Overberg Geoscientists Group
416 Mya
First Amphibians
The first Tetrapods -that is animals with 4 legs evolved from the Lobe finned fish, such as the Coelacanth and Lungfish. There is a pretty consistent fossil assemblage that shows the development of the anatomy and organs which evolved to enable the creatures to inhabit the land.
As we know there are numbers of fish in the present fauna which can spend time out of, and away from, water. Mudskippers can be seen around mangroves and eels travel overland if their streams or ponds dry up. Lungfish can remain buried in the lakebeds until the rains bring relief and the lakes fill up again.
These are all positive indications that the complete ties to water are not totally essential.
It is not known what exactly were the advantages for this migration away from water, but the fact that plants and insects had already adapted to the land may have pushed the fish to utilize this source of food. There is also evidence of large predatory fish in rocks of the same age, so escape from predation may have contributed too.
There are a number of immediate differences that fossils must show if they are to be considered as part of the transition between lobe-finned fish and tetrapod amphibians. The most obvious perhaps is the development of limbs.
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If we look at the skeletal changes, we can follow this clearly particularly the development of the legs, though there are other skeletal changes too.
This is Eusthenopteron still a lobe finned fish, but with the basic skeleton to form the basic limbs.
Tiktaalik could have come out onto land and had the basic ‘one – two – lots’ bone organization. That is:
Humerus – Radius & Ulna – lots in the fore limb
Femur – Tibia & Fibula – lots in the hind limb.
It probably used the limbs mainly underwater, but could emerge if necessary.
Acanthostega. The limbs were even better developed and especially the hind limbs, could support its weight. It still had gills, so would only emerge occasionally.
Ichthyostega. A fully adapted amphibian.
It is perhaps very difficult to say where fish end and amphibians start. Eusthenopteron is obviously a fish, Ichthyostega an amphibian.
There can be no doubt about the transitions.
Further reading:
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Shubin, N. (2008). Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body. New York: Pantheon Books
Laurin, M. (2010). How Vertebrates Left the Water. Berkeley, California, USA.: University of California Press.
Long JA, Gordon MS (2004). "The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition". Physiol. Biochem. Zool. 77 (5): 700–19.
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D J Mourant Aug 2023