Croaking Science: Evolutionary Origins Part 1-The Age of Amphibians

Becky Austin our Croaking Science Volunteer, finds out about the evolutionary origons of our species in this two part post...look out for part two on reptiles in a few weeks time.

Picture the scene: A hot, humid environment, with huge forests of mosses and ferns and swampy mangroves. It sounds like a far cry from what we know today but this was what Britain was like in the Devonian era, around 400 million years ago, when it was part of a huge land mass close to the equator. It was a time of great change: the massive drop in atmospheric CO₂ due to these early forests paved the way for fishes of the time to take to the land, and become the very first amphibians.

The Devonian is often termed “the age of fishes”, as Osteichthyes, or “bony fishes” ruled the earth’s aquatic environments. One branch, the “lobe-finned fishes”, is the ancestor group of amphibians and all other terrestrial vertebrates. The closest modern-day example is the African lungfish, which can survive long periods of drought by breathing air with a primitive lung. However, it was a group of lobe-finned fishes called the Osteolepiformes which were the pioneers of the land.

A number of fossil discoveries have helped decipher how Osteolepiformes moved from water to land. Panderichthys, from 385mya onwards, had nostrils for breathing air, and eyes on top of its head to see out of the water. A flatter body and stronger bones helped protect against the crush of gravity out of water, and it had two sets of fins. 3 million years later, Tiktaalik had wrist bones and digits on its limbs, and by 365mya the first real amphibian-like creatures roamed the land, such as Ichthyostega and the later Eryops.

These first amphibians had tough skin to avoid drying out, lungs evolved from swim-bladders for use instead of gills, and hind-limbs extending from the newly evolved pelvis. But, like modern amphibians, they still had to return to water to lay eggs, which is why they retained their aquatic lifestyle. It is possible that this path of evolution was followed to adapt to life in and between woodland swamps of the Devonian – a path which led to the formation of all terrestrial vertebrate life seen today, over 350 million years later.
 

References:

Ahlberg, P.E. and Milner, A.R. (1994). The origin and early diversification of tetrapods. Nature, 368: 507-514.

Clack, J.A. (2012). Gaining ground: the origin and evolution of tetrapods. 2^nd edition. Indiana University Press, Indiana, USA.

Retallack, G.J. (2011). Woodland hypothesis for Devonian tetrapod evolution. The journal of geology, 119: 235-258.