In this article we will discuss about the classification of class reptilia.
Classification of Class Reptilia:
1. Body covered with dry cornified skin, usually with scales or scutes. Few surface glands are present.
2. Two pairs of limbs, each typically with 5 digits, ending in horny claws and suited to running, crawling or climbing. Limbs reduced or absent in Ophidia and paddle like in marine turtles.
ADVERTISEMENTS:
3. Skull with one occipital condyle. Skeleton completely ossified.
4. Heart imperfectly four-chambered.
5. Aortic arches one pair, erythrocytes oval, biconvex and nucleated.
6. Respiration always by lungs. In marine turtles, cloacal respiration occurs..
ADVERTISEMENTS:
7. Cranial nerves twelve pairs.
8. Variable body temperature, poikilothermous.
9. Fertilization internal. Eggs are much yolky, laid on land.
10. Segmentation is meroblastic. Embryonic membranes amnion, allantois, yolk sac develop.
ADVERTISEMENTS:
11. Metamorphosis absent.
The class Reptilia is divided into five subclasses—Anapsida, Ichthyopterygia, Lepidosauria, Archosauria, and Synapsida:
A. Subclass Anapsida:
1. Skull with solid roof, without opening behind the eye.
It has two orders— Cotylosauria and Chelonia:
Order 1. Cotylosauria:
Oldest reptiles ranging from upper Carboniferous to upper Triassic.
1. Skull roofed completely. Pelvis flattened, plate-like.
2. Body size varies from 30 to 60 cm.
ADVERTISEMENTS:
Two suborders—Captorhinomorpha and
Diadictomorpha are recognized.
Examples: Captorhinus, seymouria,Diadectes, etc.
Order 2. Chelonia (Testudinata):
i. Includes turtles, terrapins, and tortoises. Body box-like, broad, with rounded dorsal carapace and ventral plastron; joined at sides, and covered by leathery skin or polygonal scutes. The head and limbs are retractile within the box. About 335 extant species live on land, fresh or marine waters.
1. The nasal opening single and anteriorly disposed.
2. Anus longitudinal. Copulatory organ single Oviparous.
3. Teeth absent in adult and jaws covered with horny sheaths.
4. The ventricle is completely divided by the inter-ventricular septum.
5. The skull is secondarily anapsid, without temporal vacuities and monimostylic. Quadrate immovably articulated. Skull bones are firmly united to each other. A bony secondary palate separates the nasal cavity from the mouth.
6. Vertebrae are procoelous or amphicoelous. The vertebrae and ribs of thoracic region are firmly fused with the body carapace. The cervical vertebrae are eight, lumber vertebrae absent, sacral two and caudal are a few in number.
7. Sternum absent. The pectoral girdle consists of ventral coracoid and dorsal scapula attached to the carapace.
The order is subdivided into four suborders Eunotosauria, Amphichelydia, Pleurodira and Cryptodira. Eunotosauria became extinct in Permian and Amphichelydia in the Lower Cretaceous.
The modern groups are placed under last two suborders:
Suborder i. Pleurodira:
Side-necked turtles. Neck folds sideways under shell. Pelvis fused to shell.
Examples: Chelys, Chelodina, Pelomedusa, Podocnemis, etc.
Suborder ii. Cryptodira:
Neck bends in vertical S-curve, if retractile. Pelvis not fused to plastron.
Examples: Chelydra, Chelonia, Chrysemys, Dermatemys, Dermochelys, Emys, Macrochelys, Lepidochelys, Trionyx, Testudo, Terrapene, Sternotherms, etc.
B. Subclass Ichthyopterygia (Parapsida):
The roof of the skull with upper opening behind eye and bounded below by supra-temporal and postfrontal.
Order 1. Mesosauria:
Fresh water lake-dwellers, slender-bodied, not more than a metre long. The hind limbs were more powerful than the forelimbs and tail laterally compressed. They were found in the late Carboniferous or early Permian beds. Example: Mesosaurus.
Order 2. Ichthyosauria:
Marine fish-like reptiles occurred between mid-Triassic and upper Cretaceous. Some were of considerable length, even up to 10- 15 metres. Skull with a single temporal vacuity.
Large head produced into elongated snout. Tail long and limbs were in the form of paddles. The vertebrae were amphicoelous and cervical ribs with double head. Abdominal ribs were present. No sacrum, the ilia had lost connection with vertebral column.
Examples: Ichthyosaurus, Ophlhalmosaurus etc.
