''Lepidodendron'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of primitive lycopodian vascular plants belonging 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 Coal forests were the vast swathes of freshwater swamp and riparian forests that covered much of the lands on Earth's tropical regions during the late Carboniferous ( Pennsylvanian) and Permian periods.Cleal, C. J. & Thomas, B. A. (2005). "Pala ...

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 The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

Period (358.9 to 298.9

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

), and persisted until the end of the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

around 252 million years ago. Sometimes erroneously called "giant club mosses", the genus was actually more closely related to modern quillworts than to modern club mosses. In the form classification system used in

paleobotany Paleobotany or palaeobotany, also known as paleophytology, is the branch of botany dealing with the recovery and identification of plant fossils from geological contexts, and their use for the biological reconstruction of past environments ( pal ...

, ''Lepidodendron'' is both used for the whole plant as well as specifically the stems and leaves.

Etymology

The name ''Lepidodendron'' comes from the Greek λεπίς ', scale, and δένδρον ''dendron'', tree.

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 A dichotomy () is a partition of a set, partition of a whole (or a set) into two parts (subsets). In other words, this couple of parts must be * jointly exhaustive: everything must belong to one part or the other, and * mutually exclusive: nothi ...

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 A spruce is a tree of the genus ''Picea'' ( ), a genus of about 40 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal ecosystem, boreal (taiga) regions of the Northern hemisphere. ''Picea'' ...

fir Firs are evergreen coniferous trees belonging to the genus ''Abies'' () in the family Pinaceae. There are approximately 48–65 extant species, found on mountains throughout much of North and Central America, Eurasia, and North Africa. The genu ...

Stem

PSM V18 D629 Lepidodendron modulatum and diplodigioides

The stem of the lycopsids had a unifacial

vascular Vascular can refer to: * blood vessels, the vascular system in animals * vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue ...

cambium, contrasting with the bifacial vascular cambium of modern trees. Though the bifacial cambium of modern trees produces both secondary

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...

and

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

, 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 In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic c ...

. 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 time series data, seasonality refers to the trends that occur at specific regular intervals less than a year, such as weekly, monthly, or quarterly. Seasonality may be caused by various factors, such as weather, vacation, and holidays and consi ...

in ''Lepidodendron'' species. The outermost cortex of oldest stems developed into the bark-like lycopodiopsid periderm. 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.

Leaves

The leaves of the lycopsid were needle-like and were densely spiraled about young shoots, each possessing only a single

vein Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...

. 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 leaf scars 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 A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will incl ...

of the leaf connected to the vascular system of the stem. This xylem bundle was composed only of primary

trachea The trachea (: tracheae or tracheas), also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all animals' lungs. The trachea extends from ...

. The two outer scars mark the forked branches of a strand of vascular tissue that passed from the

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

of the stem into the leaf. This forked strand is sometimes referred to as the "parichnos". Surrounding this strand were

parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

cells and occasionally thick-walled elements. Surrounding both conducting tissues was a broad sheath of transfusion

tracheid A tracheid is a long and tapered Lignin, lignified cell in the xylem of Tracheophyta, vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms also use another type of conductive cell, called vessel elements, to t ...

s. 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 Aerenchyma or aeriferous parenchyma or lacunae, is a modification of the parenchyma to form a spongy tissue that creates spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and ...

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 A ligule (from "strap", variant of ''lingula'', from ''lingua'' "tongue") is a thin outgrowth at the junction of leaf A leaf (: leaves) is a principal appendage of the plant stem, stem of a vascular plant, usually borne laterally above g ...

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 A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

. 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 In botany and dendrology, a rhizome ( ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...

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.

Hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

e are occasionally present in the tissues of ''Lepidodendron'' lycopsids, indicating the presence of

mycorrhiza A mycorrhiza (; , mycorrhiza, or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, the plant root system and its surroundings. Mycorrhizae play ...

l associations.

Decay

Different

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

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. 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. Rather than reproduce with seeds, ''Lepidodendron'' lycopsids reproduced with spores. The spores were stored in

sporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

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 Dendrochronology (or tree-ring dating) is the scientific method of chronological dating, dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, ...

and dormant buds indicates no seasonal growth patterns, and modern plants with similar characteristics tend to grow in

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

conditions. However, ''Lepidodendron'' species were distributed throughout

subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...

regions. The lycopsid inhabited an extensive area compared to tropical flora of the same time period, with lycopods growing as far north as

Spitsbergen Spitsbergen (; formerly known as West Spitsbergen; Norwegian language, Norwegian: ''Vest Spitsbergen'' or ''Vestspitsbergen'' , also sometimes spelled Spitzbergen) is the largest and the only permanently populated island of the Svalbard archipel ...

and as far south as

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

, in a latitudinal range of 120°.

Extinction

Euramerica Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pa ...

, ''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 Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...

File:LepidodendronOhio.jpg, External mold of ''Lepidodendron'' from the

Upper Carboniferous Upper may refer to: * Shoe upper or ''vamp'', the part of a shoe on the top of the foot * Stimulant, drugs which induce temporary improvements in either mental or physical function or both * ''Upper'', the original film title for the 2013 found f ...

Ohio Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...

. 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