In this article we will discuss about Dipnoi:- 1. Introduction to Dipnoi 2. Geographical Distribution of Dipnoi 3. Palaeozoic Records 4. Habit and Habitat 5. External Structures 6. Skeletal Structures 7. Digestive System 8. Respiratory System 9. Circulatory System 10. Nervous System 11. Endocrine System 12. Excretory System 13. Reproductive System 14. Affinities 15. Cytogenetic Evidence.
Contents:
- Introduction to Dipnoi
- Geographical Distribution of Dipnoi
- Palaeozoic Records of Dipnoi
- Habit and Habitat of Dipnoi
- External Structures of Dipnoi
- Skeletal Structures of Dipnoi
- Digestive System of Dipnoi
- Respiratory System of Dipnoi
- Circulatory System of Dipnoi
- Nervous System of Dipnoi
- Endocrine System of Dipnoi
- Excretory System of Dipnoi
- Reproductive System of Dipnoi
- Affinities of Dipnoi
- Cytogenetic Evidence of Dipnoi
1. Introduction to Dipnoi:
The dipnoans are generally called ‘lung- fishes’. The group, Dipnoi owes its name from the presence of two internal nostrils. Again the name Dipnoi, gets from its two breathing devices, gills and lungs. Standing on the basic piscine platform, dipnoans show many interesting features that the early fishes have gone through to become the land vertebrates, especially the amphibians.
This makes the systematic position of the group controversial. The dipnoans evolved during the middle Devonian and flourished well in the Permian and Triassic periods. They became rare after Triassic and are represented now by three specialized genera.
2. Geographical Distribution of Dipnoi:
The surviving dipnoans are represented by three genera (Fig. 6.37), Neoceratodus (Ceratodus), Lepidosiren and Protopterus respectively. Neoceratodus forsteri (Fig. 6.37C) lives in the Mary and Burnett rivers of Queensland, Australia.
This Australian lung-fish was, until recently, an inhabitant of the Darling river system. Lepidosiren paradoxa, the South American lung-fish, (Fig. 6.37B) lives in the tributaries of the Amazon. Protopterus is restricted to the fresh waters of Central Africa (Fig. 6.37A).
It has four species. P. aethiopicus, P. annectans, P. dolloi and P. amphibius. P. aethiopicus lives in the basins of the Nile, Congo and in the tropical Lakes Victoria, Kvoga, Edward and Albert of Eastern Africa. P. annectans occurs in West Africa, P. dolloi from the Ogoue and the Congo, and P. amphibius in the Lakes of East Africa. Protopterus and Lepidosiren belong to the same family, Lepidosirenidae and Neoceratodus is the only genus of the family Ceratodontidae. It is apparent that despite the oceanic barriers separating them, the living dipnoans have special relationship.
3. Palaeozoic Records of Dipnoi:
The earliest known fossil lung-fish, Diabolichthyes has been recorded in the early Devonian period. It possessed some features which resemble with rhipidistians, suggesting that the species reflects a transitional stage in between rhipidistians and extant lung-fishes.
Others like Dipterus (Fig. 6.59A), Pentlandia, Scaumenacia, Phaneropleuron, Rhynchodipterus and Griphognathus, of which Dipterus has been recorded from middle Devonian and the rest from the upper Devonian.
Uronemus is present in lower Carboniferous and Ctenodus from Carboniferous and Permian. Dipterus valenciennesi is the Middle Devonian form which had an elongated body, thick cycloid scales of the cosmoid type and a heterocercal tail. The skull bones have the similarity with the Osteolepis.
There were no premaxillae and maxillae. The jaw suspension resembles the autostylic condition. The broad, ridged tooth plates were developed on prevomers and palatopterygoids. Throughout the dipnoan evolution, the durophagous condition has persisted, an adaptation for feeding on hard foods like mollusces and other invertebrates with the help of broad tooth plates.
4. Habit and Habitat of Dipnoi:
The three living dipnoans inhabit the rivers and are also capable of breathing air by ‘lungs’. Protopterus lives in large lakes and rivers of tropical Africa. It can survive even when the rivers become completely dried up in summer. It can dig into the mud and aestivate (summer sleep) there for at least six months in a ‘cocoon’ made out of clay and mucus (Fig. 6.38).
The mouth of the cocoon is closed by a lid which is perforated by a minute pore for the entry and exit of air. During the period of aestivation, nutrition is derived from the stored fat. Transshipment of the cocoon is possible even when the fish is enclosed in a cocoon.
This habit of aestivation is adopted by the group since the Permian period. This fact is attested by the remains of cylindrical burrows associated with fossilized dipnoan bones. Lepidosiren has also the property of aestivation to escape death during summer. It lives in rivers which become shallow and stagnant in summer, but never dry up completely.