C. Subclass Synaptosauria (Synapsida):
Late Palaeozoic or Mesozoic aquatic reptiles having a single temporal (parapsid) vacuity high in the skull bounded above by postorbital and squamosal.
It has two orders:
Order 1. Protosauria:
Lizard-like terrestrial reptiles not exceeding 30 cm in length existed during lower Permian. The vertebrae were amphicoelous. Example: Araeoscelis.
Order 2. Sauropterygia:
Aquatic forms with single temporal vacuity in the skull bounded by a postorbital squamosal arch. The coracoids were single. Feet webbed. They existed between Permian and Triassic period.
The order includes three suborders Nothosauria, Placodontia, and Plesiosauria.
Examples: Placodus, Cyamodus, Elasmosaurus etc.
D. Subclass Lepidosauria:
Lizard-like reptiles with two temporal vacuities in the skull, although in some more specialized forms the vacuities reduced. The subclass comprises three orders.
Order 1. Eosuchia:
The easuchians are considered to be the ancestors of the squamata, and appeared in the upper Permian. They had inter-parietal and tubular bones and parietal foramen in the skull.
Example: Youngina.
Order 2. Rhynchocephalia:
The widespread representatives of this order are known from the mid-Triassic, but the sole present-day representative of this order is Sphenodon punctatum of coastal islands of New Zealand.
Sphenodon is the oldest surviving lepido-saurian and a Mesozoic fossil (Homoeosaurus) points to the continuity of the race.
1. Scales granular; a mid-dorsal row of low spines present.
2. Quadrate immovable and rami of mandible joined by ligament.
3. Teeth-acrodont.
4. Vertebrae amphicoelous and intercentra are preset.
5. The ribs are single-headed and have uncinata process.
6. Sternum and abdominal ribs present.
7. Anal opening transverse and hemi-penis absent.
8. Lungs, heart, and brain similar to Squamata.
Examples: Hyperodapedon, Sphenodon Rhynchosaurus, etc.
Order 3. Squamata:
The existing widely distributed lizards and snakes and some extinct forms are included in this order.
1. Skin with horny epidermal scales or shields.
2. Skull with temporal fossa and no inferior temporal fossa.
3. Quadrate bone movable. The maxilla, palatine and pterygoid are immovably articulated with the skull.
4. Lower jaw composed of several pieces of bones.
5. Teeth either acrodont or pleurodont.
6. The vertebrae are procoelous. Chevron bones present.
7. The ribs are single-headed.
8. The cloacal aperture transverse.
9. A pair of eversible hemipenes present.
The order is subdivided into two suborders—Lacertilia and Ophidia:
Suborder i. Lacertilia (Sauria):
1. Lizards with usually slender body and moderate mouth gape. Existing number of species 3,140.
2. Limbs are often well-developed, typically four, sometimes reduced or absent.
3. Mandible fused anteriorly.
4. Pterygoid in contact with quadrate.
5. Eyelids and a tympanum present.
6. Pectoral girdle well-developed or vestigeal in some.
7. Tongue usually entire.
Of the 30 families 26 are existing.
Examples: Hemidactylus, Calotes, Varanus, Phrynosoma, Atiolis, Coleonyx, Chamaeleo, Crotaphytus, Iguana, Lacerta, Heloderma, Anguis, Ophisaurus, etc.
Suborder ii. Ophidia (Serpentes):
Surviving species of snakes are about 2,500.
1. Elongated body and wide mouth opening.
2. Limbs, feet, ear openings, sternum, episternum and urinary bladder absent. The middle ear is degenerated.
3. The rami of the mandible joined anteriorly by ligaments at the symphysis Maxillae, palatine and pterygoids are freely movable.
4. Teeth slender, conical, usually on jaws and on roof of mouth (palatines and pterygoids).
5. No separate supra-temporal ossification.
6. Eyelids absent, eyes covered with transparent spectacles.
7. Tongue slender, bifid and protrusible.
8. Left lung reduced.
Examples: Python, Boa, Naja, Uropeltis, Anilins, Xenopeltis,Natrix, Bungarus, Crotalus, Vipera, Hydrophis, etc.
E. Subclass Archosauria:
It is a great and widely varying assembly of reptiles probably all descendants of Thecodontia.
1. The skull is diapsid without inter-parietal, tabular bones or a parietal foramen.
2. The palatal teeth were lost in almost all except few primitive forms.
3. The lower jaw with a vacuity on the outside between dentary, sub-angular and angular.
4. Antorbital vacuity in front of the orbit almost always present.
5. Some were bipedal and exhibited correspondingly striking modification in the limb girdles.
Archosaurians are arranged in two orders—Thecodontia and Crocodilia.
