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SARCOPTERYGIAN FISH

 

   There are two groups of modern bony fish: the actinopterygians and the sarcopterygians.  While the actinopterygians compose nearly all fish alive today, sarcopterygians are interesting because of their close relationship to amphibians.  Sarcopterygians are large fish in which the muscles that move the fins are actually located in the fins.  This may have been an adaptation for bottom-dwelling to allow the fish to push off solid objects. Sarcopterygians developed pectoral and pelvic fins which were more similar to each other than in other fish groups (Coates, 2003). Psarolepis and Achoania are the most primitive known sarcopterygian fossils (Zhu, 2004).

Later sarcopterygians and tetrapods possessed internal nostrils which allowed air from the nasal cavity to enter the mouth and reach the lungs.  The first sarcoptergians, however, did not have internal nostrils.  Instead, they could bring water into an anterior nostril, through the nasal cavity, and out a posterior nostril which opened onto the snout.  The fossil sarcopterygian fish Kenichthys presents and intermediate state between these two conditions in which the posterior nostril empties at the edge of the jaw, in a separation between the maxillary and premaxillary bones.  This condition is considered as the intermediate state between the lack of an internal nostril (choana) and its presence (Zhu, 2004).

sarcopterygian kenichthyes

     There were five groups of crossopterygians of which one, the coelocanths, includes a single modern species. The earliest crossopterygians are known from the Early Devonian. Another group, the rhizodonts, included giant Carboniferous predators which could reach lengths of 7 meters and a Late Devonian fish Sauripteris which possessed jointed bones beyond the radius and ulna. Both groups appear in the earliest Devonian (and coelacanths are known from the middle Devonian).  Four genera of sarcopterygians (lungfish and coelacanths) survive today).  In 1938, a coelacanth was found off the coast of Madagascar, the first evidence of their existence since the Cretaceous.  Late Paleozoic and Mesozoic coelacanths were distributed throughout the world.  In coelacanths, the notochord still extends to the pituitary gland and the ratio of brain weight to body weight is similar to that of the most primitive sharks and actinopterygians.   Megalocoelocanthus dobiei was a coelacanth from the Cretaceous of the eastern U.S. that could reach 3.5 m in length (Schwimmer, 1994).

megalocoelacanthus

When lungfish were first discovered in 1836, they were identified as reptiles. Lungfish were once diverse and included forms with duck-like bills. The earliest fossil lungfish are known from the Early Devonian. One species reached lengths of 2 meters (Long, 1995). Given the presence of lungfish since the early Devonian and the presence of an internal nostril in crossopterygians (as in amphibians) from the Middle Devonian, it is assumed that early sarcopterygian fish could breathe atmospheric air (Long, 1995).

The Devonian lungfish Dipterus is pictured below.  Lungfish are more closely related to amphibians than coelocanths (Meyer, 1990; Brinkman, 2004).

lungfish
     A variety of body shapes and fin forms of fossil lungfish are known.  The pectoral fin is capable of rotation in modern forms and apparently in fossil forms as well.  Some fossils had an unrestricted notochord (Ahlberg, 1995a). The following images are of modern and fossil lungfish.

lungfishlungfish

lungfish lungfish
pelvic fins
pectoral fins
sunfish

 

    Lungfish are named after an organ they possess which is often considered to be specific to land vertebrates: lungs.  Lungfish possess true lungs in addition to their gills which they use to breathe atmospheric air.  The lungs of lungfish are homologous to a structure in actinopterygians named the “swim bladder”.  While the primary function of swim bladders in teleost fish is for buoyancy and is not used to breathe air,

 Many actinopterygians, including primitive species, can use the structure to breathe oxygen from atmospheric air. 

Gar

swim bladder swim bladder
bowfin
bladder swim bladder

BREATHING FISH

     Is it surprising that some fish can breathe atmospheric air?  Not really.  The ability of aquatic animals to breath oxygen from the air is thought to have evolved at least 67 times (Maina, 1998).      A number of crustaceans can do so.  The sea cucumber Holothuria tubulosa can breathe in stagnant water by rising through the surface and performing gas exchange through its cloaca. (Hoar, 1983, p. 505)  There are lung-like structures in some snails.  Some oligochaete and polychaete worms perform gas exchange through their anus (Maina, 1998).. 

     A considerable number of fish can breathe oxygen from air.  The majority do so as a supplement to the oxygen transported by the gills but some are actually obligate air breathers that drown in water without access to air.  The chamber in which air is held for gas exchange with surrounding blood vessels can be simple but many fish have adapted it for more efficient exchange through the branching of the chamber to increase surface area and the development of a very thin respiratory membrane.  Increased vascularization of caudal and pelvic fins function in respiration in the walking goby (Periopthalmus) and the South American lungfish, respectively. (Weichert, 1970, p. 244)  Some fish perform gas exchange with water moved through their highly vascularized rectum.  Some aquatic turtles possess 2 sacs opening into the cloaca which can function in gas exchange (Weichert, 1970, p. 244).

