Archaeopteris

Taxobox
fossil_range = Late Devonian - Early Carboniferous
regnum = Plantae
divisio = Progymnospermophyta †
ordo = Archaeopteridales †
familia = Archaeopteridaceae †
genus = "Archaeopteris" †
subdivision_ranks = Species
subdivision =
*†"Archaeopteris fissilis"
*†"Archaeopteris halliana"
*†"Archaeopteris hibernica"
*†"Archaeopteris macilenta"
*†"Archaeopteris obtusa"

"Archaeopteris" is an extinct genus of tree-like plants with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous, and has a global distribution.

Until the recent discovery of "Wattieza", many scientists considered "Archaeopteris" to be the earliest known tree. Bearing buds, reinforced branch joints, and branched trunks similar to today's timber, it is more reminiscent of modern seed-bearing trees than other spore bearing taxa; It combines characteristics of woody trees and herbaceous ferns, and belongs to a group of extinct plants sometimes called the progymnosperms, plants with gymnosperm-like wood but that produce spores rather than seeds.

Anatomy

The trees of this genus were small to medium in size with leafy foliage reminiscent of some conifers. The large fronds were thickly set with fan-shaped leaflets on stems that inclined sharply upward. They typically grew to 10 m in height. Some species are large, with trunks that exceeded 1.5 m in diameter. The veining branches diverged dichotomously. There was also intermediate feathering at each frond node or axes.

Leafy shoots occurred in opposite arrangement in a single plane. The leaflets, or pinnules, overlapped one another and were subcircular to wedge-shaped. On fertile branches, the leaves were replaced by spore capsules.

Other modern adaptations

Aside from its woody trunk, "Archaeopteris" possessed other modern adaptations to light interception and perhaps to seasonality as well. The large umbrella of fronds seems to have been quite optimized for light interception at the canopy level. In some species, the pinnules were shaped and oriented to avoid shading one another. There is evidence that whole fronds were shed together as single units, perhaps seasonally like modern deciduous foliage or like trees in the cypress family Cupressaceae.

The plant had nodal zones that would have been important sites for the subsequent development of lateral roots and branches. Some branches were latent and adventitious, similar to those produced by living trees that eventually develop into roots. Before this time, shallow, rhizomatous roots had been the norm, but with "Archaeopteris", deeper root systems were being developed that could support ever higher growth.

Habitat

Evidence indicates that "Archaeopteris" preferred wet soils, growing close to river systems and in flood plain woodlands. It would have formed a significant part of the canopy vegetation of early forests. Speaking of the first appearance of "Archaeopteris" on the world-scene, Stephen Scheckler, a professor of biology and geological sciences at Virginia Polytechnic Institute, says, "When [Archaeopteris] appears, it very quickly became the dominant tree all over the Earth. On all of the land areas that were habitable, they all had this tree".

Scheckler goes on to say, "Its litter fed the streams and was a major factor in the evolution of freshwater fishes, whose numbers and varieties exploded in that time, and influenced the evolution of other marine ecosystems. It was the first plant to produce an extensive root system, so had a profound impact on soil chemistry. And once these ecosystem changes happened, they were changed for all time. It was a one-time thing" (sciencedaily.com).

Looking roughly like a top-heavy Christmas tree, "Archaeopteris" may have played a part in the transformation of Earth’s climate during the Devonian before becoming extinct within a short period of time at the beginning of the Carboniferous period.

Relationship to conifers

Because "Archaeopteris" reproduced by releasing spores rather than by producing seeds, paleobotanists suspect that modern trees come from a sibling line of plants they call the "progymnosperms". "Archaeopteris" is more like an ancient cousin than a direct ancestor.

Discovery and classification

"Archaeopteris" was originally classified as a fern, and it remained classified so for over 100 years. In 1911, Russian paleontologist Mikhail Dimitrievich Zalessky described a new type of petrified wood from the Donetz Basin in Russia. He called the wood "Callixylon", though he did not find any structures other than the trunk. The similarity to conifer wood was recognized. It was also noted that ferns of the genus "Archaeopteris" were often found associated with fossils of "Callixylon".

In the 1960’s, paleontologist Charles B. Beck was able to demonstrate that the fossil wood known as "Callixylon" and the leaves known as "Archaeopteris" were actually part of the same plant.Beck, CB (1960) The identity of "Archaeopteris" and "Callixylon". Brittonia 12, 351-368.] Beck, CB (1962) Reconstruction of "Archaeopteris" and further consideration of its phylogenetic position. American Journal of Botany 49, 373-382] It was a plant with a mixture of characteristics not seen in any living plant, a link between true gymnosperms and ferns.

The genus "Archaeopteris" is placed in the order Archaeopteridales and family Archaeopteridaceae. The name is similar to that of the first known feathered bird, "Archaeopteryx", but in this case refers to the feather-like nature of the plant's fronds.

See also

* Glossopteris
* "Calamites"

References

* [http://www.sciencedaily.com/releases/1999/04/990422060147.htm Earliest Modern Tree Lived 360-345 Million Years Ago, from Sciencedaily.com]
* [http://www.biologie.uni-hamburg.de/b-online/kerp/seite4.html History of Paleozoic Forests: the Early Forests and the Progymnosperms]
* [http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_65680.htm Consequences of Rapid Expansion of Late Devonian Forests, by Stephen E. Scheckler]
* Walker, Cyril and Ward, David. "Fossils". Smithsonian Handbooks. Dorling Kindersley, Inc. New York, NY (2002).
* Mayr, Helmut. "A Guide to Fossils". Princeton University Press, Princeton, NJ (1992).
* [http://www.ucmp.berkeley.edu/seedplants/progymnosperms.html Introduction to the Progymnosperms]
* Davis, Paul and Kenrick, Paul; "Fossil Plants". Smithsonian Books (in association with the Natural History Museum of London), Washington, D.C. (2004). ISBN 1-58834-156-9