It lives mostly at the bottom of the river. During summer when the water-level becomes low and toxic due to the decomposition of the organic materials, Neoceratodus thrives well by switching over to pulmonary respiration. When the water is plenty and oxygen is easily available, Neoceratodus adopts gill-respiration. Neoceratodus dies if it is taken out of water.
All the surviving general of the lung- fishes are sluggish in nature and are bottom- dwellers. They are carnivorous, although Lepidosiren sometimes feeds on plant materials. It lives mainly on snails. Protopterus is predaceous in nature and feeds on worms, crustaceans, insects, frogs and many other small animals.
These fishes are often found to attack their own species and bite off portions of the limbs, tail and other parts of the body. The lost parts are regenerated by the animals. Neoceratodus is also a carnivorous fish which devours molluscs, crustaceans and other worms.
In Lepidosiren and Protopterus, parental care is present. The eggs are laid in muddy nests. The nest is simply a tunnel of about 30 cm deep. The male usually guard the nest till the development is complete. The parental care is highly specialised in Lepidosiren because its pelvic fins become specially modified during breeding season.
The pelvic fins become greatly enlarged and highly vascularized. The significance of such modification is either to release excess of oxygen to the surrounding water for the respiration of the developing youngs or possibly to act as accessory respiratory structure to compensate aerial respiration while remaining around the nest.
5. External Structures of Dipnoi:
The three extant dipnoans have elongated piscine body covered by overlapping cycloid scales. The scales are thin, but in the fossil genus Dipterus, the scales were thick and covered by cosmine. The dorsal, anal and tail fins are continuous and are supported by partly calcified fibre-like rays called camp-totrichia. Two small dorsal fins were present in Dipterus.
The pectoral and pelvic fins are usually designated as the ‘limbs’. These are extremely elongated, filamentous structures and are devoid of fin-rays. These are highly mobile and help in ‘walking’ along the bottom by suing the so-called limbs as ‘legs’. The paired fins in all of them are typically of ‘archipterygial’ types, each having an axis with two rows of radials.
The tail is diphycercal (Protocercal or isocercal) in the living genera, but in Dipterus it was heterocercal with a small epichordal lobe. The operculum and a slit-like branchial opening are present on either side. The spiracles are absent.
The external nostrils are enclosed within the upper lip and two internal nostrils open into the mouth cavity. The lateral line sensory system is well-developed. The cloacal aperture lies at the root of the tail. Two abdominal pores usually open into the cloaca.
6. Skeletal Structures of Dipnoi:
The internal skeleton in the dipnoans is largely cartilaginous in nature. The tenancy to reduce internal ossification may be neotenous. It is composed of axial and appendicular skeleton.
Axial skeleton:
The notochord persists as an un-constricted rod and remains en-sheathed by tough fibrous covering. The vertebrae are represented by paired neural or basidorsal and basiventral cartilages which form the neural arches in the trunk and both the neural and haemal arches in the tail region respectively.
The vertebrae lack centra. Distinct joint between the skull and vertebral column is lacking. The pleural ribs, like that of other teleosts, are present in the body wall.
The skull shows tendency towards reduction of bones in the roof. The anterior part of the roof is incomplete. The roof and walls of the cranium are mostly composed of parietals and frontals. The cranial floor is formed by the Para sphenoid. The Para sphenoid is expanded and assumes rhombic shape. The premaxillae, maxillae and nasals are totally absent.
The vomer is narrow. The jaw suspension is autostylic, because the mandible is attached with the skull by a palatoquadrate. The lower jaw is composed of paired Meckel’s cartilages, tooth-bearing coronoid and supraangulars. In Neoceratodus, vestigial dentaries are present. The hyoid and branchial arches are all cartilaginous in nature. The branchial arches bear gill-rakers.
Appendicular skeleton:
The pectoral girdle consists of a stout cartilage with a pair of investing bones, the cleithra and infraclavicle. The pectoral girdle is attached to the skull by ligament. The pelvic girdle is composed of a cartilaginous plate with a long epipubic and pre-pubic processes.
Each pectoral fin consists of a long basal cartilage, a central axis with rows of jointed cartilaginous radials. The central axis is made up of small cartilaginous pieces with pre-axial and postaxial radials on the sides. The pelvic fin has a similar structure except that the pre-axial radials are lacking.
7. Digestive System of Dipnoi:
The food of the lung-fishes consists largely of the invertebrates (specially the molluscs) and decomposed vegetation. The teeth form characteristic tooth-plates for crushing the molluscan shells. The tooth-plates are formed by the fusion of many small denticles.
The tooth-plates are borne on the bony plates. Each tooth-plate produces into three major cusps. The shape and disposition of the tooth- plate is produced into three major cusps. The shape and disposition of the tooth-plates depend on the degree of adaptation to crushing and shearing the molluscan body. The alimentary canal is a simple tube. The pharynx leads into an oesophagus.
