In this article we will discuss about the classification of amphibia.
About 2,500 extant species of three divergent groups—frogs and toads, salamanders, and caecilians are included in Amphibia.
1. The skin is moist, glandular and without scales.
2. A distinct neck is absent.
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3. Typically, two pairs of limbs, no paired fins. Forelimbs with four and hind limbs with five clawless digits.
4. Two nostrils connected with the mouth cavity.
5. Skull with two occipital condyles.
6. Heart 3-chambered; two atria (auricles) and one ventricle; two other chambers— sinus venosus and conus arteriosus, present.
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7. Respiration by gills, lungs, skin or lining of buccal cavity.
8. Tympanum present; a rod-like collumela connects the tympanum and inner ear.
9. Eggs with gelatinous coverings, usually laid in water. Larvae usually aquatic.
The class Amphibia is divided into two subclasses:
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1. Apsidospondyli and
2. Lepospondyli.
(1) Subclass Apsidospondyli:
The central of vertebrae formed from cartilage blocks in units of two, which are ossified in varying degrees as anterior intercentra and posterior pleurocentra.
Apsidospondyli are placed into two superorders—Labyrinthodontia and Salientia:
(A) Superorder Labyrinthodontia (Stegocephalia):
1. The cross section of the teeth exhibits a prominent radiating in-folding of the enamel surface, hence the name.
2. Tail present; legs all of about same size.
3. Cranium and cheeks completely roofed with bony plates.
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4. Often armoured ventrally with overlapping scales.
Order 1. Ichthyostegalia:
The’ earliest known amphibians, perhaps the nearest to true prototetrapods, were prevalent from upper Devonian to upper Carboniferous.
2. The skull is about 20 cm long with three interesting features.
i. A preopercular and possibly subopercular persisted on the back of squamosal and quadratojugal bones.
ii. The septomaxilla formed part of the dermal covering of the skull.
iii. Like Dipnoi and some early crossopterygians, the nose bears a pit on the under-side of the skull, bridged by a process of the maxilla and divided into anterior and posterior part.
Examples: Ichthystega, Ichthystegopsis, etc.
Order 2. Rhachitomi:
Typical labyrinthodonts flourished during the Permian and Triassic, became subsequently extinct.
1. About 150 cm long with short powerful legs and five toes.
2. Large depressed skull with large inter-pterygoid cavities and otic notch.
3. The rhachitomous vertebrae had a semilunar wedge-like inter-centrum and one or two posterior pleurocentra in addition to a vertical neural arch.
Example: Eryops .
Order 3. Stereospondyli:
Transitional types arose from rhachitome stock, lived in the early Triassic waters, became extinct towards the end of Triassic.
1. Heavy-headed, shallow-water, bottom- dwellers of large size.
2. Vertebrae with pleurocentra removed, the intercentra joining almost the whole structure.
3. Skull flattened, ossification much reduced, except in the exoccipitals, which formed a double condyle.
4. Para sphenoid highly developed and apparently had firm union with pterygoids.
5. Eyes dorsally placed.
6. Massive supporting pectoral plate formed from Clavicles and interclavicles. Examples: Capitosaurus, Cyclotosaurus, Paracyclotosaurus, Brachyops, etc.
Order 4. Embolomeri:
Fish-like labyrinthodonts appeared in the Carboniferous, became abundant during the Pennian and extinct at the end of Triassic. Some were superficially crocodile-like.
1. The teeth had the dentine in-folded at the base into labyrinthine grooves.
2. The vertebrae were embolomerous, consisting of a neural arch resting on two notochordal centra—an anterior inter-centrum and a posterior pleurocentrum.
3. The shoulder girdle with increased size.
4. The pelvic girdle not directly attached to the vertebral column but held in place by ligament.
Examples: Eogyrinus, Palaeogyrinus, etc.
Order 5. Seymouriamorpha:
The group represents a stem apparently developed astride the transition line between Amphibia and Reptilia at the lower Permian.
