Several ecological changes played a major role in the Devonian water-to-land evolutionary transition of tetrapods.
These changes appear as a consequence of the colonisation of land by vascular plants, a process begun in the Silurian period (435-410 milions of years ago), with the origin and adaptive radiation of the rhyniophytes.
You can download here the 2002 version of the Global Stratigraphic Chart of the IUGS (International Union of Geological Sciences).

Some fossil remnants are even known from the uppermost Ordovician strata, about 440 millions of years ago (Arens et al., 1998: UCMP).

Transitional environments, at the interface between water and land, offered to these autotrophs more light availability, higher concentrations of oxygen and carbon dioxide in the subaerial medium, more rapid gas diffusion rates, and soils that are usually richer of minerals (Raven et al., 1981).

The origin and expansion of vascular plants on land took place at the water edge, probably in low energy, soft-bottomed habitats like estuaries, swamps, or the banks of the lower tracts of rivers, were the first rudimentary radical structures could be more easily anchored.

During the Early and Middle Devonian, the first appearance and expansion of the two major groups of terrestrial vascular plants, namely the lycophytes and the euphyllophytes, was followed by the diversification of a number of increasingly terrestrial arthropod lineages, including the scorpions (Clack, 2002; Murphy, 2005: Opportunity knocked).

During the Late Devonian several plant taxa underwent an extensive adaptive radiation in semiterrestrial habitats; the vegetation coverage of land went through a dramatic expansion: the 'Devonian explosion'.

Euphyllophytes such as the first massive arboreal progymnosperms (e.g. Archaeopteris sp.) rapidly expanded their ranges from the water's edge into more terrestrial ecosystems.

This led to radical changes in the environment.

Left: an Early Devonian landscape; right: two early tetrapods along the banks of a Late Devonian stream: Acanthostega gunneri (foreground) and Ichthyostega stensioei (in the background).
Left: © Z. Burian (In: Špinar & Burian, 1972); right: illustration: J. Sibbick © PBS (In: A Brief History of Life, 2006), with permission

Mean net primary production (grams of dry matter/m^2/yr) recorded for those modern ecosystems that are assumed to be similar to Late Devonian ones for their general ecological characteristics.
dsx= deserts (rock, sand, ice); os= open ocean; cs= continental shelf; l-r= lake and stream; upw= upwelling zone; tsf= tropical seasonal forest; es= estuaries; tpf= tropical pluvial forest; abr= algal bed and reef; sw= swamp and marsh
Data from Whittaker & Likens, 1973; and from Bullini et al., 1998

These newly formed wetland ecosystems were likely to have been among the most productive ones in the Late Devonian, especially at tropical latitudes and where supplied by the subsidiary energy of tides, that promote a rapid nutrients' turnover (Odum, 1983).

This hypothesis is based on a comparison of the net primary productivities in extant ecosystems.

The assumption is that during the Late Devonian terrestrial ecosystems were already sufficiently similar to some modern ones to postulate that similar general ecological dynamics were operating (Odum, 1983; Bullini et al., 1998; Clack, 2002; Murphy, 2005).

Several geological models show that the Devonian period witnessed a rapid and dramatic increase in atmospheric oxygen, together with a corresponding decrease in carbon dioxide (Graham et al., 1997).

This was one of the most important consequences of the increase in land plants covering the Earth.

The evolution of rigid and resistant structures both to sustain their weight out of water and to conduct nutritive substances to all the parts of their bodies allowed these increasingly terrestrial autotrophs to evolve bigger and bigger bodies.

Their structural framework contained organic compounds such as cellulose and lignin, which are resistant to microbial and other biochemical degradation (Odum, 1983).
This resulted in a rapid increase in the accumulation rate of organic carbon in subaerial sediments.


For the first time since the origin of life on Earth, large amounts of organic carbon began to accumulate above the sea level.

Fluctuations of percent oxygen and carbon dioxide according to different models through Paleozoic and Mesozoic Eras.
All the models indicate a sharp increase in the oxygen level and a contemporaneous decrease of carbon dioxide level at the end of the Devonian period (D). PAL= Present Atmospheric Level. Modified from Graham et al., 1997, with permission from the authors

Both organic carbon and nutrient sediment concentration increase with decreasing grain size.
TOC (wt%)= weight percentage of total organic carbon; TN (wt%)= weight percentage of total nitrogen; %mud= mud weight percentage in the sediment.
Data taken from a modern wave dominated estuary; analogous results have been obtained in drowned river valleys. Modified from Logan & Longmore, In: Radke et al., 2003, with permission.