The lung-fishes lack distinct stomach. The posterior region of the oesophagus shows slight dilatation which is regarded by many as a part of the stomach (Fig. 6.39A). The cavities between the stomach and intestine are separated by a flap-like pyloric valve. The intestine is ciliated and contains a spiral valve running along the entire length of the intestine and makes about six and a half turns (Fig. 6.39C).
The spiral valve ends at a short distance ahead of the cloacal aperture. This portion is designated as rectum which opens into a small cloaca. The cloaca extends upward to the urinogenital papilla and then forwards as a closed bladder (rectal gland). The cloacal aperture is situated on the left side of the median fin.
The liver is a single massive gland which is slightly divided into two unequal lobes. The liver lies ventrally to the right side of the stomach. The gall-bladder is large and situated on the left margin of the liver. The pancreas remains embedded within the walls of the gut. The islets of Langerhans are not seen in the pancreas of the dipnoans.
The spleen is composed of vascular tissue and is attached to the right dorsolateral wall of the stomach. The alimentary canal exhibits little histological difference and the whole of the gut is lined by columnar, ciliated and goblet cells. The gut, particularly its posterior portion, is kept in position by a ventral mesentery which is attached to the ventral body wall.
8. Respiratory System of Dipnoi:
Both gill and pulmonary respiration take place in the lung-fishes. Although the dipnoans possess the gills as well as lungs, they use mostly the lungs. The nostrils help in aerial respiration. The external nostrils lie at the margin of the mouth and the internal nostrils open into the buccal cavity.
A slit-like glottis is present in the floor of the oesophagus which opens into a short trachea. The trachea passes into the lungs around the right side of the oesophagus. The glottis is provided with a fibro cartilaginous plate which resembles the epiglottis.
The swim-bladder is modified into the ‘lung’ which is similar to that of other tetrapods in structure and function. The main difference lies in its topography. The lung is placed dorsal to the gut while in the tetrapods it is ventral. The dorsal position of the lung in the dipnoans is regarded to be the result of shifting from its original ventral to the dorsal side.
The walls of the lungs contain muscle fibres and the internal cavity produces numerous alveoli which lead into minute alveolar sacs. In Protopterus and Lepidosiren the supply of blood to the lungs is elaborate. The blood is supplied to the lungs by the sixth embryonic right afferent branchial arch as seen in amphibians. The blood from the lungs is returned by a special pulmonary vein.
Aquatic respiration takes place through the gills. The gill-structure is similar in many respects to that of other crossopterygians except the absence of mandibular pseudo- branch and having only a vestigial mandibular pouch.
Neoceratodus, the most aquatic of the dipnoans, possesses an hyoidean hemi branch and four holobranchs. In the lung-fishes which depend more on the aerial respiration, the gills help in the excretion of carbon-dioxide. Protopterus and Lepidosiren obtain 98% of their oxygen from the air.
9. Circulatory System of Dipnoi:
The circulatory system is well developed. The blood is composed of all the cellular entities observed in higher vertebrates. The introduction of pulmonary respiration in the lung-fishes results in the attainment of much complicacy in the cardiac structure. The heart is enclosed in a stiff pericardium.
The heart is situated somewhat posterior to the gills. The heart of Protopterus and Lepidosiren is typically built on the same plan, but in Neoceratodus the heart is slightly different (Fig. 6.40). The heart of the lung-fishes consists of four chambers, the sinus venosus, auricle, ventricle and conus arteriosus.
The sinus venosus is incompletely divided into two parts. It opens into the auricle by a broad sinuauricular opening. The auricle becomes dilated on either side of a thin and perforated inter-auricular septum, i.e., the cavity of the auricle is almost divided.
As the septum contains pores, the blood of the two auricular cavities becomes mixed up. The cavity of the right auricle receives venous blood from the sinus venosus while the left cavity gets oxygenated blood from the pulmonary vein. The pulmonary vein from the lungs passes through the sinus venosus. The auricles are communicated with the ventricle by a large auriculoventricular aperture.
This aperture is plugged by a large fibrous cushion, called auriculoventricular cushion. It is continued into the ventricular cavity as an incomplete inter-ventricular septum.
Although the ventricle appears to be divided into two parts by the presence of septum, the ventricular cavity is single and lies anterior to the so-called inter-auricular septum. The presence of auriculoventricular cushion is peculiar and the auriculoventricular aperture may be opened or closed by raising or lowering the cushion.