     Some fish perform gas exchange with air through their buccopharyngeal apparatus (mouth and throat), an opercular structure, the swim bladder, the intestine, pharyngeal lungs, the skin, and the gastrointestinal tract which allow gas exchange using swallowed air.  Some of these fish are obligate air breathers and may travel for short distances over land.(Maina, 1998, p. 220).  An air-breathing species of the family Loricaridae is pictured in the following photos.

breathing fish breathing fish
breathing fish

While the fine filaments of  most fish gills collapse in air, a few fish possess modified gills which can breathe atmospheric air (Johansen, Kjell from Hoar, 1970). Some placoderms seem to have possessed lunglike structures, although placoderm organs are of unknown homology  (Perry, 2001).  The air/swim bladder of bony fish functions as a respiratory organ in lungfish and in basal actinopterygians, suggesting that this was its primitive function in the ancestral bony fish.  In other words, swim bladders seem to have evolved from lungs rather than lungs from swim bladders.  Basal actinopterygians even possess blood vessels which correspond to the pulmonary circuit of lungfish and tetrapods, although there is no separation of this blood from that of the systemic circuit in the heart (Johansen, Kjell from Hoar, 1970). 

     Note that there are capillaries in the swim bladder of fish which bring the blood close enough to the epithelial lining of the bladder to perform gas exchange with the air contained within.

swim bladder swim bladder

An African catfish (Channallabes apus) not only can come onto land, its modified neck allows it to capture prey on land despite the lack of strong pectoral fins. Most of this fish’s diet consists of terrestrial insects (Van Wassenbergh, 2006).

 

LUNGS

     If a lung is defined as a vascularized air sac, then lungs are known in some spiders, decapods, chilopods, isopods, and snails (and in all scorpions).  (Hoar, 1983, p. 508). The ancestors of bony fish evolved lungs as an outpocket of the esophagus (Romer, p. 363).   The gar pike and Amia will suffocate without atmospheric air; as will sarcopterygian lungfish (Romer, p. 360, 364).  The placoderm Bothriolepis possessed a pair of pharyngeal pouches which extended posteriorly and probably functioned as simple lungs.  Some sharks possess a vestigial structure which may be related to this. (Weichert, 1970; Kardong). 

      Among vertebrates, lungs are not unique to tetrapods, they actually evolved in fish before the adaptation to life on land.  Some fish alive today, appropriately called lungfish, possess true lungs, the homologs of tetrapod lungs.  The Australian lungfish, the most primitive of the three lungfish, possesses a single lung while the African and South American lungfish possess paired lungs.  Once African lungfish (Protopterus) reach more than 100g, more than 85% of their oxygen is obtained through gas exchange in the lung (Orgeig, 1995).  While the Australian lungfish is an opportunistic air breather, African and South American lungfish are primary air breathers during several months of the year which they spend on land. (Hoar, 1970)  The African lungfish secretes a mucus cocoon where it waits in the mud for the next rainy season.  During this time, it depends on oxygen from the air which is breathed through its lungs.  Experimentally, these lungfish have been able to survive 5 years in this state.(Webster, 1974, p. 372)  The lungs of lungfish actually function better than those of many amphibians. (Weichert, 1970, p. 221).  The following picture of a lungfish shows both its gills and lungs.

lungs lungs
lungs
lungs
salam
In lungfish, the lungs are long structures which join with the esophagus (just as the swim bladder of actinopterygian fish joins with the esophagus). The lungs of a salamander are also long organs which join with the esophagus, as seen in the photo to the right.
     Why would a fish evolve lungs?  Lack of oxygen in water can cause mortality in fish and still waters can have very low levels of dissolved oxygen (especially at warmer temperatures).  Any fish with lungs has an advantage in these environments since it can supplement its oxygen supply from the gills.  A variety of modern fish can breathe oxygen from atmospheric air (which is why a goldfish in a fishbowl doesn’t need an air pump to survive). 

