| Lepidodendron | |
|---|---|
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| Life reconstruction | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Lycophytes |
| Class: | Lycopodiopsida |
| Order: | †Lepidodendrales |
| Family: | †Lepidodendraceae |
| Genus: | †Lepidodendron Sternberg, 1820 |
| Species | |
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| Synonyms | |
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Lepidodendron is anextinctgenus of primitivelycopodian vascular plants belonging the orderLepidodendrales. It is well preserved and common in the fossil record. Like other Lepidodendrales, species ofLepidodendron grew as large-tree-like plants in wetlandcoal forest environments. They sometimes reached heights of 50 metres (160 feet),[1] and the trunks were often over 1 m (3 ft 3 in) in diameter. They are often known as "scale trees", due to their bark having been covered in diamond shaped leaf-bases, from which leaves grew during earlier stages of growth. However, they are correctly defined as arborescentlycophytes. They thrived during theCarboniferous Period (358.9 to 298.9million years ago), and persisted until the end of thePermian around 252 million years ago. Sometimes erroneously called "giantclub mosses", the genus was actually more closely related to modernquillworts than to modern club mosses. In theform classification system used inpaleobotany,Lepidodendron is both used for the whole plant as well as specifically the stems and leaves.
The nameLepidodendron comes from theGreekλεπίςlepis, scale, andδένδρονdendron, tree.
Lepidodendron species were comparable in size to modern trees. The plants had tapering trunks as wide as 2 m (6.6 ft) at their base that rose to about 40 m (130 ft)[2] and even 50 m (160 ft),[1] arising from an underground system of horizontally spreading branches that were covered with many rootlets. Though the height of the lycopsids make the plants similar to modern trees, the constantdichotomy of branches created ahabit that contrasts with that of modern trees. At the ends of branches were oval-shapedstrobili calledLepidostrobus that had a similar shape to modern cones of aspruce orfir.[3]
The stem of the lycopsids had aunifacialvascular cambium, contrasting with thebifacial vascular cambium of modern trees. Though the bifacial cambium of modern trees produces both secondaryphloem andxylem, the unifacial cambium ofLepidodendron lycopsid produced only secondary xylem. As the lycopods aged, the wood produced by the unifacial cambium decreased towards the top of the plant such that terminal twigs resembled youngLepidodendron stems. Compared to modern trees, the stems and branches of the lycopsids contained little wood with the majority of mature stems consisting of a massivecorticalmeristem. The nearly-uniform growth of this cortical tissue indicates no difference in growth during changing seasons, and the absence of dormantbuds further indicates the lack ofseasonality inLepidodendron species.[4] The outermost cortex of oldest stems developed into the bark-likelycopodiopsidperiderm.[5] The bark of the lycopsid was somewhat similar to that ofPicea species, asleaf scars formed peg-like projections that stretched and tore as the bark stretched. To resist the bending force of wind,Lepidodendron depended on their outer bark rather than their vascular tissues, as compared to modern trees that rely mostly on their central mass of wood.[3]
The leaves of the lycopsid were needle-like and were densely spiraled about young shoots, each possessing only a singlevein. The leaves were similar to those of a fir in some species and similar to those ofPinus roxburghii in others, though in general the leaves ofLepidodendron species are indistinguishable from those ofSigillaria species. Thedecurrent leaves formed a cylindrical shell around branches. The leaves were only present on thin and young branches, indicating that, though the lycopsid were evergreen, they did not retain their needles for as long as modern conifers. The leaf-cushions were fusiform and elongated, growing at most to a length of 8 cm (3 in) and a width of 2 cm (3⁄4 in). The middle of leaf-cushions were smooth, whereleaf scars were created when anabscission layer cut a leaf from its base. Each leaf scar was composed of a central circular or triangular scar and two lateral scars that were smaller and oval-shaped. This central scar marks where the mainvascular bundle of the leaf connected to the vascular system of the stem. This xylem bundle was composed only of primarytrachea.