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.
Figure 24.02
16.2
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.
Figure 24.01
16.1
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
http://people.eku.edu/ritchisong/RITCHISO//fins2.gif
Lobe-finned fishes: Class
Sarcopterygii
 Primitive
Sarcopterygians were 20-70 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
http://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
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 fanshaped 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
http://www.amtra.de/images/Lungenfisch415.jpg
South American Lungfish
http://www.ucmp.berkeley.edu/vertebrates/sarco/lungfish1.jpg
Australian Lungfish
http://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 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 1938.

The curator of a small museum, M. CourtneyLatimer, 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 CourtneyLatimer 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.
Figure 24.23
16.20
Coelacanths

Coelacanths are readily identified from their fins.




The caudal fin has a small median 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”).
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.