Where were these large amounts of plant material produced at the water's edge likely to accumulate after death, and what would have been the ecological consequences?

Again, we can assume that during the Devonian the same general sedimentological dynamics that are currently at work were operating.

Sites of organic matter accumulation are controlled to a large extent by processes that govern the transport and deposition of fine sediment, because organic matter adsorbs onto mineral surfaces and has a high affinity for fine-grained sediment (Logan & Longmore, 2003).

On the other hand, fine sediment is deposited only in conditions of low environmental energy, such as lakes, alluvial plains, estuaries, tidal flats and deep sea basins (Ricci Lucchi & Mutti, 1980).

That is, with the exception of the latter one, exactly at the water's edge: right where this material was produced in great amounts during the Late Devonian.

Furthermore, the rapidly expanding vegetation exerted a drag force in shallow water currents and decreased the depositional energy of the colonised transitional habitats (not unlike the modern mangrove forests: Mazda et al., 1997a,b), thereby increasing the sedimentation rate of fine sediment.
Roots also stabilised sedimentary deposits, decreasing erosion rates.

Under the water, higher sedimentation rates reduced the contact time between organic matter and dissolved oxygen, inhibiting its aerobic decomposition and contributing to the accumulation of higher concentrations of organic carbon and nutrients in sediment (Logan & Longmore, 2003).

On land, high organic detrital inputs in the soil and the short burial time implied that microbial oxygen consumption was more rapid than the rate of oxygen diffusion through the sediment. Again, this resulted in higher deposition rates due to the slower decomposition by anaerobic microorganisms (Odum, 1983). Moreover, gas diffusion rates are positively correlated with sediment granulometry, making anaerobic decomposition in water-logged and fine sediments largely predominant.

Furthermore, these first arboreal plants increased the proportion of finer particles in the soil by the direct and indirect production and delivery of organic acids via leaching of surface litter and penetration of the sediments by deeper roots (Murphy, 2005: Plants and soils).
These processes, also known as biological weathering, greatly increased the proportion of finer particles in the sediments, especially clays.

Right, above: median TOC (wt%) (weight percentage of total organic carbon) in the sediment of some modern southwestern Australian estuaries versus percent catchment cleared. Sediment TOC is inversely correlated to the amount of native vegetation that has been removed from the catchment.
Modified from Logan & Longmore, In: Radke et al., 2003, with permission.

Right, below: visual model of a rhizosphere, the volume surrounding the root of a vascular plant, where the biological weathering takes place.
Elaboration: Prusinkiewicz P., 1991 (© Algorithmic Botany, 2006), with permission.

In this way, these first semiterrestrial ecosistems, that occupied brackish and freshwater shallow waters, and were characterised by low environmental energy and deposition of fine sediment, both greatly expanded and became sinks of nutrients and organic carbon.

Here more and more complex detrital food webs developed, as more animal species of previously exclusively marine groups began to exploit these massive trophic resources.

These processes led to the formation and expansion of wide, gently sloping deposits of mud, that formed wide ecological gradients from water to land, where a whole array of gradually changing environmental conditions from aquatic to subaerial conditions was realised.

These ecosystems were rapidly colonised by an increasingly rich and diverse aquatic vertebrate fauna.

The adaptive radiation along these ecological gradients was patterned by synecological and autoecological factors, such as the availability of water and dissolved oxygen, or the inter- and intraspecific competition.

Along the coastline these gradients were further extended onshore by specific tidal sedimentological processes (Ricci Lucchi & Mutti, 1980).

The Middle and Late Devonian expansion of vascular plants in semiterrestrial habitats induced dramatic changes in the composition, productivity and complexity of soft-bottomed estuarine and freshwater ecosystems.

A striking increase in abundance and diversity among estuarine and freshwater animals is documented by the Late Silurian and Devonian fossil record (Murphy, 2005: Going Upstream).

Lungfishes (dipnomorphs) underwent an adaptive radiation during the Devonian as they invaded the increasingly rich and productive brackish and freshwater ecosystems.
The species from the Early Devonian, like Dipnorhynchus sp. (A), were marine.
During the Late Devonian lungfishes became increasingly restricted to estuarine and freshwater habitats, acquiring a paedomorphic body plan (e.g. Phaneropleuron sp.: B) that would be conserved up to the present with few changes (e.g. Protopterus sp.: C).
Note the visible reduction of the opercular bone from A to B and C, as more specialised air breathing structures were developed.
A: redrawn by G. Polgar from Long, 1995, with permission; B: modified from Saxon, 1975 © Fish Out of Water (11/2005); C: drawing from Eccles, 1992 © Froese & Pauly (2005): Fishbase. Drawings not in scale

On the other hand, the increase in atmospheric oxygen resulted in a thickening of the Devonian ozone layer, already formed by the photosynthetic action of unicellular autotrophs during the Proterozoic eon (2500-540 millions of years ago: Bullini et al., 1998).

