Monday, 8 June 2015

A Look At The Early Tetrapods

Most amphibians to life on both land and in the water
Amphibians, by definition, straddle the aquatic and terrestrial worlds. While different species may lean towards one environment more than the other, generally speaking they all have the capability to move about on both land and in water.

Their skins must remain moist but they can tolerate periods out of water in order to hunt, mate or move to new pools as old ones dry up. This is, of course, the modern state of amphibians. It has been assumed that early amphibians were small and greatly favoured water over the land. The fossil record of the first amphibians is sparse - a phenomenon known as Romer's Gap - and so our view of them is limited. Yet the specimens we do have indicate this was the case.

The first amphibians which were fully adapted for terrestrial locomotion are several tens of millions of years younger. Analysis of a 333 million year old limb from the amphibian Ossinodus pueri, however, has shown that the adaptations for a more terrestrial lifestyle may have evolved further back than the fossil record suggests. Conducted by researchers from Monash University, the Queensland Museum and the Queensland University of Technology, this revealed that the limb had been fractured in life and was in the process of healing.

'The nature of the fracture suggests the bone broke under high-force impact,' said Dr Matthew Phillips from the Queensland University of Technology. 'The break was most plausibly caused by a fall on land because such force would be difficult to achieve with the cushioning effect of water. Indeed, the fracture is somewhat reminiscent of people falling on an outstretched arm and the humerus crashing into and fracturing the radius.'

A false colour scan of the 333 million year old Ossinodus limb,
where the colour indicates the stress placed on the limb
The re-modelling of the internal bone structure during the healing process was in accordance with the forces and stresses generated by walking on land. Additionally the blood vessels appeared to enter the bone at low angles, which would aid in reducing mechanical stress in the bones supporting the body weight. The high degree of ossification in the bone suggests that it was adapted to support the creature's weight. 

'This specimen of Ossinodus is our oldest vertebrate relative shown biomechanically to have spent significant time on land. It is two million years older than the previous undoubtedly terrestrial specimens found in Scotland, which were less than 40 centimetres long,' said Dr Phillips.

Its size combined with its age suggests that the process of adapting to the new biomechanical challenges posed by an increasingly terrestrial lifestyle began before Ossinodus evolved. The timeline of early tetrapod evolution bears closer scrutiny. Other fossils of Ossinodus show that it was up to 1.5 in length and 20 kilograms in weight. To have moved about on land its skeleton must have been adapted to support such a bulk.