02 September 2005

Let the best lineage win

A paper on Ichthyostega, one of the oldest known amphibian fossils, was published in yesterday's Nature1. For more information on the significance of this fossil for our understanding of the development of terrestriality among vertebrates, I recommend the posts at afarensis and Pharyngula.

What got me interested in this subject was the accompanying News & Views article by Robert L. Carroll2. In the last paragraph of his essay, Carroll points out that among the at least 11 lineages of lobe-finned fish and early amphibians from the Devonian, only one appears to be a close relative of the extant land vertebrates. All other lineages, including that of Ichthyostega, seem to have died out. Carroll also mentions that similar "ultimately unsuccessful experiments" took place during other major transformations in the evolution of vertebrates, including the evolution of several species of Homo, only one of which remained.

The development of terrestriality was also a major event during the evolutionary history of gastropods. Several lineages of marine gastropods independently invaded land3 and, unlike the early vertebrate pioneer lineages that ultimately failed, many of these gastropod lineages seem to have survived. An incomplete list of extant gastropod families that appear to have become terrestrial independently of each other include the Cyclophoridae, Pomatiasidae, Helicinidae and the Truncatellidae. Of course, the most successful group of them all is the stylommatophoran pulmonates. These are the land snails that carry their eyes on the tips of the upper pair of their two pairs of tentacles (for an exception to the usual two pairs of tentacles, see this previous post of mine on Vertigo).

The stylommatophorans have evolved the most number of species of terrestrial snails and invaded most terrestrial habitats, including deserts. In these respects, they have, therefore, been more successful than the other gastropod groups that became terrestrial. Many of the non-stylommatophoran land snails either have fewer species or are much restricted in their habitat preferences.

Thus, it seems that during the early evolution of the major animal groups, such as the vertebrates or the gastropods, several groups independently attempted the same major lifestyle change, but ultimately either only one group succeeded or several groups succeeded with one of them becoming much better at the new lifestyle than the rest.

This is certainly worth developing further, but it is too late in the night for that.

1. Ahlberg P.E., Clack J.A., Blom H. 2005. The axial skeleton of the Devonian tetrapod Ichthyostega. Nature, 437:137-140.
2. Carroll, R.L. 2005. Between water and land. Nature, 437:38-39.
3. Barker, G.M. 2001. Chapter 1 in The Biology of Terrestrial Molluscs (Barker, G.M., ed.). CABI Publishing.

1 comment:

afarensis said...

Interesting - and informative - take on the subject.