   The fossil sarcopterygian Styloichthys seems to be intermediate between the most primitive sarcopterygians (Psarolepis and Achoania) and the more advanced rhipidistians (Zhu, 2002)

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The common ancestor of Glyptolepis and later rhipidistians possessed a series of bones in its fins.

glyptolepis

Rhipidistians were the dominant late Paleozoic freshwater sarcopterygians that became extinct in the Permian Period. There are a number of characteristics they share with the first amphibians, which seem to have descended from them. They have a large number of bones (up to 30) around the eye in a sclerotic ring while ray finned fish have only 4 such bones. The nostrils open into the oral cavity via an internal naris while in ray finned fish the nasal cavity formed a blind sac. There is a tube that is comparable to the lacrimal duct (tear duct) of land vertebrates. They possess lateral line organs (involving grooves along the skull) for detecting aquatic prey that were still present in the first amphibians. The organization of the skull bones was extremely similar to that of the first amphibians. The pineal foramen is very prominent between the two parietal bones, as it was in the early amphibians. Although the notochord is still large and functions in support, thin walled vertebral centra surround it. Since living sarcopterygians have lungs, it is presumed that their ancient relatives which would evolve into amphibians had them also. (Carroll, 1988; Yu, 1998)

osteolepis
glyptolepis
 

 

    Species of the Late Devonian genus Eusthenopteron are known from all northern continents (Long, 1995). There are a number of rhipidistians which show great similarities to the early amphibians.  Although Eusthenopteron is perhaps the best known, there are some that were even closer to amphibian lineage. 

EusthenopteronEusthenopteron

The skull of Osteolepis shows closer affinities to the amphibians than that of Eusthenopteron but unfortunately fossils do not include the fins and girdles.  Panderichthyes is closer still in its skull characteristics and the solidification of its vertebrae.  Elginerpeton, Ventastega, Obruchevichthyes, and Metaxygnathus, although represented by incomplete fossils, seem to be even closer to the origin of amphibians although they retain sarcopterygian features such as large teeth in the coronoid bone of the lower jaw, outside the row with the rest of the teeth.  They both had tetrapod-like girdles (shoulder and hip) and Elginerpeton had tetrapod-like limb bones (and probably had toes) (Ahlberg, 1995; Carroll, 1988).

osteolepis skullpalate of osteolepis

The first vertebrates to evolve a humerus were the rhipidistian sarcopterygians which preceded the amphibians. They possessed a joint in the region of the elbow, although the rest of the fin was fairly stiff (Carroll, p. 161). Before the evolution of amphibians, rhipidistian fish evolved two bones to articulate with the humerus, the radius and ulna.The fish ancestors of amphibians possessed small bones in the region of the wrist and cartilaginous fin rays in the region of tetrapod fingers. Rhipidistians possessed an ulnare and intermedium of the wrist (Carroll, p. 163). Rhipidistian fish possessed a femur which included a joint in the area of the knee, although the rest of the fin was fairly stiff (Carroll, p. 161). In rhipidistian fish, the tibia and fibula possessed a hingelike connection with the femur (Carroll, p. 163). Rhipidistian fish possessed a fibulare and intermedium homologous to two tarsal bones of the tetrapod foot (Carroll, p. 164)

The common ancestor of Panderichthys and tetrapods evolved the rudiments of ribs. The shoulder differed from that of tetrapods in that the glenoid cavity was smaller and more medial and the scapulocoracoid was bigger. The ulna had increased in size in these fish and was intermediate between earlier fish and tetrapods. The fin rays were still composed of dermal bone, unlike the endochondral bone which would compose tetrapod digits (Clack, 2002). The fin of Panderichthys is depicted below with bones homologous to those of tetrapods (such as the humerus in blue, radius in red, and ulna in green).

The fish Panderichthys possessed a middle ear similar to that of early tetrapods and unlike those of more primitive sarcopterygian fish. It possessed a hyomandibula whose modifications made it similar to the stapes of early tetrapods (Brazeau, 2006). Panderichthys possessed a middle ear which was intermediate between that of more primitive rhipidistians such as Eusthenopteron and the early amphibians. Although it lacked a true stapes, the hyomandibula (from which the stapes is derived) was modified to become very similar to the stapes (Brazeau, 2006).

ear in fish and amphibians

panderichthys skullpanderichthys

Front fin of Panderichthys: arm panderichtys

The fossil Elpistostege is thought to represent a fish more closely related to amphibians than Panderichthys.

skull skull

Conclusion:

In the Devonian, sarcopterygian fish evolved which had lungs, skulls with bones homologous to those which composed early tetrapod skulls, and fin bones homologous to the bones of tetrapod limbs. Terrestrial environments possessed a diversity of arthropods without large vertebrate predators to feed on them. The adapatation to terrestrial environments would also have offered a way to escape predators and escape from drying bodies of water. Fossil, anatomical, and genetic evidence suggest that these fish were the ancestors of the vertebrates which adapted to life on land, the tetrapods.

fish

fish

fish near land

fish on land

fish on rock

amphibian

amphibian