Lepidodendron is a genus of tree-like lycopod plants that were prolific in the Carboniferous period (359-300 million years ago), but existed well into the Mesosoic Era. Its fossils have dates that span over one hundred million years (383.7 to 205.6 million years old) (paleobiodb.org). Related to modern-day club mosses and quillworts, Lepidodendron was comparatively gigantic, reaching heights of 130 ft (40 m), with massive supporting trunks that frequently grew over 2 m (6 ft) in diameter (Thomas & Watson 1976). The trunk was not woody, but composed instead of soft tissue. It had a characteristic scaly texture, caused by diamond-shaped “leaf scars” which are among the most common Carboniferous fossils. The leaf scars were composed of photosynthetic tissue that would have been green, unlike the trunks of modern trees. Structural support was provided by a bark-like region that sheathed the trunk and thickened to support the growing plant without flaking off. Lepidodendron spent most of its 10-15 year life cycle as an unbranched trunk. When it was fully grown, it branched in a binary fashion and was crowned with a series of bifurcating branches bearing long, thin leaves in a spiral arrangement. The ends of the branches bore cone-like structures. It is believed that Lepidodendron was monocarpic (reproduced only once during the end of its life), and used encapsulated spores instead of seeds.
Lepidodendron grew in high densities in very moist, swamp environments. Their density would have been made possible by the fact that they did not branch until fully grown. Because they were so prolific, these organisms were a major contributor of biomass to the Carboniferous coal seams found all over the world (DiMichele et al. 1985). Their populations fell sharply at the end of the Carboniferous period (Kerp 2000), but specimens from China show they survived into the Permian. One record from the Eastern US is dated to the Triassic. Theories about their decline abound, and include climate change (DiMichele and Phillips 1996) and tectonic activity (Kerp 1996, 2000; Cleal & Thomas 2005). Their descendants survive only in diminutive forms (e.g., club mosses).
Fossils of Lepidodendron have been found throughout North America, Europe, and Asia (paleobiodb.org). At least one collection has been made from the Permian period of Australia and Indonesia. The ages of its fossils range between 383.7 and 205.6 million years.
Leaf Scar Fossils
Impressions of the stem surfaces of Lepidodendron are among the most common Carboniferous fossils from the central US. This distinctive feature is a scale-like pattern formed by closely-packed, diamond-shaped leaf scars (see media). These leaf scars, or sometimes leaf cushions (the expanded base of the leaf) are formed when the long leaves of Lepidodendron drop off, leaving an imprint. Each leaf scar has three small pits where the stem of the leaf attached, the central one representing the vascular bundle that fed the leaf, and the others are parichnos channels (part of the plant’s aerating system) (Taylor et al 2009).
Lepidodendron was an arborescent (tree-like) plant. It grew to great heights (130 ft, or 40 m) and its trunk frequently grew thicker than 2 m (6 ft) in diameter (Thomas & Watson 1976). The trunk had a characteristic scaly texture, caused by diamond-shaped “leaf scars” which are among the most common Carboniferous fossils. The leaf scars were composed of photosynthetic tissue that would have been green, different from the trunks of modern trees. Structural support was provided by a thick, barklike section of the trunk called the periderm (Taylor et al 2009). As the stem thickened, the outer cortical tissues peeled off, leaving the periderm on the outside of the lower trunk and the leaf scar pattern on only the upper trunk and branches (Eggert 1961). Lepidodendron was a vascular plant (having a central water/nutrient transportation system), the center of the stem contained xylem tissues (tubular transport structure) (Taylor et al 2009). The roots were shallow and radiated horizontally from four main axes.
Lepidodendron spent most of its 10-15 year life cycle as an unbranched trunk. When it reached maturity, it branched in a binary fashion and was crowned with a series of bifurcating branches bearing clusters of thin, linear leaves in spiral arrangements. The ends of the branches bore cone-like structures (DiMichele and Bateman 1992). It is believed that Lepidodendron was monocarpic (reproduced only once during the end of its life), and used encapsulated spores instead of seeds.
Lepidodendron grew in high densities in very moist, swamp environments (Taylor et al 2009). Their density would have been made possible by the fact that they did not branch until fully grown.
