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The Lineage of the Coelacanth
Dr Eric Anderson
What are the coelacanths and why is their evolutionary position so
important? Is there a link between coelacanths (dubbed as "old fourlegs"
and the origin of land
vertebrates?
Much attention has been given in the news media concerning the South African
Coelacanth Conservation and Genome Resource Programme, a research project
centering around the discovery of a population of these important, primitive
fishes off Sodwana Bay, located about 75 km south of the border with Mozambique.
This multi-partnered, multi-million rand effort was launched on 12 April 2002
and includes submersible dives, oceanographic research, a genome mapping project
and other related investigations.
What are coelacanths and why is their evolutionary position important? Ever
since the description of the first fossil coelacanth in 1839 scientists have
supported and refuted their lineage and the origin of land vertebrates, or
tetrapods. Over about the last 10 years it has become abundantly clear that, not
only were coelacanths not on the main line to tetrapods, but that they are part
of an early, primitive lobe-fin fish group.
The bony fishes are divided into two main groups, the ray-fins and lobe-fins.
The ray-fins include the thousands of familiar sport and commercial fishes, but
of the lobe-fins, only eight species survive, six lungfishes and two
coelacanths.
Tetrapods evolved from the more advanced lobe-fins of the Devonian Period
(about 410 to 355 million years ago), the so-called Rhipidistians, two of which
are pictured here in an evolutionary progression (bottom to top). At bottom is Eusthenopteron
foordi from Canada, long thought to be in the direct line to tetrapods since
its description in 1881. In the middle is Panderichthys rhombolepis,
member of a recently elucidated order, the Elpistostegiformes. These fishes had
lost the dorsal fins and most of the gill arches, breathed air and had many
skull characters of primitive tetrapods, like Ichthyostega stensioei from
Greenland (top; arrow points to remnant of gill opening). Ichthyostega
had seven digits in the feet and still retained some gill arch rudiments and fin
rays in the tail. They appeared in the Late Devonian.
The coelacanths have now been shown to share some advanced cheek and skull
characters with an extinct order called the Onychodontiformes. Neither the
earliest onychodont, Strunius walteri from Europe (pictured above), nor
earliest coelacanth, Gavinia syntrips from Australia had lobed fins. But,
on the basis of their unique head structures, scales, teeth and fin positions,
and the sharing of these with an even more primitive form from China called Psarolepis,
we can classify the onychodont-coelacanth line as primitive within the
lobe-fins, with Psarolepis as the stem lobe-fin; that is, the most
primitive with respect to all the rest.
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