Tetrapods – Why bother coming onto the Land?

Following our publication of an article providing information on the recent discovery of Tetrapod trace fossils in a Polish quarry, we have had the question posed to us – why evolve into cumbersome land dwelling Tetrapods when as fish you are extremely well adapted to a life in water?  Good question and one that continues to puzzle scientists.

To read the report on the trace fossils: From the Water onto the Land 35 million years Earlier

The Devonian Period is often referred to as the “Age of Fishes”, there certainly was evolutionary radiation in some of the fish clades that existed during this geological period.  The Actinopterygians or the ray-finned fish diversified extensively, indeed this group is still represented today by something like 25,000 species.  However, clades declined and many important fish groups that had evolved during the early Devonian did not survive to see the Carboniferous.  Jawless fishes, once common in the Late Silurian and up until the mid Devonian, went into steep decline, clades such as the Osteostracans and the Heterostracans were all but extinct some 370 million years ago.  Just a few families of jawless fish represented these once diverse and abundant groups as the Devonian drew to a close – sort of “dead clades walking”.

On the subject of walking, the Devonian saw the invasion of the land by the first four-legged vertebrates – the Tetrapods had arrived.  In the light of the article published regarding the discovery of Tetrapod trace fossils in Poland we received some information from Dr. Zerina Johanson, from the Natural History Museum in London.

Dr. Johanson, has published a number of papers on Devonian vertebrates, here are her comments:

The article on the Polish trackways is very important because of its age – Middle Devonian (Eifelian) based on Conodonts (extinct, jawless marine creatures whose fossils are used to help date rock strata – biostratification), found 20 metres above the trackways.

As the authors note, this is 18 million years older than the previously known Tetrapods, and also older than their nearest fish ancestors, the Elpisostegalids.  This has the effect of pushing the fish to Tetrapod split (evolution of Tetrapods from fish ancestors) from the Late Devonian (based on body fossils) to the Middle Devonian.  Also as the authors note, this period of time is characterised by more typical Sarcopterygian fish, rather than those that are currently recognised as the nearest ancestors to Tetrapods. These include the very famous Tiktaalik, which has a flattened skull and eyes rotated to the top of the skull. This morphology does not characterise Middle Devonian Sarcopterygian fish.

Also important is that the Polish trackway locality is interpreted as an “extremely shallow marine tidal, perhaps lagoonal, environment”, and the authors suggest that some of the track makers were swimming.  So, these earliest Tetrapods did not necessarily move onto land.  Other Devonian Tetrapods, such as Acanthostega from East Greenland, clearly lived in a river environment while possessing hands and feet.

So, your reader’s question about why Tetrapods evolved limbs and moved onto land, is a good one, and perhaps one that we can’t answer at the moment, because limbs seem to have evolved in the water. Sometimes it’s difficult to say exactly why a feature evolved – in this case we could say that the evolution of hands and feet was a preadaptation for life on land.  As to why Tetrapods moved onto land, the authors of the Polish study noted that:

“The intertidal environment provides a ready food source of stranded marine animals on a twice-daily basis, in the immediate vicinity of the sea, and would thus have allowed marine ancestors of Tetrapods gradually to acquire terrestrial competence while accessing a new and essentially untouched resource.”

In the Polish researcher’s scenario, Tetrapods were moving from a shallow marine environment onto land to access new food resources, although presumably they lived primarily in the water.  Fully terrestrial Tetrapods don’t appear until the Carboniferous.

Our thanks to Dr. Johanson for helping to explain the importance of the new research from Poland.  It does make you think, after all, as we put together this blog, in essence we are nothing more than highly evolved fish.

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