| Archaeopteris | |
|---|---|
 | |
| Archaeopteris hibernica | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Class: | †Progymnospermopsida |
| Order: | †Archaeopteridales |
| Family: | †Archaeopteridaceae |
| Genus: | †Archaeopteris Dawson (1871) |
| Species | |
| |
Archaeopteris is anextinctgenus ofprogymnosperm tree withfern-like leaves. A useful index fossil, this tree is found instrata dating from the UpperDevonian to LowerCarboniferous (382 to 323 million years ago), the oldest fossils being 385 million years old,[1] and had global distribution.
Until the 2007 discovery ofWattieza, many scientists consideredArchaeopteris to be the earliest known tree. Bearing buds, reinforced branch joints, and branched trunks similar to today'swoody plants, it is more reminiscent of modern seed-bearing trees than otherspore-bearingtaxa. It combines characteristics of woody trees andherbaceous ferns, and belongs to the progymnosperms, a group of extinct plants more closely related toseed plants than to ferns, but unlike seed plants, reproducing using spores like ferns.
John William Dawson described the genus in 1871. The name derives from theancient Greekἀρχαῖος (archaīos, "ancient"), andπτέρις (ptéris, "fern").Archaeopteris was originally classified as afern, and it remained classified so for over 100 years. In 1911,Russian paleontologistMikhail Dimitrievich Zalessky described a new type ofpetrified wood from theDonets Basin in modernUkraine. He called the woodCallixylon, though he did not find any structures other than the trunk. The similarity toconifer wood was recognized. It was also noted that ferns of the genusArchaeopteris were often found associated withfossils ofCallixylon.
In the 1960s,paleontologist Charles B. Beck was able to demonstrate that the fossil wood known asCallixylon and the leaves known asArchaeopteris were actually part of the same plant.[2][3] It was a plant with a mixture of characteristics not seen in any living plant, a link between truegymnosperms and ferns.
The genusArchaeopteris is placed in theorderArchaeopteridales andfamilyArchaeopteridaceae. 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'sfronds.
Archaeopteris is a member of a group of free-sporing woody plants called theprogymnosperms that are interpreted as distant ancestors of thegymnosperms.Archaeopteris reproduced by releasing spores rather than by producing seeds, but some of the species, such asArchaeopteris halliana wereheterosporous, producing two types of spores. This is thought to represent an early step in the evolution ofvascular plants towards reproduction by seeds,[4] which first appeared in the earliest, long-extinct gymnosperm group, the seed ferns (Pteridospermatophyta). Theconifers or Pinophyta are one of four divisions of extant gymnosperms that arose from the seed ferns during the Carboniferous period.
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The trees of this genus typically grew to 24 m (80 ft) in height[5] with leafy foliage reminiscent of some conifers. The large fern-likefronds were thickly set with fan-shaped leaflets or pinnae. The trunks of some species exceeded 1.5 m (5 ft) in diameter. The branches were borne in spiral arrangement, and a forked stipule was present at the base of each branch.[5] 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).
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[citation needed] that whole fronds were shed together as single units, perhaps seasonally like moderndeciduous foliage or like trees in the cypress familyCupressaceae.
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 withArchaeopteris, deeper root systems were being developed that could support ever higher growth.
Evidence indicates thatArchaeopteris preferred wetsoils, growing close toriver systems and infloodplain woodlands. It would have formed a significant part of the canopy vegetation of early forests. Speaking of the first appearance ofArchaeopteris on the world-scene, Stephen Scheckler, a professor of biology and geological sciences atVirginia 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".[6] One species,Archaeopteris notosaria, has even been reported from within what was then the Antarctic Circle: leaves and fertile structures were identified from theWaterloo Farm lagerstätte in what is now South Africa.[7]
Scheckler believes thatArchaeopteris 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."[8]
Looking roughly like a top-heavyChristmas tree,Archaeopteris may have played a part in the transformation of Earth'sclimate during the Devonian before becoming extinct within a short period of time at the beginning of the Carboniferous period.
