Lobe-finned fishesLobe-finned fishes

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

16.2

Figure 24.02

Lobe-finned fishes: Class Lobe-finned fishes: Class SarcopterygiiSarcopterygii

Primitive Sarcoptrygians were abundant in Primitive Sarcoptrygians were abundant in the Devonian, but have since declined to a the Devonian, but have since declined to a handful of species that includes today the handful of species that includes today the lungfishes and coelacanths.lungfishes and coelacanths.

16.1

Figure 24.01

Lobe-finned fishes: Class Lobe-finned fishes: Class SarcopterygiiSarcopterygii

Unlike in the actinopterygians (where the Unlike in the actinopterygians (where the rays fan out from the base of the fin) the rays fan out from the base of the fin) the rays of the paired fins in Sarcopterygians rays of the paired fins in Sarcopterygians extend from a central shaft of bones to extend from a central shaft of bones to support the fin web.support the fin web.

Fin structurehttp://people.eku.edu/ritchisong/RITCHISO//fins2.gif

Lobe-finned fishes: Class Lobe-finned fishes: Class SarcopterygiiSarcopterygii

Primitive Sarcopterygians were 20-70 cm Primitive Sarcopterygians were 20-70 cm long and cylindrical.long and cylindrical.

They possessed two dorsal fins, paired They possessed two dorsal fins, paired pelvic and pectoral fins that were fleshy, pelvic and pectoral fins that were fleshy, scaled and possessed a bony central axis. scaled and possessed a bony central axis. The heterocercal caudal fin had a The heterocercal caudal fin had a epichordal lobe.epichordal lobe.

Fossil Sarcopterygianhttp://www.pc.gc.ca/progs/spm-whs/images/miguasha/mig6b.jpg

http://animals.nationalgeographic.com/staticfiles/NGS/Shared/StaticFiles/animals/images/1024/coelacanth-swimming.jpg

Lobe-finned fishes: Class Lobe-finned fishes: Class SarcopterygiiSarcopterygii

Sarcopterygian fishes also had massive Sarcopterygian fishes also had massive jaw muscles in comparison to those of jaw muscles in comparison to those of actinopterygians.actinopterygians.

In addition, early sarcopterygians were In addition, early sarcopterygians were covered with a dentine-like material called covered with a dentine-like material called cosmine.cosmine.

Lobe-finned fishes: Class Lobe-finned fishes: Class SarcopterygiiSarcopterygii

Today the sarcopterygians are a very Today the sarcopterygians are a very small group that includes only six species small group that includes only six species of lungfishes (Dipnoi) and two species of of lungfishes (Dipnoi) and two species of coelacanths (Actinistia).coelacanths (Actinistia).

However, all of the tetrapods (four-legged However, all of the tetrapods (four-legged vertebrates) are descended from an vertebrates) are descended from an extinct group of sarcopterygian fishes extinct group of sarcopterygian fishes known as the rhipidistians.known as the rhipidistians.

LungfishesLungfishes

There are six species of lungfishes: one There are six species of lungfishes: one South American, one Australian and four South American, one Australian and four African species.African species.

As their name suggests, these fish, as all As their name suggests, these fish, as all sarcopterygians do, possess alveolar sarcopterygians do, possess alveolar lungs and can breathe air.lungs and can breathe air.

LungfishesLungfishes

Extant Dipnoi have lost the articulating toothed Extant Dipnoi have lost the articulating toothed premaxillary and maxillary bones of the other premaxillary and maxillary bones of the other Osteichthyes.Osteichthyes.

They have crushing dental plates with fan-They have crushing dental plates with fan-shaped ridges and teeth scattered over the shaped ridges and teeth scattered over the palate. In addition, strong muscles attach the palate. In addition, strong muscles attach the lower jaw to the chondrocranium. Lungfishes lower jaw to the chondrocranium. Lungfishes are thus specialized to feed on hard foods such are thus specialized to feed on hard foods such as crustaceans and molluscs.as crustaceans and molluscs.

LungfishesLungfishes

The dorsal, caudal and anal fins have fused into The dorsal, caudal and anal fins have fused into a single continuous fin that extends around the a single continuous fin that extends around the entire rear third of the body.entire rear third of the body.

The change in body form of the lungfishes may The change in body form of the lungfishes may be an example of paedomorphosis.be an example of paedomorphosis.

They were initially considered to be salamanders They were initially considered to be salamanders when first described.when first described.

LungfishesLungfishes

The Australian lungfish can gulp air and survive The Australian lungfish can gulp air and survive being in oxygen poor water, but cannot live out being in oxygen poor water, but cannot live out of water. of water.