[citation needed] The two outer scars mark the forked branches of a strand of vascular tissue that passed from thecortex of the stem into the leaf. This forked strand is sometimes referred to as the "parichnos". Surrounding this strand wereparenchyma cells and occasionally thick-walled elements. Surrounding both conducting tissues was a broad sheath of transfusiontracheids. Below the leaf scar the leaf-cushion tapered to a basal position. In this tapering area, circular impressions with fine pits were present. These impressions were continuous with the parichnos scars near the top of the tapering portion. This is because the impressions are formed byaerenchyma tissue that developed in closely with the parichnos. Above the leaf scar was a deep triangular impression known as the "ligular pit" for its similarities to theligule ofIsoetes. In some leaf-cushions a second depression was present above the ligular pit. Though its purpose is unclear, it has been suggested that the depression may mark the position of asporangium. As the branch of aLepidodendron lycopsid grew the leaf-cushion only grew to a certain extent, past which the leaf-cushion stretched. This stretching widened the groove that separated the leaf-cushions, creating a broad, flat channel.[3]
The underground structures ofLepidodendron and similar lycopsid species known from the fossil record includingSigillaria are assigned to the form taxon,Stigmaria. The rootlets were dichotomously branched from therhizomes similar toIsoetes. These rhizomorphic axes were shoot-like, and dichotomous branching of the rootlets structured the stigmarian systems. Rootlet scars can be seen fromStigmaria fossils where the root hairs used to be attached.[6]Hyphae are occasionally present in the tissues ofLepidodendron lycopsids, indicating the presence ofmycorrhizal associations.[7]
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Differentfossil genera have been described to name the various levels of decay inLepidodendron bark fossils. The nameBergeria describes stems that have lost their epidermises,Aspidiariu is used when cushions have been removed by deep decay, andKnorria is used when the leaf cushions and the majority of cortical tissues has decayed, with a shallow "fluted" surface remaining. However, it has been suggested that these are more likely growth forms than preserved bark types, as entire fossilized trunks have been discovered with dissimilar forms; if decay is assumed to be constant throughout the trunk, then different forms indicate growth rather than levels of decay. It is likely that the trunk ofLepidodendron lycopsids were subject to the growth formsKnorria,Aspidiaria, andBergeria progressing up the trunk, respectively.[8]
During the early stages of growth,Lepidodendron grew as single, unbranched trunk, with leaves growing out of the scale leaf bases (cushions). Towards the end of the lycopod growth, the leaves on the lower part of the trunk were shed, and inLepidodendron, the upper part of the trunkdichotomously branched into acrown.[9] The rate of growth of arborescent lycophytes is disputed, some authors contended that they had a rapid life cycle, growing to their maximum size and dying in only 10 to 15 years, while other authors argue that these growth rates were overestimated.[9] Rather than reproduce with seeds,Lepidodendron lycopsids reproduced with spores. The spores were stored insporangia situated on fertile stems that grew on or near the main trunk. The fertile stems grew together in cone-like structures that clustered at the tips of branches.[10]
The lack ofgrowth rings and dormant buds indicates no seasonal growth patterns, and modern plants with similar characteristics tend to grow intropical conditions. However,Lepidodendron species were distributed throughoutsubtropical regions. The lycopsid inhabited an extensive area compared to tropical flora of the same time period, with lycopods growing as far north asSpitsbergen and as far south asSouth America, in alatitudinal range of 120°.[4]
InEuramerica,Lepidodendron became extinct at the end of the Carboniferous,[11] as part of a broader pattern of ecological change, including the increasing dominance ofseed plants in lowland wetland forests, and increasingly arid-adapted vegetation across western Pangea.[12] However, in theCathaysia region comprising what is now China, wet tropical environmental conditions continued to prevail, withLepidodendron (in its broad sense) only becoming extinct around the end of the Permian, around 252 million years ago, as a result of the extreme environmental disturbance caused by thePermian-Triassic extinction event.[11][13]
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