Order 1. Thecodontia:
1. Carnivorous small reptiles arose in the Triassic period.
2. The hind legs were longer, slim skulls, and sharp teeth in sockets along the jaw edges.
3. Some were aquatic with features like modern crocodiles.
4. The external nostrils were sometimes set above the skull level, posteriorly placed near the eyes, enabling the animal to breathe white in water.
Examples: Omithosuchus, Aetosaurus, etc.
Order 2. Crocodilia:
The amphibian, carnivorous, extant crocodiles, alligators, caimans, gavialis and their allies are included in this order and are the largest existing reptiles, the only remnants of a once widespread Mesozoic archosaurian stock.
1. The elongated body with powerful long heavy compressed tail, four limbs, five digits in the forelimbs and four digits in the hind limbs and forelimbs webbed.
2. Body covered with rows of sculptured bony scutes. Epidermal scales present.
3. The head large and long with powerful long jaws, rimmed with numerous bluntly conical teeth.
4. The external nostrils occur at the end of the snout which can be closed or opened by muscular system during submersion.
5. Tympanic membrane is protected by two scaly flaps which close the external meatus at the time of diving. Eustachian system present.
6. Cloacal opening longitudinal. A median erectile grooved penis in male and a clitoris in female is present.
7. The Quadrate is immovable.
8. The secondary palate is formed by grown out portions of maxillae-palatines and pterygoids and internal nares open into the throat.
9. The centra of vertebrae are either amphicoelous, fait at each end or procoelous. The anterior therecic vertebrae have elongated, bifid transverse processes. The sacral vertebrae are two. The first caudal vertebra with convex ends.
10. The ribs are bifid at vertebral end.
11. A sternum and abdominal ribs present.
12. The pelvis has pubis extended from the acetabulum.
13. The ventricle of the heart is completely divided, but the aortic arches two. The roots of the left and right aortic arches are communicated by an aperture, the foramen of Panizzae.
Examples: Crocodylus, Gavialis, Alligator, Caiman, Tonistoma, etc.
Order 3. Pterosauria (Pterodactyla):
1. Toothed, tailed flying fossit reptiles with pnenmatic skeleton, like birds. Fore limbs were converted to wings; fourth finger highly enlarged to support membranous wing, other fingers were small. The sternum was keeled, the external nares placed far back. The eyes bordered with sclerotic plates.
Examples: Pterauodon, Pterodactylus. Orinothodesmuns, etc.
Order 4. Saurischia:
Pelvic girdle was triradiate, teeth were on the preniaxillae. There were two or one antorbital vacuities in the skull.
Examples: Tyrannosaurus, Branchiosaurus, Gorgosatirus, etc.
Order 5. Ornithischia:
1. Extinct bird-like herbivorous dinosaurs with a pelvis, essentially like that in modern birds, in which the pubis lay parallel with an extended ischium.
2. The presence of predentary bone in mandible might have supported the beak.
Examples: Iguanodon, Demetrodon, Nodosaurus, Ankylosaurus, Stegosaurus, Anatosailrus, etc.
F. Sub-Class Synapsida (Synapto- Sauria):
1. Mammal-like replites, skull roof with lower opening and lateral temporal vacuity behind eye, bounded above by postorbital and squamosal.
2. The lower jaw flat with heterodont teeth.
3. Pre-coracoids and coracoids were present in shoulder girdles.
The sub-class has three orders— Pelycosauria, Therapsida and Ictidosauria:
Order 1. Pelycosauria:
1. Herbivorous or carnivorous forms existed during upper Carboniferous and Lower Permian period.
2. Skull with temporal vacuity. Neural spines much elongated. Limbs small.
Examples: Dimetodon, Varauosaurus, Edaphosaurus, etc.
Order 2. Therapsida:
1. Large, heavy, mammal-like reptiles existed between Permian and lower Triassic, with double occipital condyle in some, big temporal vacuity in skull, reduced quadrate and quadratojugal bones.
2. In some forms secondary palate was present.
3. Teeth distinctly differentiated into incisors canines and molars.
Examples: Lycosaurus, Cynognathus
Dicynodon, Endotlriodon, etc.
Order 3. Ictidosauria:
1. Mosaic reptilian group with most of the pro-mammalian features. Existed between upper Traissic and mid Jurassic time.
2. The large temporal opening confluent with the orbit.
3. Quadrate and articular bones reduced.
4. Dentition of heterodont type.
Examples: Tritylodon, Bienotherium, etc..