Archaeopteris is an extinct genus of progymnosperm tree with fern-like leaves. A useful index fossil, this tree is found in Stratum dating from the Upper Devonian to Lower Carboniferous (), the oldest fossils being 385 million years old,[ Fossilized Roots Are Revealing the Nature of 385-Million-Year-Old Forests] and had global distribution.
Until the 2007 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 Woody plant, it is more reminiscent of modern seed-bearing trees than other spore-bearing Taxon. It combines characteristics of woody trees and ferns, and belongs to the progymnosperms, a group of extinct plants more closely related to Seed plant than to ferns, but unlike seed plants, reproducing using spores like ferns.
Taxonomy
John William Dawson described the genus in 1871. The name derives from the
ancient Greek ἀρχαῖος]] (
archaīos, "ancient"), and πτέρις]] (
ptéris, "fern").
Archaeopteris was originally classified as a
fern, and it remained classified so for over 100 years. In 1911,
paleontologist
Mikhail Zalessky described a new type of
petrified wood from the
Donets Basin in modern
Ukraine. 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
of
Callixylon.
In the 1960s, Paleontology 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.
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 fern-like nature of the plant's .
Relationship to spermatophytes
Archaeopteris is a member of a group of free-sporing woody plants called the
that are interpreted as distant ancestors of the
.
Archaeopteris reproduced by releasing spores rather than by producing seeds, but some of the species, such as
Archaeopteris halliana were
Heterospory, producing two types of spores. This is thought to represent an early step in the evolution of
towards reproduction by seeds,
which first appeared in the earliest, long-extinct gymnosperm group, the seed ferns (Pteridospermatophyta). The
or Pinophyta are one of four divisions of extant gymnosperms that arose from the seed ferns during the Carboniferous period.
== Description ==
The trees of this genus typically grew to in height
with leafy foliage reminiscent of some conifers. The large fern-like
were thickly set with fan-shaped leaflets or pinnae. The trunks of some species exceeded in diameter. The branches were borne in spiral arrangement, and a forked stipule was present at the base of each branch.
[ Within a branch, leafy shoots were in opposite arrangement in a single plane. On fertile branches, some of the leaves were replaced by sporangia (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, Rhizome 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 , growing close to river systems and in floodplain 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 Tech, 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".[Anderson, H. M., Hiller, N. and Gess, R. W.(1995). Archaeopteris (Progymnospermopsida) from the Devonian of southern Africa. Botanical Journal of the Linnean Society 117, 305–320.]
Scheckler believes that Archaeopteris had a major role in transforming its environment. "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."[Virginia Tech, "Earliest Modern Tree Lived 360-345 Million Years Ago," ScienceDaily, 22 April 1999]
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.
See also
External links
-
History of Paleozoic Forests: the Early Forests and the Progymnosperms
-
Consequences of Rapid Expansion of Late Devonian Forests, by Stephen E. Scheckler
-
Walker, Cyril and David Ward. Fossils. Smithsonian Handbooks. Dorling Kindersley, Inc. New York, NY (2002).
-
Mayr, Helmut. A Guide to Fossils. Princeton University Press, Princeton, NJ (1992).
-
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).
