|The evolutionary progression of tails in primitive and derived |
fish, showing the place of Aetheretmon valentiacum
Primitive relatives of teleosts, the chrondrosteans, carry a dual tail structure into adulthood. What this suggests is that the loss of the upper tail during teleost development was an example of ontogeny (embryological development) recapitulating phylogeny (evolutionary development). Yet without fossils of a primitive two-tailed ancestor this theory is somewhat weakened.
Now a recent study of 350 million year old fish fossils from Scotland has provided conclusive proof that the theory is incorrect, providing a more suprising view of vertebrate evolutionary history. Lauren Sallan, an assistant professor in the School of Arts & Science's Department of Earth and Environmental Science, studied fossils of an extinct teleost relative Aetheretmon valentiacum which had resided, largely unstudied, in fossil collections across Scotland. The smallest specimens, barely over an inch in length, were of juveniles at an intermediate stage of development. If ontogeny had recapitulated phylogeny, then these juveniles should have been similar in form to adults. Yet the juveniles had a tail like that of modern teleost juveniles, while the adults had asymmetrical tails more like chondrosteans.
|The asymmetric tail of Aetheretmon valentiacum (front) |
versus the symmetrical tail of modern teleosts (back)
This argument extends to tetrapods which are descended from the same class of fish as the teleosts. As such, the tails of tetrapods and modern fish are not truly the same structure adapted for different purposes; instead in the former the lower caudal tail is lost and the vertebral tail enlarged, while in the latter the upper tail is reduced and the caudal fin enlarged.
'It tells us why we have all this diversity in fins and limbs in past and present,' added Sallan. 'There might have been some lineages that favored one form over another for functional or ecological reasons. If a fish couldn't adapt this trait, which is so vital for swimming, they might have gone extinct.'
The next step in studying this quirk of evolution would be to confirm the molecular pathways underlying the dual tail development in tetrapods and fish.