Well it didn’t take long for avid readers to spot the oddity in Friday’s posted picture of the early tetrapod, Acanthostega. This strange animal did not have the ‘usual’ number of digits, instead it had eight. Having more than five fingers or toes is called polydactyly and is a rare condition.
Few examples of polydactyly occur in the animal kingdom today, the panda’s thumb though, is one classic example. The panda has five digits on its paw plus an opposable 'thumb' but this thumb is not a sixth digit like the others, but actually an unusual outgrowth of a wrist bone. Thus even the panda’s thumb is not truly an example of more than five digits.
So what about Acanthostega and his buddies? The Late Devonian was a busy time, plant life was diversifying and insect life was gaining ground on land. Tetrapods, were making their first steps on to land and living semi-aquatic lifestyles about 375 million years ago. These animals had many characteristics advantageous to aquatic life, such as streamlined bodies, webbed feet and tail fins. But they also had weight-bearing limbs with which they could lift themselves out of the water. Acanthostega had eight digits on its front and hind limbs and two other early tetrapods, Ichthyostega and Tulerpeton, also had more than five digits.
So how could these animals have had such a wide variety of limb structures when all of their descendants seem to have a variation upon the five digit structure? It is thought that pentadactyly evolved in an animal that was ancestral to all present day tetrapods (amphibians, reptiles, birds and mammals) and that this event event happened 340 million years ago in the Lower Carboniferous (about 35 million years after the first tetrapods evolved).
To be honest, we don#t understand how these animals could have been so experimental while their descendants were so conventional. And why 5 anyway? No one knows. And we have no examples to examine. There are few animals who have extra digits. The most common is the novelty polydactyl cat, but this species is the result of selective breeding of animals with a genetic anomaly.
In a general sense, we know that it easier to ‘lose’ a trait then to gain it, hence the large number of animals who have reduced digits. But the striking lack of polydactyl examples in the long history of tetrapods since the Devonian implies there may be an evolutionary constraint at work. For example, pleiotropy is the multiple effects of genes on more than one physical characteristic. Hand-Foot-Genital syndrome illustrates such a condition. This rare disorder malforms limbs and the urinary system because the same defective genes pattern both systems. So perhaps the constraint on tetrapod limb structure is part of a greater pattern.