In contrast, the South American and African In contrast, the South American and African species can survive out of water for long periods species can survive out of water for long periods of time. of time.

The African species live in seasonal steams and The African species live in seasonal steams and ponds that dry out, but the lungfish survives by ponds that dry out, but the lungfish survives by burrowing into the mud and forming a cocoon in burrowing into the mud and forming a cocoon in which it survives until the water returns.which it survives until the water returns.

African lungfishhttp://www.amtra.de/images/Lungenfisch415.jpg

South American Lungfishhttp://www.ucmp.berkeley.edu/vertebrates/sarco/lungfish1.jpg

Australian Lungfishhttp://www.nhm.ac.uk/about-us/news/2007/october/images/Australian%20lungfish%20copyright%20Jean%20Joss-370_12548_1.jpg

Figure 24.22

The discovery of living coelacanthsThe discovery of living coelacanths

Coelacanths were believed to have been extinct Coelacanths were believed to have been extinct for perhaps 50 million years (there are fossils for perhaps 50 million years (there are fossils identical in appearance that are 70 million years identical in appearance that are 70 million years old) when one was caught by a South African old) when one was caught by a South African fishing boat in 1938.fishing boat in 1938.

The curator of a small museum, M. Courtney-The curator of a small museum, M. Courtney-Latimer, recognized the fish was unusual and Latimer, recognized the fish was unusual and she brought it to the attention of the icthyologist she brought it to the attention of the icthyologist J.L.B. Smith who after some delay in arriving J.L.B. Smith who after some delay in arriving identified the fish. identified the fish.

The discovery of living coelacanthsThe discovery of living coelacanths

Unfortunately, the delay in arriving meant the fish had Unfortunately, the delay in arriving meant the fish had badly decomposed and many important structures had badly decomposed and many important structures had been lost.been lost.

Smith named the fish (Smith named the fish (LatimeriaLatimeria) in honor of Courtney-) in honor of Courtney-Latimer and then embarked on a 14-year quest to find Latimer and then embarked on a 14-year quest to find another coelacanth. another coelacanth.

But it wasn’t until 1952 that a second was caught off the But it wasn’t until 1952 that a second was caught off the Comoro Islands, north of Madagascar, which is where Comoro Islands, north of Madagascar, which is where the fish occur naturally (the 1938 fish apparently had the fish occur naturally (the 1938 fish apparently had drifted far from its normal range). The story is told in drifted far from its normal range). The story is told in Smith’s book “The search beneath the sea.”Smith’s book “The search beneath the sea.”

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

CoelacanthsCoelacanths

In 1998 another population of In 1998 another population of LatimeriaLatimeria [but a [but a different species] was discovered off Indonesia different species] was discovered off Indonesia (10,000km east of the Comoros).(10,000km east of the Comoros).

Coelacanths are large fish up to about 5 feet Coelacanths are large fish up to about 5 feet long, blue-grey in color with white spots.long, blue-grey in color with white spots.

They live in deep (70-400m) cold water and are They live in deep (70-400m) cold water and are predators feeding mainly on lanternfish.predators feeding mainly on lanternfish.

16.20

Figure 24.23

CoelacanthsCoelacanths

Coelacanths are readily identified from their fins. 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 Each of the paired fins is very mobile and has a long

fleshy basal lobe.fleshy basal lobe. The anterior dorsal fin’s fleshy lobe is reduced and it The anterior dorsal fin’s fleshy lobe is reduced and it

possesses long protective hollow spines (coelacanth possesses long protective hollow spines (coelacanth means “hollow spine”).means “hollow spine”).

When they swim coelacanths move their pelvic and When they swim coelacanths move their pelvic and pectoral fins in the same pattern that tetrapods walk.pectoral fins in the same pattern that tetrapods walk.

CoelacanthsCoelacanths

Because coelacanths possess an unusual suite Because coelacanths possess an unusual suite of characters including fat-filled lungs, a high of characters including fat-filled lungs, a high level of urea in the blood, a liquid filled level of urea in the blood, a liquid filled notochord, lobed fins, ventral kidneys and a notochord, lobed fins, ventral kidneys and a reduced brain there has been debate about their reduced brain there has been debate about their phylogenetic affinities.phylogenetic affinities.

The consensus today is that coelacanths are a The consensus today is that coelacanths are a sister group to the Rhipidistia which gave rise to sister group to the Rhipidistia which gave rise to the lungfish and tetrapods.the lungfish and tetrapods.

We will discuss the origins of the tetrapods We will discuss the origins of the tetrapods shortly.shortly.