The conus arteriosus becomes spirally twisted and the cavity becomes complicated by the presence of valves. A spiral veins is present. It begins ventrally and extends forward to the anterior end of the conus. Three rows of proximal valves are present in the conus of Protopterus and Lepidosiren. But in Neoceratodus the conus lacks the spiral valve and a series of semilunar valves marks the course.
There are a few rows of proximal and two rows of distal valves in the conus of Neoceratodus. The valves in the conus are so arranged that the blood from the right side of the auricle is directed into the last two branchial arches and that from the left side into the first two. By this way a mechanism towards the separation of systemic and pulmonary circulations is achieved.
The lung-fishes possess inconspicuous ventral aorta. The four afferent branchial arteries arise from the anterior end of the conus immediately outside the pericardium (Fig. 6.41). The afferent artery carrying blood to the hyoidean hemi branch originates from the first one.
The arches bearing gills are provided with afferent and efferent branchial branches. Two efferent branchial arteries from each gill- bearing arch join to form four epibranchial arteries. These four epibranchial arteries of either side join to form a single median dorsal aorta.
The pulmonary arteries carry the blood to the lungs. Neoceratodus is peculiar by lacking the second efferent branchial vessel. In all the lung-fishes, a coronary artery is present which arises from the anterior efferent branchial arches.
The venous system presents certain advanced features which are not observed in other fishes (Fig. 6.42). The blood from the anterior part of the body is returned to the heart by two Percivals or ductus Cuvieri. Each precaval is formed of an inferior jugular, anterior cardinal and subclavian veins. But the left precaval, in addition to the three veins, receives the posterior cardinal vein.
The posterior parts of both the posterior cardinal veins constitute the renal portal veins and are connected by interconnecting cross-channels. Presence of an inferior vena cava is a characteristic feature in the venous system of lung- fishes. This vessel collects majority of the blood from the posterior part of the body and conveys it directly to the sinus venosus.
The inferior vena cava enters into the liver and also receives the hepatic veins. The hepatic portal drains the intestine by the sub-intestinal and infra intestinal veins. Both the inferior vena cava and the left posterior cardinal vein have been formed by the renal veins draining blood from the kidneys. Both these veins receive segmental and genital veins on their way to the heart.
The blood from the tail region is carried by a caudal vein which is bifurcated into two renal portal veins. The renal portal veins break up into capillaries inside the kidneys. In Neoceratodus, the caudal vein opens into the inferior vena cava and not into the renal portal veins as seen in other two genera.
In Neoceratodus, the renal portal vein is connected by a ventral abdominal vein and the caudal vein by the lateral cutaneous veins. The pulmonary veins carrying blood from the lungs unite to form a common pulmonary vain and opens into the left auricle. The venous system presents many peculiarities and the posterior portion shows an intermediate stage between that of Elasmobranchs and Amphibia.
10. Nervous System of Dipnoi:
The brain of lung-fishes (Fig. 6.43) shows many characteristics resembling that of amphibians. The telencephalon becomes evaginated into a pair of well-marked cerebral hemispheres. These are elongated structures and are united at the posterior ends but remain distinct anteriorly.
The olfactory lobes are sessile and lie dorsal to the anterior ends of the cerebral hemispheres. The roof of the cerebral hemispheres is thin but nervous in nature. The diencephalon is relatively small and its roof is formed of a large mass of choroid tissue, the saccus dorsalis.
A pineal body is present on the saccus dorsalis and its stem extends back towards the posterior commissure. The hypothalamus bears small inferior lobes. The optic lobes are slightly developed and become fused to form single oval mass in front of the cerebellum. The cerebellum is small and forms a narrow transverse ridge.
The roof of the diencephalon is covered over by anterior plexus which projects into-the third ventricle while the fourth ventricle is formed of choroid plexus. The brain of Neoceratodus is slightly different and the cerebral hemispheres are smaller in size and the roof is membranous. In Neoceratodus, the optic lobes are slightly separated and form paired lobes.
A peculiarly lobes saccus endolymphaticus or endo-lymphatic sacs lies above the medulla oblongata. This is formed by the backward extension of the internal ear and the significance of this structure is not known. The cranial nerves are basically similar to that of teleosts. A sympathetic nervous system is associated with the vagus nerve. An ill- developed lateral line sense organ is present.
In the living dipnoans this system resembles closely that of amphibian larvae. The sensory organs are placed in grooves in Protopterus, while in others these are lodged inside the canals of the skin. But the pattern of development of the lateral line sensory system is basically same in the lung-fishes.
11. Endocrine System of Dipnoi:
The adrenal of dipnoans are represented by inter-renal and chromaffin tissues. These tissues remain intermingled and are situated along with the venous channels on the ventral aspect of the kidneys. These tissues function like the adrenals of higher forms, specially that of mammals.