Lepidodendron spent most of its 10-15 year life cycle as an unbranched trunk. It branched in a binary fashion and was crowned with a series of bifurcating branches bearing thin, linear leaves in a spiral arrangement. The ends of the branches bore cone-like structures (DiMichele & Bateman 1992). It is believed that Lepidodendron was monocarpic (reproduced only once during the end of its life). The cones contained spores rather than seeds. Lepidodendron is noted for the production of megaspores (large, elaborately sheathed spores), though some of its species produced microspores similar to those seen today (Taylor et al 2009).
|This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (April 2009)|
Lepidodendron — known as scale trees — is an extinct genus of primitive, vascular, arborescent (tree-like) plant related to the lycopsids (club mosses). They were part of the coal forest flora. They sometimes reached heights of over 30 metres (100 ft), and the trunks were often over 1 m (3.3 ft) in diameter, and thrived during the Carboniferous Period (about 359.2 ± 2.5 Mya (million years ago) to about 299.0 ± 0.8 Mya). Sometimes erroneously called "giant club mosses", they were actually more closely related to today's quillworts than to modern club mosses.
The name Lepidodendron comes from the Greek lepido, scale, and dendron, tree.
Description and biology
Lepidodendron had tall, thick trunks that rarely branched and were topped with a crown of bifurcating branches bearing clusters of leaves. These leaves were long and narrow, similar to large blades of grass, and were spirally-arranged. The vascular system was a siphonostele with exarch xylem maturation.
The closely packed diamond-shaped leaf scars left on the trunk and stems as the plant grew provide some of the most interesting and common fossils in Carboniferous shales and accompanying coal deposits. These fossils look much like tire tracks or alligator skin.
The scars, or leaf cushions, were composed of green photosynthetic tissue, evidenced by the cuticle covering and being dotted with stomata, microscopic pores through which carbon dioxide from the air diffuses into plants. Likewise, the trunks of Lepidodendron would have been green, unlike modern trees which have scaly, non-photosynthetic brown or gray bark.
Lepidodendron has been likened to a giant herb. The trunks produced little wood, being mostly soft tissues. Most structural support came from a thick, bark-like region. This region remained around the trunk as a rigid layer that grew thicker, but did not flake off like that of most modern trees. As the tree grew, the leaf cushions expanded to accommodate the increasing width of the trunk.
Lepidodendron likely lived in the wettest parts of the coal swamps that existed during the Carboniferous period. They grew in dense stands, likely having as many as 1000 to 2000 giant clubmosses per hectare. This would have been possible because they did not branch until fully grown, and would have spent much of their lives as unbranched poles.
The branches of this plant ended in cone-structures. Lepidodendron did not produce true seeds like many modern plants. Instead, it reproduced by means of elaborate, encapsulated spores. It is estimated[who?] that these plants grew rapidly and lived 10–15 years. Most species were probably monocarpic, meaning they reproduced only once toward the end of their life cycle.
Decline and extinction
By the Mesozoic era, the giant lycopsids had died out and were replaced by smaller Quillworts, probably due to competition from the emerging woody gymnosperms and other plants. Lepidodendron is one of the more common plant fossils found in Pennsylvanian (Late Carboniferous) age rocks. They are closely related to other extinct Lycopsid genera, Sigillaria and Lepidendropsis.
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In popular culture
In the 19th century, due to the reptilian look of the diamond-shaped leaf scar pattern, petrified trunks of Lepidodendron were frequently exhibited at fairgrounds by amateurs as giant fossil lizards or snakes.
|Wikimedia Commons has media related to Lepidodendron.|
- Davis, Paul; Kenrick, Paul (2004). Fossil Plants. Washington, DC: Smithsonian Books. ISBN 1-58834-181-X.
- Morran, Robin C. (2004). A Natural History of Ferns. Portland: Timber Press. ISBN 0-88192-667-1.
- "Plant fossils of the British Coal Measures" by Christopher J.Cleal and Barry A.Thomas, publ. The Palaeontological Association, London, 1994, 222 pages, ISBN 0-901702-53-6.
Names and Taxonomy
Lepidodendron is a member of the lycophytes. Lycophytes are one of the two major lineages of vascular plants. Its sister group, containing seed plants and ferns, is known by the somewhat unofficial name of euphyllophytes. The two lineages have evolved independently for hundreds of millions of years, but Lycophytes have evolved many convergent features with euphyllophytes, including leaves, roots, cones and tree-like plants like Lepidodendron. Today’s lycophytes are represented by about 1600 species of club mosses and spike mosses (Taylor et al 2009).
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