In this article we will discuss about the classification of class chondrichthyes.
1. The class includes sharks, rays and chimaeras. They are mostly predaceous and marine.
2. The skin is tough, possesses minute placoid scales and mucous glands.
3. Skeleton cartilaginous and notochord persistent. The vertebrae are complete and separate.
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4. The median and paired fins are supported by cartilaginous fin rays. The pelvic fins in male are provided with claspers.
5. The tail is heterocercal.
6. Ventrally placed mouth with rows of teeth. Spiral valve is present in the intestine.
7. The swim bladder absent.
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8. The nostril one or two, never open in the mouth cavity.
9. The heart is with a contractile conus arteriosus and bears rows of valves.
10. Five to seven pairs of gills, each being placed in a separate cleft. No operculum over the gills except in Holocephali.
11. Sexes are separate. Gonads typically paired and gonoducts open into the cloaca.
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12. Eggs large and yolky. Fertilization internal.
The Class Chondrichthyes (Elasmobranchii) is divided into two subclasses, Selachii and Bradyodonti:
A. Subclass Selachii:
1. Modern sharks. The gills are placed in separate clefts and a spiracle is present behind each eye.
Order 1. Cladoselachii (Pleuropterygii) Devonian-Permian:
1. Shark-like, fusiform Chondrichthyes became extinct in Devonian to Permian periods. It is the ancestor of all modern sharks.
2. Tail heterocercal with a large lower lobe, and a horizontal keel-like fin on each side of the caudal peduncle.
3. Pectoral, pelvic fins and two dorsal fins were very prominent, sometimes with a spine in front of the first skeleton of fins consist of parallel cartilages of simple structure.
4. The large sub-terminal mouth, long sharp teeth, relatively enormous forwardly directed eyes, body covered with small denticles, amphistylic jaw suspension, a well- developed postorbital process, paired nostrils and absence of claspers are prominent features.
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Examples: Cladoselache, Cladodus, Symmorium, Diademonus, etc.
Order 2. Pleuracanthodii (Ichthyotomi): Devonian-Triassic:
1. Slender, fresh water, shark-like fishes having dorsal fin extending along the body and tail. It became extinct in Triassic period.
2. The pectoral fins are paddle-like and ‘archipterygium’ type, having long axis with pre- and post-axial rays. This type of fin is supported to be ancestral type of all fins.
3. The tail is diphycercal with equally developed blades of fin above and below the notochord.
4. A long spine on the head and claspers are present in male.
Example: Pleurocanthus.
Order 3. Protoselachii: Devonian-Cretaceous:
1. The pectoral fin is notched at the posterior margin.
2. Claspers in male present.
3. Two types of teeth, one sharply pointed, located anteriorly and the other flat type placed behind for crushing molluscan shells.
4. They flourished in Palaeozoic seas and became extinct in Permian to Triassic period. Some belonging to the genus Heterodontus are still surviving. From Protoselachii all the euselachians arose in the Mesozoic period.
Examples: Hybodus, Heterodontus, etc.
Order 4. Euselachii: Jurassic-Recent:
1. Sharks and rays constituting this group are abundant in equatorial and temperate seas.
2. Teeth numerous, developed in continual succession.
3. Pectoral fins have three basal pieces, the pro-, meso- and metapterygium, from which a number of pre-axial radials spread out.
This order is divided into two suborders:
Suborder i. Pleurotremata (Squaliformes):
1. It includes about 225 living species of sharks with spindle-shaped body and heterocercal tail.
2. Gill-slits 5 to 7 pairs—laterally placed.
3. The anterior margin of the pectoral fin is free from the body. The pectoral radials are simple and of a few segments, and, as a rule, only the anterior ones reach the free edge of the fin. The two halves of the pectoral arch are well-separated above.
4. Paired pre-orbital cartilages attached to the olfactory capsules are often well-developed.
5. The pterygoquadrate has no articulating process to the skull.
6. The hyomandibular and ceratohyal both bear cartilaginous rays, and both take part in supporting the first gill. The ceratohyal is a single cartilage and attached to the lower end of the hyomandibular.
The suborder includes four superfamilies:
a. Heterodontoidea.
Example: Heterodontus.
b. Notidanoidea.
Examples: Chlamydoselachus, Heptranchius, Hexanchus.
c. Gaboidea.
Examples: Lamm, Carcharodon, Carcharinus, Sphyrnn, Scyliorhimts, etc.
d. Squaloidea.
Examples: Squalus, Squantina, Laticaudus (Scoliodon), etc.
Suborder ii. Hypotremata (Batoidea, Rajiformes):
1. About 300 extant species including rays and skates with depressed body; chiefly bottom-dwellers in shore waters.
2. Pectoral fins enlarged, joined to the sides of the body or to the head. The pectoral radials are numerous, multi-segmented and bifurcated at the ends, all of them reaching the free edge of the fin. Swim by flapping.
3. The halves of the pectoral girdle either fuse with each other or else both fuse to the vertebral column.
4. Gills ventral, five pairs.
5. Spiracle highly functional.
6. Paired pre-orbital cartilages attached to the olfactory capsules, which are often very well-developed.
7. The pterygoquadrate has no articulating process to the skull.
8. The hyomandibular lacks rays, and takes no part in the support of the gill; ceratohyal is segmented and attached to the hyomandibular—either high up or not at all.
The hypotremata is divided into two groups—the Narcobatoidea including electric rays and the Batoidea including all other skates and rays.
Examples: Pristis, Raja, Rhinobatos, Torpedo, Dasyatis, Aetobatus, Manta, etc.
B. Subclass Bradyodonti:
1. This aberrant group diverged from the main cartilaginous stock at the Carboniferous period.
2. Vertebrae are reduced to nodules. Teeth are firmly attached to the jaws and modified into crushing plates.
3. Notochord remains un-constricted.
Order 1. Eubradyodonti: Devonian- Palaeozoic:
1. Primitive bradyodonts, remarkable for their dentition. The teeth are few and fuse to form flattened crushing teeth.
2. The nature of vertebrae, mode of jaw suspension and cleavage pattern are primitive.
3. They became extinct at the end of Palaeozoic era.
Examples: Cochliodus, Psammodus, Helodus, etc.
Order 2. Holocephali: Jurassic-Recent:
1. The surviving species are mollusceating, deep sea forms with claspers in male and large egg cases in females like those in elasmobranchs.
2. The general anatomy is similar to that of Euselachii.
3. The skin naked except for a few denticles restricted to the head claspers.
4. Jaw suspension holostylic, the upper jaw being immovably united with cranium. Neither skull nor jaw are attached to hyoid arch.
5. The extra claspers are on the head and in front of the ordinary pair on the abdomen.
6. Cloaca and spiracle absent.
7. An operculum covers the gills.
8. The dentition is highly peculiar in the composition of the tooth plates as well as in the shape.
9. The teeth are in the form of plates. The arrangement of these plates, producing the ‘parrot-like’ beak, has contributed largely to the modification of the whole skull.
10. The first dorsal fin is far forward with strong spine.
11. The tail is slender, whip-like.
12. About 25 species are found in cold ocean waters down to 1,800 meter.
Examples: Chimaera, Callorhynchus, Harriotta, etc..