Lepidodendron

Lepidodendron, from Ancient Greek λεπίς (lepís), meaning "scale", and δένδρον (déndron), meaning "tree", is an extinct genus of primitive lycopodian vascular plants belonging to the order Lepidodendrales. It is well preserved and common in the fossil record. Like other Lepidodendrales, species of Lepidodendron grew as large-tree-like plants in wetland coal forest environments. They sometimes reached heights of , and even , 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 constant dichotomy of branches created a habit that contrasts with that of modern trees. At the ends of branches were oval-shaped strobili called Lepidostrobus that had a similar shape to modern cones of a spruce or fir.

Stem

thumb|upright|Leaf scars shown on a Lepidodendron. The "diamond shape" or scale impressions are common indicators of the leaf scars from Lepidodendron lycophytes.

The stem of the lycopsids had a unifacial vascular cambium, contrasting with the bifacial vascular cambium of modern trees. Though the bifacial cambium of modern trees produces both secondary phloem and xylem, the unifacial cambium of Lepidodendron 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 young Lepidodendron stems. Compared to modern trees, the stems and branches of the lycopsids contained little wood with the majority of mature stems consisting of a massive cortical meristem. The nearly-uniform growth of this cortical tissue indicates no difference in growth during changing seasons, and the absence of dormant buds further indicates the lack of seasonality in Lepidodendron species. The bark of the lycopsid was somewhat similar to that of Picea species, as leaf 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. Hyphae are occasionally present in the tissues of Lepidodendron lycopsids, indicating the presence of mycorrhizal associations.

Decay

thumb|Lepidodendron sp. bark from the [[Estonian Museum of Natural History.|213x213px]]

Different fossil genera have been described to name the various levels of decay in Lepidodendron bark fossils. The name Bergeria describes stems that have lost their epidermises, Aspidiaria is used when cushions have been removed by deep decay, and Knorria 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 of Lepidodendron lycopsids were subject to the growth forms Knorria, Aspidiaria, and Bergeria progressing up the trunk, respectively.

Growth and reproduction

left|thumb|upright|Reconstruction of a juvenile Lepidodendron, showing the unbranched trunk with leaves

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 in Lepidodendron, the upper part of the trunk dichotomously branched into a crown. Rather than reproduce with seeds, Lepidodendron lycopsids reproduced with spores. The spores were stored in sporangia 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.

Distribution

The lack of growth rings and dormant buds indicates no seasonal growth patterns, and modern plants with similar characteristics tend to grow in tropical conditions. However, Lepidodendron species were distributed throughout subtropical regions. The lycopsid inhabited an extensive area compared to tropical flora of the same time period, with lycopods growing as far north as Spitsbergen and as far south as South America, in a latitudinal range of 120°.

Extinction

In Euramerica, Lepidodendron became extinct at the end of the Carboniferous, as part of a broader pattern of ecological change, including the increasing dominance of seed plants in lowland wetland forests, and increasingly arid-adapted vegetation across western Pangea. However, in the Cathaysia region comprising what is now China, wet tropical environmental conditions continued to prevail, with Lepidodendron (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 the Permian-Triassic extinction event.

Gallery

File:Lepidodendron sp. (fossil lycopod) (lower Pottsville Group, Lower Pennsylvanian; Irish Ridge East roadcut, near Trinway, Ohio, USA) 2 (32394265633).jpg|Lepidodendron sp. bark from the Pottsville Group, Lower Pennsylvanian

File:Lepidodendron elegans.JPG|Lepidodendron elegans

File:Lepidodendron aculeatum.jpg|Lepidodendron aculeatum

File:Lepidodendron lycopodioides.jpg|Lepidodendron lycopodioides

File:PSM V18 D630 Restoration of a lepidodendron.jpg|Restoration of Lepidodendron with leafy branches

File:Joggins Lepidodendron bark imprint.jpg|Lepidodendron bark from Joggins, Nova Scotia, Canada

File:Fossil Tree Stumps at Fossil Grove Glasgow 1977.jpg|Lepidodendron fossil stumps from Fossil Grove, Glasgow, Scotland

File:Lepidodendron sp. - Impressão do caule MN 01.jpg|Lepidodendron sp. stem impression displayed at a collection held in the National Museum of Brazil

File:Description of the coal flora of the Carboniferous formation in Pennsylvania and throughout the United States (Plate LXIII) (21489161993).jpg|Various Lepidodendron diagrams from the Geological Survey of Pennsylvania

File:LepidodendronOhio.jpg|External mold of Lepidodendron from the Upper Carboniferous of Ohio.

File:Stigmaria Heimans.jpg|1911 reconstruction of a mature Lepidodendron, showing dichotomous branching at the top of the trunk

File:Lepidodendron PAMuseum.jpg|Trunk fragment, showing leaf base scars

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Stem

Decay

Growth and reproduction

Distribution

Extinction

Gallery

See also

References

Further reading