A Tail Of Two Histories

The basic vertebrate body plan is a tube with a head at one end and a tail at the other. Over millions of years it has been ornamented with all manner of adaptations, including limbs, fins, spines, teeth, eyes and even secondary loss of such features, yet the basic plan has persisted. Heads are crucial to the body, providing the bulk of sensory information and neurological processing, and adaptations for specific modes of feeding. Tails are similarly important. In virtually all marine vertebrates it is the primary means of propulsion. In many species of primate it serves as a fifth limb for rapid maneuvering in tree tops, while in several big cats it is used to balance the forces acting on the body during a chase. Clearly the tail is a vital element of vertebrate evolutionary history.

The evolutionary progression of tails in primitive and derived
fish, showing the place of Aetheretmon valentiacum

The largest group of fish alive today are the teleosts. The embryos of these fish have an asymmetrical 'dual tail' consisting of an upper scaly branch containing the vertebral column and a lower fleshy lobe, the caudal fin. The growth of this upper tail is stunted during early development, becoming part of the body, while the caudal fin grows into a symmetrical tail in the adult form.

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)

'What this shows is that ancient fish and modern fish had the same developmental starting point that has been shared over 350 million years,' said Sallan. 'It's not the ancestral tail showing up in modern teleost larvae; it's that all fish have two different structures to their tail that have been adjusted over time based on function and ecology for all of these species.'

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.