1. Head relatively small, pointed; dorsal nostrils; a comparatively thick body and a short tail.
2. All its limbs were pentadactyl, short, muscular and thrust from the midline.
3. The atlas and axis were formed from anterior cervicals.
4. Interclavicle long and a ventral coracoid ossification in the shoulder girdle.
5. The pelvic girdle was anchored by means of two sacral vertebrae.
6. The ilium was dorsally expanded for attachment of strong walking muscles.
7. The shoulder girdle lay beneath the jaw.
8. The ribs were larger and double-headed.
Example: Seymouria
(B) Superorder Salientia (Anura, Batrachia):
1. Tailless frogs and toads with short and broad bodies and long hindlimbs.
2. The earliest fossil records are found as early as Jurassic period.
The Salientia is divided into three orders- Eoanura, Proanura and Anura.
Order 1. Eoanura:
1. These are “dawn frogs”—ancestral to the modern frogs and were prominent in upper Carboniferous.
2. Cranial, vertebral and other features are labyrinthodoritine.
Examples: Amphibamus, Miobatrachus, etc.
Order 2. Proanura:
They were ancestral form of Anura in the Triassic period in Madagascar.
1. The trunk was longer than that of the modern frog.
2. The skull unlike that of modern frog.
3. The postcranial skeleton, including caudal vertebrae, was not otherwise anuran; the ilia were elongated
Example: Protobatrachus.
Order 3. Anura (Salientia):
Tailless modern toads and frogs with more than 2,200 extant species arranged in ten families;
1. Skull thin without solid roof; bones few.
2. Vertebrae few, and last is a slender urostyle.
3. Ribs reduced or none.
4. Egg deposition and fertilization usually external.
Examples: Bufo, Rana, Rhacophorus, Xenopus, Ascaphus, Hyla, etc.
(2) Subclass Lepospondyli:
1. These are small late Palaeozoic amphibians, ranging from a few inches to a few feet in size.
2. The- vertebrae lepospondylous, i.e., centra formed directly of bone around notochord, not preceded by cartilage.
3. A few of these animals are superficially snake-like.
Order 1. Aistopoda:
Small sized, body snake-like with as many as 100 vertebrae and with forked ribs, found in Carboniferous deposits.
Example: Ophiderpeton.
Order 2. Nectridia:
1. One type was superficially snake-like, limbless and another type with small limbs – and a flattened skull, occurred in the Upper Carboniferous.
2. The skull bore an extraordinary armament formed by the outgrowth of the tabulars.
3. Cranium was largely unossified.
Examples: Sauropleura, Diplocaulus, etc.
Order 3. Microsauria (Adelospondyli):
1. Permian urodeles but retained too many labyrinthodont features to be closely related to caudata.
2. The neural arches were loosely attached to centrum.
3. Skull incompletely roofed; circumorbital bones lost, which left the orbit open below.
4. Gill breathing in adult continued.
Example: Lysorophus.
Order 4. Phyllospondyli:
1. The broad, heavily roofed skull with a highly diagrammatic arrangement of the investing bones.
2. Two occipital condyles were present in the skull.
3. The notochord and nerve cord remained in the same cavity.
Example: Brafichiosaurus.
Order 5. Urodela (Caudata):
1. Newts and salamanders; degenerate animals with a determinable ancestry from the Cretaceous period. More than 250 species are existing today.
2. The head, trunk and tail usually distinct.
3. Limbs are about equal-sized.
4. Limb girdles largely cartilaginous.
5. No dermal shoulder girdle.
6. Larvae resemble adults in form and have teeth in both jaws.
Modern urodels are placed in eight families.
Examples: Cryptobranchus, Necturus, Atnphiuma, Siren, Typhlomolge, Ambystoma, Trilototriton, Salamandra, etc.
Order 6. Apoda (Gymnophiona, Caecilia):
1. Aberrant, blind, worm-like, limbless, tropical amphibians with a very short tail and intromittant organ in the male.
2. About 55 species are existing today and without fossil record.
3. The body is usually grooved transversely, and in the grooves are often series of minute scales, a legacy from carboniferous ancestors.
4. The eyes are lidless and sometimes covered by cranial bones.
5. A protrusible sensory tentacle between nostril and orbit present.
6. The vent is almost terminal.
7. The skull compact, roofed with bones.
8. The vertebrae amphicoelous and many.
9. The limb girdles absent.
10. The eggs large and yolky, laid on land.
11. Cleavage meroblastic.
Examples: Typhlonectes, Rhinotrema, Coecilia, Ichthyophis, Caymnopis, Ureotyphlus, etc.