12. Excretory System of Dipnoi:
The excretory system comprises of a pair of elongated kidneys which are separate anteriorly but are usually fused at their posterior ends. But in Lepidosiren, the posterior portions of the kidneys remain separate. The kidneys are of mesonephric type and remain in intimate contact with the gonads.
As the spermatozoa passes out through the kidney tubules, view. C. Diagrammatic longitudinal sectional view after this particular type of kidney is designated by Greham Kerr as the opisthonephros. The kidneys extend throughout the greater part of the visceral cavity.
Two thick-walled ducts, one from each kidney, may unite in Neoceratodus or may remain separate in Protopterus and Lepidosiren before opening into the cloaca. A cloacal bladder is present on the dorsal side. The lung-fishes normally excrete 30-70% of nitrogenous waste products through the gills in the form of ammonia.
But during aestivation, the nitrogenous waste products are stored in the form of non-toxic urea because the cloacal aperture is made closed by the cocoon.
13. Reproductive System of Dipnoi:
The sexes are separate. The sexual dimorphism is absent excepting Lepidosiren where the males develop vascular papillae on the pelvic fins during breeding season.
Female reproductive organs:
The ovaries are also paired and elongated bodies (Fig. 6.44A). The ovaries are typically like that of other fishes and are kept in position in Protopterus by mesovarium but in Neoceratodus these are attached with the dorsal body wall. The oviducts are located on the lateral side of the ovaries. Each oviduct (Mullerian duct) opens anteriorly into the body cavity by a fringed slit-like opening.
The anterior part of the oviduct is greatly convoluted and the posterior part is slightly expanded to form the uterus. The uteri join posteriorly to form a median vagina which opens into the urinogenital sinus. In non-breeding stage, the oviducts are nearly straight but in breeding season these become highly convoluted. The eggs are shed free into the body cavity and carried out by the oviducts.
Male reproductive organs:
There are two elongated testes in lung-fishes (Fig. 6.44B). In Lepidosiren and Protopterus, the testes are narrow bodies and appear round in cross- section. But in Neoceratodus the testes are thick and triangular in cross-section.
The testes are enclosed by fatty tissues and lie on the ventrolateral sides of the kidneys. In Protopterus the anterior half of the testis remains in contact with the body wall while its posterior half is suspended by mesorchium.
The testes of Protopterus are proportionately larger in size than that of Lepidosiren and extend the whole length of the body cavity. The testes are attached with the kidneys dorsally. The anterior end of the right testis in the dipnoans is attached with the liver while the left one extends far forward.
The vasa deferentia (numbering about six in Lepidosiren and Neoceratodus) open into the kidney ducts. But in Protopterus two vasa deferentia unite at the posterior end.
The Mullerian ducts are usually reduced. In Lepidosiren and Protopterus the anterior end of the Mullerian duct bears an opening in the larval stage while in adult it is closed. These ducts do not open into the urinary sinus. In Neoceratodus, the Mullerian ducts of the two sides fuse to form a. common sinus and end blindly in the cloaca.
Development:
The eggs are minute (measuring 3.5-4 mm in Protopterus aethiopicus) and shelless. In Lepidosiren the first three cleavage planes are vertical (Fig. 6.45) and are restricted to the animal pole which subsequently extends around the vegital pole.
Gradually the egg gets divided into cells. The cells on the vegital side are larger in size (mega-meres) and contain yolk and that on the animal end are smaller (micromeres). A blastula is produced which contains a blastocoel inside.
During the onset of gastrulation, the mega meres invaginate through the blastopore and the micromeres spread over the entire embryo. The gastrula elongates, neural folds are produced and the developing embryo transforms into the larva. Each larva after hatching, attaches itself to the substratum by the ventral sucker. The larva is a non-feeding form and derives nutrition from the yolk.
In lung-fishes, the egg contains limited quantity of yolk, as a result fully-formed larva is very small. Four pairs of external gills are visible in the larvae of Protopterus and Lepidosiren, while in Neoceratodus the opercular folds develop prior to the formation of the external gills. The external gills degenerate with the development of the lung.
Vestiges of the external gills may persist in adult stage. The lungs in the dipnoans, like that of other tetrapods, develop as an outgrowth from the floor of the pharynx which becomes eventually shifted to the dorsal side of the gut.
In Protopterus and Lepidosiren a peculiar structure of unknown function called Pinku’s organ is present. This structure is derived from the spiracular rudiment. Development of such a structure is not found in Neoceratodus.
I. Primitive Features:
a. Diphycercal tail.
b. Ventral inferior nostril.
c. Spiral valve in the intestine.
d. Persistent notochord without any constriction.
e. Cartilaginous autostylic skull.
II. Degenerated Features:
a. Reduced scales.
b. Slender paired fins (archipterygial type), seen in some primitive sharks such as Pleura canthus.
c. Dorsal fins reduced or fused.
d. Premaxilla, maxilla and nasal bones degenerated.
e. Some anterior vertebrae fused with the skull.
