae do not have gills
 undergo internal fertilization
Anapsids, Diapsids, and Synapsids
Amniotes are often described and grouped by the number of openings (fenestrae) that are
present in the temporal region of their skull. The three groups that have been identified on this
basis include the anapsids, diapsids, and synapsids. Anapsids have no openings in the temporal
region of their skull.
The anapsid skull is characteristic of the earliest amniotes. Diapsids have two pairs of openings
in the temporal region of their skull. Diapsids include birds and all modern reptiles. Turtles are
also considered diapsids (although they have no temporal openings) because it is thought that
their ancestors were diapsids. Synapsids, which include mammals, have a single pair of
temporal openings in their skull.
The temporal openings characteristic of amniotes are thought to have developed in conjunction
with stronger jaw muscles, and it was these muscles that enabled early amniotes and their
descendants to more successfully capture prey on land.
Classification
Amniotes are classified within the following taxonomic hierarchy:
 Birds (Aves) - There are about 10,000 species of birds alive today. Members of this group
include game birds, birds of prey, hummingbirds, perching birds, kingfishers, buttonquail,

loons, owls, pigeons, parrots, albatrosses, waterfowl, penguins, woodpeckers and many others.
Birds have many adaptations for flight such as lightweight, hollow bones, feathers, and wings.
 Mammals (Mammalia) - There are about 5,400 species of mammals alive today. Members of
this group include primates, bats, aardvarks, carnivores, seals and sea lions, cetaceans,
insectivores, hyraxes, elephants, hoofed mammals, rodents, and many other groups. Mammals
have several unique adaptations including mammary glands and hair.
 Reptiles (Reptilia) - There are about 7,900 species of reptiles alive today. Members of this group
include crocodiles, snakes, alligators, lizards, caimans, tortoises, worm lizards, turtles, and
tuataras. Reptiles have scales that cover their skin and are cold-blooded animals.
REFERENCES
 Modern text book of Zoology – Vertebrates; Professor R.L. Kotpal; Rastogi publication; Third
Edition
 Vertebrate Zoology; E.L. Jordan and P.S. Verma
 A manual of Zoology, Vol. II Vertebrata; Ayyar, M. Ekambaranath
 Vertebrate Zoology – Volumes of different Phyla; Hyman L.H.
 Vertebrate Zoology for Degree students; V. K. Agarwal; S.Chand Publication; 2012
 Vertebrate Zoology, Vol.II; Parker and Haswell
 Minor phyla – General information; Professor R.L. Kotpal; Rastogi Publication; Fifth Edition
 Vertebrate Comparative Anatomy, Function, Evolution; K.V.Kardong; Fourth Edition; Tata
McGraw Hill
 The life of Vertebrates; J.Z. Young; ELBS - Oxford University Press
 Practical Zoology; Second Edition; Dr. K.C. Ghose & Dr. B. Manna; New Central Book Agency
Pvt. Ltd. , Kolkata; 1999.

Chordata gnathostomata: TAXONOMY: T. Y. B.Sc. Zoology, Sem. VI; Course Code-USZO601; Unit-II: University of Mumbai 2017-18

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Chordata gnathostomata: TAXONOMY: T. Y. B.Sc. Zoology, Sem. VI; Course Code-USZO601; Unit-II: University of Mumbai 2017-18