Early Tetrapods Thrived in Woodlands

In a study published in the most recent edition of the scientific publication “The Journal of Geology”, University of Oregon based researchers are claiming that the first land dwelling Tetrapods originated in humid, forested habitats.  The scientists claim that they have evidence to suggest that the first types of fish to move around on land lived on wooded flood plains.

Up until the early 1980’s most palaeontologists accepted the hypothesis that fish such as the Panderichthyids, from which Tetrapods are believed to be descended made the move out of water onto land as a result of many water courses drying up.  This theory stated that as inland sea, lakes and rivers gradually shrank and eventually dried up, as the climate warmed, the fish that lived in these habitats evolved terrestrial habits and adapted to a life in which greater and greater amounts of time were spent out of the water.  Evidence for the hot Devonian climate can still be seen in many places today.  As water evaporated mineral deposits such as salt were left behind.  These were readily oxidised in contact with the air and this led to the formation of great, red sandstone belts that can be seen in many parts of the world.  The sandstone sea stack, known as the “Old Man of Hoy” is an example of Devonian aged sandstone.

One of the most prominent exponents of this “fish onto dry land” theory was Alfred Romer, who died in 1973 after a long and distinguished academic career which saw him serve in the faculties of both Harvard University and the University of Chicago.

Eusthenopteron – A Candidate for the Ancestor of Tetrapods

The name Eusthenopteron means “good strong fin” – ideal for a tramp in the woods?

Picture Credit: Parc National de Miguasha

However, there have been a number of challenges to this theory.  Many scientists now believe that the first Tetrapods evolved in much more humid and wetter habitats.   According to Gregory J. Retallack (University of Oregon), his discoveries at numerous sites in Maryland, New York and Pennsylvania suggests that Romer and his supporters got it wrong when they postulated on fish adapting to a drier climate.

Professor Retallack, believes that his own studies show that the first land-dwelling Tetrapods evolved in much more wet and humid environments – wooded floodplains.

He stated:

“Such a plucky hypothetical ancestor of ours probably could not have survived the overwhelming odds of perishing in a trek to another shrinking pond.”

During the Late Devonian, there was no immense struggle to escape drying water courses in order to ensure survival, according to the Professor.  In this new study, he argues for a very different explanation.  The Professor examined numerous buried soils in rocks yielding footprints and bones of early transitional fossils between fish and amphibians of Devonian and Carboniferous geological age.  What he found raises a major challenge to Romer’s theory and supports later theories put forward over the last thirty years or so that suggest that limbs did not develop from the paired fins of the Panderichthyids to help creatures walk on land, but they probably evolved to help these animals crawl over plant stems and logs in clogged up water courses.

The Professor commented:

“These transitional fossils were not associated with drying ponds or deserts, but consistently were found with humid woodland soils.  Remains of drying ponds and desert soils also are known and are littered with fossil fish, but none of our distant ancestors.  Judging from where their fossils were found, transitional forms between fish and amphibians lived in wooded floodplains.  Our distant ancestors were not so much foolhardy, as opportunistic, taking advantage of floodplains and lakes choked with roots and logs for the first time in geological history.”
During the Late Devonian and Early Carboniferous, extensive swamps and lowland forests evolved.  It is the preserved remains of these huge areas that have helped form the immense coal deposits found in Europe and elsewhere in the world.
The Professor’s study supports the theory of a wet, watery world from which the first Tetrapods evolved to exploit these forested habitats.  Limbs would prove “handy” (no pun intended), for negotiating fallen logs and other obstacles, whilst flexible necks allowed for feeding in shallow water.  By this new woodland hypothesis, the limbs and necks, which distinguish salamanders from fish, did not arise from “reckless” adventure in deserts, but rather were nurtured by a newly evolved habitat of humid, wooded floodplains.

The findings, Retallack said, dampen both the old desert hypothesis of Romer and a newer inter-tidal theory put forth by Grzegorz Niedbwiedzki and colleagues at the University of Warsaw, after the discovery of some remarkable Late Devonian trace fossils of early Tetrapods found in ancient, preserved, lagoonal mud found in southeastern Poland.

To read about the Polish discovery: From the Water and Onto the Land – Thirty-Five Million Years Earlier

In 2010, the Polish team published their discovery of evidence suggesting the existence of 2.5 metre long, 395-million-year-old Tetrapods in Late Devonian strata.   The track-ways they uncovered suggest that some types of animal may have been capable of venturing out onto land many tens of millions of years earlier than previously thought.

As with the desert pond theory, postulated by Romer,  this new hypothesis also proposes that fish gradually developed an adaptation of their existing swim bladder into a rudimentary lung that allowed them to survive for longer periods out of water.  That happened about the same time as paired fins were developing into true limbs.

Another advantage that the new Tetrapods had was a lack of any land-based predators to discourage their early perambulations.   Given that these were no “small fry” but in some cases, creatures almost a metre long, they faced no danger from bigger fish in the sheltered inlets and oxbows of a flooded woodland.

Fish in woodland habitats, whatever next?

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