Order Dipnoi is dividid into 2 suborders.
Suborder 1 . Monopneumona :- Lung single :- Lateral jointed rays of archipterygium (paired fins) well developed.
Example :-  Living Australia n Neoceratodus foresteri and extinct Triassic ceratodus.

Suborder 2. Dipneumona :-Lung double :- Lateral rays of archipterygium
(paired fin) vestigeal or absent.
Examples :- African Protopterus and Australian Lepidosiren paradoxa.


Affinities of Dipnoi

With special features of their own, the Dipnoi combine characteristics in which they resemble different groups of fishes as well as Amphibia.

Affinities with fishes

Lung fishes are true fishes beyond doubt as they resemble them in general in the following features :

  1. Body spindle-shaped and streamlined.
  2. Locomotory appendages fins.
  3. Diphycercal caudal fin.
  4. Largely ossified, slender dermal fin rays.
  5. Body covered by overlapping cycloid scales.
  6. Notochord persistent.
  7. Vertebrae without centra.
  8. Skull with little ossification and with several investing bones.
  9. Branchial arches 4 to 6 pairs present.
  10. Aquatic respiration by gills.
  11. Lateral line sensory system.

Affinities with Elasmobranchii

Lung fishes resemble cartilaginous fishes in the following primitive characters :

  1. Intestine with a spiral valve.
  2. Nephrostomies lacking in kidney tubules.
  3. Similar dienccphalon.
  4. Similar female reproductive system.
  5. Each gill arch with 2 efferent arteries.
  6. Similar conus arteriosus.

The main differences from elasmobranchs are presence of diphycercal tail, opercula and lungs, absence of claspers and external fertilization.

Affinities with Holocephali

According to Jarvick (1964, 1967), dipnoans and holocephalians resemble remarkably with each other as follows :

  1. Teeth fused into dental plates on jaws.
  2. Gills covered by operculum.
  3. Operculo-gular membranes of both sides fused.
  4. Intestine with a spiral valve.
  5. Jaw suspension autostylic.
  6. Excurrent nostrils opening into mouth cavity.
  7. Similar kidneys, gonads and ducts.
  8. Similar cranial nerves.
  9. Each gill arch with 2 efferent arteries.

Main differences from holocephalians are presence of lungs, absence of claspers and external fertilization

Affinities with Actinopterygii.

Lung fishes resemble subclass Actinopterygii in the following characters :

  1. Blunt snout.
  2. Body covered with cycloid scales.
  3. Paired inferior jugular veins.
  4. Powerful palatine and splenial teeth.
  5. Presence of swim bladder.
  6. Presence of operculum over gills.

However, the Actinopterygii belong to a separate evolutionary line. They have thin, broad fins modified for swimming, and the external nostrils never penetrate into the mouth. Most of them are small in size, have reduced snout, large eyes, single separate dorsal fin and a homocercal caudal fin.

Affinities with Crossopterygii

The two orders, Dipnoi and Crossopterygii were included under the subclass Sarcopterygii by Romer (1959). Instead of specializing for aquatic habitats (as did the Actinopterygii), they have adapted for a semiaquatic existence. They show many close similarities as follows :

  1. Powerful leg-like lobate fins.
  2. Caudal fin diphycercal.
  3. Internal nostrils piercing roof of mouth cavity in some cases.
  4. Similar skull bones.
  5. Vertebral column reaching upto the end of caudal fin.
  6. Air bladder modified as a lung with good pulmonary circulation,
  7. Contractile conus arteriosus.
  8. Larval forms in some cases with external gills as accessory respiratory organs.

Besides, the fossil crossopterygians (rhipidistians such as Devonian Osteolepis) and fossil dipnoans (such as Devonian Dipterus) show even closer affinity in :

  1. Similar body shapes and sizes (20 to 70 cm).
  2. Separate 2 dorsal, 1 anal and 1 heterocercal caudal fin, supported by dermal bony rays.
  3. Paired fins somewhat lobate with a fleshy scaly central axis. Pectoral fins placed high.
  4. Presence of internal nares in some.
  5. Cycloid scales modified from cosmoid type.
  6. Similar number and disposition of dermal bones on skull and pectoral girdle.
  7. No vertebral centra.
  8. Similar opercular and gular bones.
  9. Similar lateral line sensory system.
  10. Comparable lower jaw.

This led to the belief that the dipnoans are degenerate descendants of the crossopterygians, which the early dipnoans closely resembled. Bat Jarvik (1968) and others throw doubt upon this belief. According to them, certain structures are more specialized in dipnoans than in crossopterygians. Their basic differences are in structural organisation of food crushing apparatus, tin skeleton, vertebral column visceral skeleton, neural tndocranium, snout, dii:.ion of heart, atrium, blood supply of swim bhddec etc,

Affinities with Amphibia

Dipnoans resemble amphibians in several features such as ;

  1. Semiaquatic or marshy habitat
  2. Internal nostrils piercing roof of mouth cavity.
  3. Multicellular skin glands.
  4. Dermal scales present m Gymnophiona.
  5. Lungs capable of pulmonary respiration.
  6. Spiracles lacking.
  7. Vomerine teeth present.
  8. Auricle and sinus venosus partially divided into right and left halves.
  9. Conus arteriosus spirally twisted and longitudinally partitioned.
  10. Ventral aorta short.
  11. Presence of anterior abdominal vein, posterior vena cava, pulmonary artery and vein,
  12. Pericardium is thin-walled.
  13. Jaw suspension autostylic.
  14. Brain similar in structure of cerebrum and cerebellum.
  15. Sperms carried through excretory part of mesonephric kidney.
  16. Similar structure of egg and development.
  17. Larval stages having suckers and external gills.

This close similarity led early workers to conclude that lung fishes gave rise to amphibians, a view no longer held now-a-days. According to Dolo, these similarities probably were due to convergent evolution on account of similar habits and habitat. On the other hand, the lung fishes have the following special features by which they differ from amphibians.

  1. Lobate fins instead of limbs for locomotion.
  2. Fin skeleton not like that of primitive tetrapods.
  3. Peculiar crushing plates instead of teeth.
  4. Skull mainly cartilaginous with little ossification.
  5. Few anterior vertebrae fused with skull.
  6. Lungs located dorsal to gut.
  7. Urinary bladder develops from dorsal wall of cloaca in Dipnoi but from ventral wall in Amphibia.


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