The consequent decrease in ultraviolet (UV) radiation reaching land probably facilitated the vertebrate colonisation of subaerial environments (Raven et al., 1981). Exposure to UV radiation can induce DNA damage, and air absorbs it less efficiently than water.

Three types of UV radiation can be distinguished. In order of increasing energy and activity on DNA: UVA, UVB, and UVC.

At present, the ozone layer, located 15 to 60 km above the Earth, completely absorbs the extremely active UVC (200-280 nm), preventing it from reaching the Earth's surface.
UVB radiation (280-320 nm) is also strongly absorbed, though a small fraction of it reaches the surface. UVA radiation (320-400 nm) is only weakly absorbed by ozone. Some of this radiation is scattered near the Earth's surface.

Present solar UV flux (watt/cm^2/nm) versus wavelength (nm) for various altitudes (top of the atmosphere; 30 km; 20 km; and Earth surface).
The red line shows the surface flux for a 10% reduction of the ozone. The blue line shows the DNA action spectra (for man), that is the probability of DNA damage by UV radiation at various wavelengths.
Modified from Newman, 2002 (© NASA GFSC SEES), with permission

How these dramatic changes affected the conditions of these shallow aquatic environments?

At the water's edge, where vegetation was abundant, the high oxygen demand of the organic matter and the low permeability of fine sediments made these warm shallow waters chronically or periodically highly hypoxic.

This may have happened both in marine and freshwater environments: daily (e.g. in tide pools), or seasonally (e.g. in alluvial plains and in riverine or lacustrine systems, due to floods or droughts).

These environmental conditions must have rewarded any air-breathing adaptation in the aquatic vertebrates that began to invade these ecosystems (Horn et al., 1997; Inger, 1957).

Also in many modern fishes, environmental hypoxia often seems to have been a crucial factor for the evolution of air breathing (see also Bimodal Respiration).

Complex structures such as internal palatal nostrils, or choanas, had originated independently in the Middle Devonian both in dipnomorphs (lungfishes) and in tetrapodomorph osteolepiforms (probable ancestors of all tetrapods) (Long, 1995; Zhu & Ahlberg, 2004; Murphy, 2005).

These strongly convergent anatomical changes are thought to correspond with the ability to breathe air, as suggested by the comparative anatomy of modern terrestrial vertebrates. It is reasonable that such structures were strongly adaptive during the invasion of brackish and freshwater habitats.

During the Late Devonian, a great evolutionary opportunity beckoned for air-breathing fishes living in tropical shallow waters.

That is, abundant trophic, environmental and metabolic resources, unexploited by vertebrate competitors, as well as habitats free from aquatic predators, were potentially available to any fish lineage that could increase its terrestriality (Clack, 2002).

Evolution of the choanas in the tetrapodomorphs.
During the Devonian the tetrapodomorph posterior nostril shifts onto the palate through breaching of the maxillary-premaxillary arcade, giving birth to choanas.
A coeval but independent process took place in dipnomorphs through loss of the arcade and inrolling of the lip (Zhu & Ahlberg, 2004).
These structures enabled to gulp air though nostrils while remaining under the water surface, or with a prey into the mouth (Romer & Parsons, 1986). All modern tetrapods and dipnoans have choanas.
A= Holoptychius sp., a porolepiform: ancestral condition (Late Devonian); B= Kenichthys campbelli, a basal tetrapodomorph: intermediate stage (Early Devonian); C= Eusthenopteron sp., an osteolepiform: derived condition (Late Devonian);
Left: skulls without mandibles; right: ventral partial views of the palates;
an= anterior nostril; ch= choana; m= maxilla; n= external nostril (naris); pm= premaxilla; pn= posterior nostril; v= vomer.
Note that the porolepiform Holoptychius sp., though being coeval to the derived form and even more recent than the intermediate one, retained the ancestral condition.
Adapted from: Long, 1995* (skull of Holoptychius sp.); Zhu & Ahlberg, 2004 (palatal sketches, skull of Kenichthys campbelli, © Nature, Macmillan Publ. Ltd*); and Clack, 2002* (skull of Eusthenopteron sp.). Drawings not to scale
*with permission