III. Specialized and Advanced Characters:
a. Internal nares, possibly help in breathing through the nose.
b. Respiration by lungs (modified air bladder) in addition to gill-respiration.
c. Auricle is partly divided into two and nearly three-chambered.
d. One of the paired auricles receives oxygenated blood through a special pulmonary arch from the lungs.
e. Conus arteriosus spirally twisted and contractile in nature.
f. Separation of pulmonary and systemic circulation.
g. Large paired cerebral hemispheres.
h. Well-developed Mullerian duct.
i. Presence of characteristic tooth plates, used for crushing of shelled invertebrates.
j. Loose bones absent in the jaw.
14. Affinities of Dipnoi:
From the anatomical standpoint, the dipnoans form a sort of structural bridge between the fishes and the amphibians. Like that of other fishes, the dipnoans possess gills and spend life in water. But unlike fishes, the dipnoans or lung-fishes can survive long dry spells by respiring through the air-breathing lungs like that of amphibia.
The paired fins are also used much the same way as the amphibians employ their limbs. Because of this fact many workers postulate that the dipnoans hold the ancestry of the amphibia. Before going into the details of the phylogenetic consideration, the affinities with the different groups of fishes and also with the amphibia are considered first.
Three genera of the living lung-fishes, Protopterus, Lepidosiren and Neoceratodus have similar structural construction. These forms differ in minor points which are primarily due to the different modes of adaptation.
Relationship with fishes (in general):
The lung-fishes show close similarity with the fishes and their piscine affinity is well- established.
The characteristics by which the lung-fishes resemble with the fishes are:
a. The notochord is persistent.
b. The vertebrae lack centra.
c. The skull shows little ossification and is with many investing bones.
d. Four to six branchial arches are present.
e. The dermal fin-rays are slender and are highly ossified.
f. The body is covered by overlapping cycloid scales.
g. The tail fin is diphycercal.
h. The gills constitute the respiratory organs to absorb the oxygen which remains dissolved in water.
i. The lateral line sense organs are present.
Remarks:
The relative position of the lung-fishes amongst the bony fishes remains still controversial because they bear many primitive features by which they resemble the elasmobranchs.
Relationship with elasmobranchs:
The lung-fishes resemble the elasmobranchs by having:
a. Similar conus arteriosus,
b. Spiral valve in the intestine,
c. Similar construction of the female reproductive system,
d. Identical diencephalon and
e. No nephrostome in the kidney tubules.
Remarks:
The above similarities are not sufficient enough to establish any relationship. The presence of lung and lack of claspers put difficulty in such contention. However, the similarities speak about the primitiveness of the dipnoans.
Relationship with holocephalians:
Jarvik (1964, 1967) has demonstrated that the dipnoans agree with the holocephalians in a remarkable way especially in the structural plan of the upper and lower jaw. The mandibular and palatine tooth-plates are the main biting elements in both the groups.
The other similarities are:
a. Similar histological picture of teeth.
b. Similar cranial muscles.
c. Fusion of the operculogular membranes of both sides.
d. Similar kidneys, gonads and the ducts.
e. Lack of stomach and presence of spiral valves.
f. Two efferent arteries in the gill-arches.
g. Shifting of ex-current nostril into the mouth cavity.
Remarks:
Despite such similarities it is extremely difficult to establish any relationship between dipnoans and holocephalians. Some of the features are shared by other groups. The dipnoans differ from holocephalians by having lungs and absence of claspers.
Relationship with rhipidistians (Crossopterygii):
The dipnoans and rhipidistians show close similarities and it is regarded by many workers that the two groups should be placed under a common systematic status.
The alleged similarities are:
a. Westoll (1949) has shown that the dermal Ipones and scales have similar histological organisation, especially having cosmine.
b. Watson and Gill (1923) established the resemblances between Middle Devonian members particularly in their body form and median fins. Presence of two separate dorsal fins, single anal fin and a hetero- cercal tail, etc., are some of the notable similarities.
c. Romer (1955) suggested that they had fleshy lobate fins with scales and well- developed endoskeleton.
d. Save-Soderbergh (1934) gave the support basing on the similarities in the number and disposition of dermal bones on the skull.
e. Similar opercular and gular bones as advocated by Watson and Gill (1923).
f. Similar general plan of laterosensory system.
g. Presence of nostril in the roof of the mouth cavity.
h. Comparable lower jaw.
Remarks:
Jarvik (1968) and many authors express doubt on the resemblances between the two groups. Most of the similarities, according to him, are either insignificant or non-existent. The structural organisation of vertebral column, neural endocranium, visceral endoskeleton, snout in the dipnoans is fundamentally different from that of the rhipidistians. So the question of establishing a close relationship is untenable.
