Opportunity Knocked:
The Devonian Transformation

Devonian fishes

The Devonian Period (410 to 356 million years ago) has been traditionally refered to as the "Age of Fishes." This appelation is appropriate given the dramatic evolutionary changes in all of the major "fish" lineages during this period. The lobe-fin and ray-fin fishes first appear during the Devonian, and the lobe-fins diversified rapidly and experienced their peak diversity during this and the following (Carboniferous) periods. Acanthodians and placoderms first appear in the Silurian, but reach their peak diversity in the Devonian. In fact, the overwhelming majority of placoderms are restricted to it. Shark-like scales occur as early as the early Silurian, but the first unequivocal shark fossils come from the early Devonian and their variety increase considerably throughout the period. Even agnathans, a polyphyletic group of jawless vertebrates which may have first appeared as early as the Cambrian, experienced substantial diversification as well as major extinctions during the Devonian.

vertebrate diversity over time cooksonia

But this emphasis on fish evolution obscurs other —more significant— changes. These changes center on the emergence and diversification of semi-aquatic and terrestrial tracheophytes (vascular plants). Fossil spores belonging to bryophytes (nonvascular plants such as liverworts and mosses) are known from as early as the Middle Ordovician. The trilete spores of pre-trachaeophytes first occur in the late Early Silurian as does the famous early fossil plant Cooksonia. These and other early plants, which are referred to as the rhyniophytes, diversify from the late Silurian through the Middle Devonian into the two major trachaeophyte clades: lycophytes (barinophytes, zosterophyllophytes and lycopsids) and euphyllophytes (trimerophytes, ferns, sphenopsids, progymnosperms, and seed plants).

vascular plant diversity over time archaeopteris frond

The diversification and expansion of trachaeophytes from the Late Silurian through the Devonian transformed the landscape. Vegetation at the beginning of this interval consisted of low-lying plants restricted to a narrow zone along the ocean's edge. By the Late Devonian, trachaeophytes expanded inland to form extensive marshes and extended upstream to form floodplain forests dominated by large trees. Plants may even have colonized drier habitats much farther from the water's edge. Terrestrial plant production had increased to the point that now coal was being formed and natural fires could be substained.

The ramifications of vegetational expansion were dramatic. These plants transformed the biosphere by transforming the terrestrial environment and linking it more closely with the aquatic realm. The first forests created a totally new biome. Terrestrial invertebrates responded to floral changes. Soil formation was accelerated and aquatic habitats became more diverse and stable. Freshwater and estuarine life became more diverse and productive. The effects of these plants are also implicated in global carbon cycling and the Devonian mass extinction.

These changes also set the stage for the evolution of tetrapods and their colonization of the land.

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Hans Kerp's web page on the earliest land plants:
Hans Steur's web page on the earliest land plants:
Ralph Taggart's web page on the first vascular land plants:
Gensel, P.G. and H.N. Andrews. 1987. "The Evolution of Early Land Plants." American Scientist 75: 478-489.
Scientific Papers:
>Bateman, R.M., P.R. Crane, W.A. DiMichele, P.R. Kiendrick, N.P. Rowe, T. Speck and W.E. Stein. 1998. "Early Evolutionof Land Plants: Phylogeny, Physiology, and Ecology of the Primary Terrestrial Radiation." Annual Review of Ecology and Systematics 29: 263-292.
Beerbower, J.R. 1985. "Early development of continental ecosystems." pp. 47-92. In: B.H. Tiffney (ed.), Geological Factors and the Evolution of Plants. Yale Univ. Press: New Haven.
Beerbower, J.R., J.A. Boy, W.A. DiMichele, R.A. Gastaldo, R. Hook, N. Hotton, III, T.L. Phillips, S.E. Scheckler, and W.A. Shear. 1992. "Paleozoic terrestrial ecosystems." pp. 205-235. In: A.K.Behrensmeyer, J.D. Damuth, W.A. DiMichele, R.Potts, H.-D. Sues and S.L. Wing (eds.) Terrestrial Ecosystems through Time. Chicago: Univ. Chicago Press.
Gensel , P.G. and D. Edwards (eds.). 2001. Plants Invade the Land: Evolutionary and Environmental Approaches. New York: Columbia Univ. Press.
Kendrick, P and P.R. Crane. 1997. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington DC: Smithsonian Institution Press.
Kendrick, P. and P.R. Crane. 1997. "The origin and early evolution of plants on land." Nature 389: 33-39.
Image Credits:
Images are copyrighted © 2002, Dennis C. Murphy, (see Terms of Use).

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