Lobe-finned fishes  The lobe-finned fishes are the fishes most closely related to the tetrapods and they are the group from which the amphibians and later the other tetrapod groups evolved.

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Lobe-finned fishes: Class Sarcopterygii  Primitive Sarcoptrygians were abundant in the Devonian, but have since declined to a handful of species that includes today the lungfishes and coelacanths.

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Lobe-finned fishes: Class Sarcopterygii  Unlike in the actinopterygians (where the rays fan out from the base of the fin) the rays of the paired fins in Sarcopterygians extend from a central shaft of bones to support the fin web.

Fin structure

Lobe-finned fishes: Class Sarcopterygii  Primitive Sarcopterygians were cm long and cylindrical.  They possessed two dorsal fins, paired pelvic and pectoral fins that were fleshy, scaled and possessed a bony central axis. The heterocercal caudal fin had a epichordal lobe.

Fossil Sarcopterygian

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Lobe-finned fishes: Class Sarcopterygii  Sarcopterygian fishes also had massive jaw muscles in comparison to those of actinopterygians.  In addition, early sarcopterygians were covered with a dentine-like material called cosmine.

Lobe-finned fishes: Class Sarcopterygii  Today the sarcopterygians are a very small group that includes only six species of lungfishes (Dipnoi) and two species of coelacanths (Actinistia).  However, all of the tetrapods (four-legged vertebrates) are descended from an extinct group of sarcopterygian fishes known as the rhipidistians.

Lungfishes  There are six species of lungfishes: one South American, one Australian and four African species.  As their name suggests, these fish, as all sarcopterygians do, possess alveolar lungs and can breathe air.

Lungfishes  Extant Dipnoi have lost the articulating toothed premaxillary and maxillary bones of the other Osteichthyes.  They have crushing dental plates with fan- shaped ridges and teeth scattered over the palate. In addition, strong muscles attach the lower jaw to the chondrocranium. Lungfishes are thus specialized to feed on hard foods such as crustaceans and molluscs.

Lungfishes  The dorsal, caudal and anal fins have fused into a single continuous fin that extends around the entire rear third of the body.  The change in body form of the lungfishes may be an example of paedomorphosis.  They were initially considered to be salamanders when first described.

Lungfishes  The Australian lungfish can gulp air and survive being in oxygen poor water, but cannot live out of water.  In contrast, the South American and African species can survive out of water for long periods of time.  The African species live in seasonal steams and ponds that dry out, but the lungfish survives by burrowing into the mud and forming a cocoon in which it survives until the water returns.

African lungfish

South American Lungfish

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The discovery of living coelacanths  Coelacanths were believed to have been extinct for perhaps 50 million years (there are fossils identical in appearance that are 70 million years old) when one was caught by a South African fishing boat in  The curator of a small museum, M. Courtney- Latimer, recognized the fish was unusual and she brought it to the attention of the icthyologist J.L.B. Smith who after some delay in arriving identified the fish.

The discovery of living coelacanths  Unfortunately, the delay in arriving meant the fish had badly decomposed and many important structures had been lost.  Smith named the fish (Latimeria) in honor of Courtney- Latimer and then embarked on a 14-year quest to find another coelacanth.  But it wasn’t until 1952 that a second was caught off the Comoro Islands, north of Madagascar, which is where the fish occur naturally (the 1938 fish apparently had drifted far from its normal range). The story is told in Smith’s book “The search beneath the sea.”

Images from the rediscovery of the Coelacanth off the Comoros 1952.

Coelacanths  In 1998 another population of Latimeria [but a different species] was discovered off Indonesia (10,000km east of the Comoros).  Coelacanths are large fish up to about 5 feet long, blue-grey in color with white spots.  They live in deep (70-400m) cold water and are predators feeding mainly on lanternfish.

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Coelacanths  Coelacanths are readily identified from their fins. The caudal fin has a small median lobe. The caudal fin has a small median lobe. Each of the paired fins is very mobile and has a long fleshy basal lobe. Each of the paired fins is very mobile and has a long fleshy basal lobe. The anterior dorsal fin’s fleshy lobe is reduced and it possesses long protective hollow spines (coelacanth means “hollow spine”). The anterior dorsal fin’s fleshy lobe is reduced and it possesses long protective hollow spines (coelacanth means “hollow spine”). When they swim coelacanths move their pelvic and pectoral fins in the same pattern that tetrapods walk. When they swim coelacanths move their pelvic and pectoral fins in the same pattern that tetrapods walk.

Coelacanths  Because coelacanths possess an unusual suite of characters including fat-filled lungs, a high level of urea in the blood, a liquid filled notochord, lobed fins, ventral kidneys and a reduced brain there has been debate about their phylogenetic affinities.  The consensus today is that coelacanths are a sister group to the Rhipidistia which gave rise to the lungfish and tetrapods.  We will discuss the origins of the tetrapods shortly.