Relationship with struniiformes:
Jessen (1966) tried to establish relationship of the dipnoans with the group struniiformes. But the fundamental plan of the operculogular series and lower jaw establishes its relation with the rhipidistians than others. The dipnoans differ widely from the struniiformes and they are not closely allied.
Relationship with Teleostomi:
The lung-fishes and ray-finned bony fishes show close affinities. But as regards the affinities of the group, the relationship with the subclass Crossopterygii is quite close.
The lung- fishes resemble the subclass Actinopterygii by having:
a. Similar lobate paired fins.
b. Presence of cosmine covering the cycloid scales and skull bones in Dipterus.
c. Presence of paired inferior jugular veins.
d. Presence of powerful palatine and splenial teeth.
e. Presence of blunt snout with ventral nostril.
f. Presence of swim-bladder, although modified into ‘lungs’ in dipnoans.
g. Presence of operculum.
But the similarities with other crossoptergians are very close.
The common features are:
a. Presence of internal nostrils.
b. The tail fin is diphycercal.
c. Existence of cosmine covering the scales.
d. Similar skull bones.
e. Presence of swim-bladder functioning as the lung.
f. The intestine contains spiral valves.
g. The conus arteriosus is contractile.
h. External gills are present in the larvae.
Remarks:
The close similarity existing between the groups suggests the contention that the lung- fishes are phylogenetically related with the other crossopterygians.
Relationship with amphibia:
The lung-fishes resembles the amphibians by many characteristics.
The common features are:
a. The presence of vomerine teeth.
b. The lungs are capable of pulmonary respiration. A pulmonary artery and a pulmonary vein are present.
c. The circulatory system of dipnoans resembles that of amphibian by many points, viz., the spirally twisted conus arteriosus is divided into two by a longitudinal partition. The auricle and the sinus venosus are imperfectly divided into two parts. The pericardium is thin-walled. The ventral aorta is short. Similar inferior vena cava and the presence of anterior abdominal vein.
d. The skin glands are multicellular.
e. Presence of dermal scales in Gymnophiona.
f. The brain of the lung-fishes resembles that of amphibia specially in the structure of the cerebrum and cerebellum.
g. The spermatozoa are carried through the excretory part of the mesonephros.
h. The structure of egg and the development are similar.
i. The larvae of Lepidosiren and Protopterus possess the external gills and sucker. In some species of Protopterus external gills may be persistent.
j. The jaw suspension is autostylic.
Although the dipnoans resemble the amphibians by a number of features, there are many individual specialised characteristics by which they are separated from the amphibians.
These are:
a. The skull is largely cartilaginous with little ossification.
b. The maxillae and premaxillae are absent.
c. Some anterior vertebrae have become fused with the skull.
d. Presence of characteristic tooth plates.
e. The lungs are located dorsal to the gut.
Remarks:
The common characteristics present in the lung-fishes and amphibians are due to their evolutionary convergence and have possibly resulted from adaptive modifications to a similar environmental condition. Most authorities are not inclined to draw a relationship between Dipnoi and Amphibia. A number of views may be mentioned in this respect.
a. Goodrich (1909, 1930):
Dipnoi gave origin to Amphibia and Amphibia leads a transitional life from water to land.
b. Newman (1939):
As Dipnoi is degenerated group, so Amphibia could not have evolved from Dipnoi.
c. Romer (1949):
The lung fishes are not ancestor but collateral uncle of land dwellers. Recently the trend has reversed as the ancestor of amphibians. Considering the morphological and anatomical point of views, D. Rosen et al. (1981), Gardiner (1982, 1983) and Duellman and Trueb (1986) consider that the nearest living relatives of recent amphibians are lung-fishes and the hypothesis is that the tetrapods arose from lung-fish.
But Jarvik (1981), Holmes (1985) and Caroll (1987) have vigorously criticised the hypothesis. They consider that the lung-fishes are highly specialized, and from such specialized group less specialized earliest amphibians cannot be developed.
In particular, dermal bones of the lung-fish skull have a different pattern from those of early amphibians. The pre-maxillary and maxillary bones are absent in the lung-fish which are important tooth bearing bones in the upper jaw of tetrapods.
15. Cytogenetic Evidence of Dipnoi:
The dipnoans differ fundamentally from the teleosts for the large size of their chromosomes. They resemble the urodeles in this respect and have high DNA values. Lepidosiren paradoxa has n = 19 metacentrics. Protopterus annectans has n = 17 and Neoceratodus forsteri has also n = 18 or 19 (Wickbom, 1945).
As compared with dipnoans, coelacanths possess a small amount of DNA value. From the above study it may be admitted that a dipnoan origin of Amphibia rather than the crossopterygian one, indicated by molecular evidence.
