Lepidodendron, from Ancient Greek λεπίς ( lepís), meaning "scale", and δένδρον ( déndron), meaning "tree", is an extinct genus of primitive Lycopodiopsida 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 the trunks were often over 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 arborescent lycophytes. They thrived during the Carboniferous Period (358.9 to 298.9 million years ago), and persisted until the end of the Permian around 252 million years ago. Sometimes erroneously called "giant club mosses", the genus was actually more closely related to modern Isoetes than to modern club mosses. In the form classification system used in paleobotany, Lepidodendron is both used for the whole plant as well as specifically the stems and leaves.
]
Description and biology
Overview
Lepidodendron species were comparable in size to modern trees. The plants had tapering trunks as wide as at their base that rose to about [
] 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 strobilus called Lepidostrobus that had a similar shape to modern cones of a spruce or fir.
Stem
The stem of the lycopsids had a unifacial Vascular cambium cambium, contrasting with the Bifacial cambium 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 further indicates the lack of seasonality in Lepidodendron species.
Leaves
The leaves of the lycopsid were needle-like and were densely spiraled about young shoots, each possessing only a single Leaf vein. The leaves were similar to those of a fir in some species and similar to those of Pinus roxburghii in others, though in general the leaves of Lepidodendron species are indistinguishable from those of Sigillaria species. The decurrent 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 and a width of . The middle of leaf-cushions were smooth, where were created when an abscission 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 main vascular bundle of the leaf connected to the vascular system of the stem. This xylem bundle was composed only of primary Vessel element. The two outer scars mark the forked branches of a strand of vascular tissue that passed from the cortex of the stem into the leaf. This forked strand is sometimes referred to as the "parichnos". Surrounding this strand were parenchyma cells and occasionally thick-walled elements. Surrounding both conducting tissues was a broad sheath of transfusion . 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 by aerenchyma 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 the ligule of Isoetes. 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 a sporangium. As the branch of a Lepidodendron 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.[
]
Underground Structures
The underground structures of Lepidodendron and similar lycopsid species known from the fossil record including Sigillaria are assigned to the form taxon, Stigmaria. The rootlets were dichotomously branched from the rhizomes similar to Isoetes. These rhizomorphic axes were shoot-like, and dichotomous branching of the rootlets structured the stigmarian systems. Rootlet scars can be seen from Stigmaria fossils where the root hairs used to be attached.
Decay
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, Aspidiariu 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
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.
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,
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
See also
Further reading