Phylogeny of dipnoans:
The affinities and origin of the dipnoans are still an unsolved problem on which diverse opinions exist. The similarities with the amphibians are due to evolutionary parallelism. The dipnoans exhibit similarities with other fishes particularly with the elasmobranchs and holocephalians.
There are reasons to believe such relationship, but their mutual interrelationship becomes difficult to establish. So in the present state of knowledge which of the surviving vertebrate group is the sister group of the dipnoans remains far from the solution.
The lung-fishes possess real lungs and Neoceratodus even ‘walks’ across the muddy bottoms of the river by using the paired fins like ‘legs’. This fact tempts one to think that these fishes have some direct connection with the forms which led to the origin of land vertebrates.
From the anatomical standpoint, the dipnoans exhibit close similarities with the bony fishes, specially with the rhipiditians on one hand and the amphibians on the other. This transitional status of the dipnoans makes this group of fishes interesting.
The contention of holding the direct ancestry of the land vertebrates is still controversial nowadays. But their relation with the ancestral stock of the land vertebrates is supported by numerous evidences.
The dipnoans, although survive by only three genera living in widely separated parts of the earth, have left behind a complete geological succession of the extinct dipnoans leading to the modern forms. All the dipnoans are united under a common superorder for a number of similar characteristics.
These features are:
(a) The presence of internal nostrils.
(b) The scales and bones are covered by cosmine.
(c) The autostylic mode of jaw suspension.
(d) The peculiar pattern of teeth.
Presence of these features also testify the biological truth that the lung-fishes have evolved from a common ancestor with the other crossopterygians. The earliest known fossil dipnoan is represented by the genus, Dipterus (Dipnorhynchus is supposed by many to be more primitive than Dipterus).
Dipterus possesses the following primitive features:
(a) The body is covered by stout cosmine covered cycloid scales.
(b) Two small dorsal fins are present.
(c) The tail fin is of heterocercal type with a small epichordal lobe.
(d) Numerous conical teeth are present.
During the course of evolution the modern lung-fishes have witnessed the following changes:
(a) Fusion of median fins to form a continuous one.
(b) Reduction in the number of dermal plates in the skull.
(c) Changeover from the heterocercal to diphycercal tail.
(d) Fusion of the conical teeth to form characteristic tooth-plates with ridges.
(e) Reduction in the number of opercular bones.
Dipterus is fundamentally akin to the most primitive crossopterygian, Osteolepis. The scales and the head bones of Dipterus are strikingly similar to that of the contemporary crossopterygians. The resemblances in some cases are so close that their relative position within the separate classes becomes sometimes confusing.
A survey of past geological history suggests that the rhipidistian line and the dipnoan line converge back as we follow them further backward. This is evidenced by the presence of close similarity in the earlier representatives of both the above groups.
The dipnoans constitute a very primitive race and possess many primitive characters in comparison to the then crossopterygians. This primitive organisation nullifies the concept of derivation of the dipnoans from the rhipidistian stock. However, both the groups after emerging from a common ancestral stock become diverged widely in course of time.
Phylogenetic significance:
The phylogenetic significance of the dipnoans as regards their holding the direct ancestry of the amphibian is questionable. Because this view is confronted with serious objections from anatomical standpoint and is difficult to interprete.
The most important obstacle is the derivation of the terrestrial limb from the peculiar and specialised archipterygial paired appendages of the dipnoans. The development of cloacal bladder is different. In the dipnoan, the cloacal bladder develops from the dorsal wall of the cloaca but in amphibians it develops from the ventral wall.
The rhipidistians, on the other hand, present fewer of such obstacles. The earliest fossil amphibians show deeper similarities with Osteolepis (primitive rhipidistian) of Devonian period than any extinct and surviving dipnoans (Fig. 6.46).
It is universally accepted that the amphibians have originated either directly. from some rhipidistians or from an unknown group closely related to the rhipidistians but never from the dipnoan source. This trend has reversed in recent years [see Rosen et al.; (1981), Gardiner (1982, 1983) and Dueliman and Trueb, 1986.
It can best be suggested that the dipnoans have diverged very early from the remote basic stock from which the amphibians actually originated. The rhipidistians and the dipnoans are the divergent offshoots of a common piscine ancestral stock.
If the contention of the emergence of the amphibians from the rhipidistian forms be taken as a fact, the dipnoans can be regarded as the ‘grand-uncle’ of the land-dwellers but not the ‘grand-father’.
Besides all these complications regarding the phylogeny of the land- dwellers, the modern dipnoans give us a glimpse of the conditions that have caused the vertebrates to conquer the land and the possible ways how the difficulties have been solved by the early tetrapods through structural adaptations.