Lepidodendron

Lepidodendron; Temporal range: Early Carboniferous–Late Permian PreꞒ Ꞓ O S D C P T J K Pg N
	Trunk fragment, showing leaf base scars
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Lycophytes
Class:	Lycopodiopsida
Order:	† Lepidodendrales
Family:	† Lepidodendraceae
Genus:	† Lepidodendron ; Sternberg, 1820
Species
	L. aculeatumSternberg 1820; L. batoviiChachlov 1948; L. obovatumSternberg 1820; L. whitehillianumAnderson & Anderson 1986;
Synonyms
	Dimicheleodendron B.A.Thomas & C.J.Cleal;

Last updated April 11, 2024

Lepidodendron is an extinct genus 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 environments. They sometimes reached heights of 50 metres (160 feet),^[1] and the trunks were often over 1 m (3.3 ft) 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 quillworts 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.

Etymology

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

Description and biology

Overview

Restoration Lepidodendron.png — Restoration

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 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.^[3]

Stem

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.^[4] The outermost cortex of oldest stems developed into the bark-like lycopodiopsid periderm.^[5] 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.^[3]

Leaves

The leaves of the lycopsid were needle-like and were densely spiraled about young shoots, each possessing only a single 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 8 cm (3.1 in) and a width of 2 cm (0.79 in). 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 of the leaf connected to the vascular system of the stem. This xylem bundle was composed only of primary trachea.^{[ citation needed ]} 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 tracheids. 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.^[3]

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.^[6] Hyphae are occasionally present in the tissues of Lepidodendron lycopsids, indicating the presence of mycorrhizal associations.^[7]

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.^[8]

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.^[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 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.^[10]

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°.^[4]

Extinction

In Euramerica, Lepidodendron became extinct at the end of the Carboniferous,^[11] 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.^[12] 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.^[11]^[13]

Gallery

Lepidodendron sp. bark from the Pottsville Group, Lower Pennsylvanian
Lepidodendron elegans
Lepidodendron aculeatum
Lepidodendron lycopodioides
Life restoration
Restoration of Lepidodendron with leafy branches
Lepidodendron bark from Joggins, Nova Scotia, Canada
Lepidodendron fossil stumps from Fossil Grove, Glasgow, Scotland
Lepidodendron sp. stem impression displayed at a collection held in the National Museum of Brazil
Various Lepidodendron diagrams from the Geological Survey of Pennsylvania
External mold of Lepidodendron from the Upper Carboniferous of Ohio.
1911 reconstruction of a mature Lepidodendron, showing dichotomous branching at the top of the trunk

Related Research Articles

Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching stems bearing simple leaves called microphylls and reproduce by means of spores borne in sporangia on the sides of the stems at the bases of the leaves. Although living species are small, during the Carboniferous, extinct tree-like forms (Lepidodendrales) formed huge forests that dominated the landscape and contributed to coal deposits.

<span class="mw-page-title-main">Lycophyte</span> Broadly circumscribed group of spore bearing plants

The lycophytes, when broadly circumscribed, are a group of vascular plants that include the clubmosses. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineages of extant (living) vascular plants; the group contains extinct plants that have been dated from the Silurian. Lycophytes were some of the dominating plant species of the Carboniferous period, and included the tree-like Lepidodendrales, some of which grew over 40 metres (130 ft) in height, although extant lycophytes are relatively small plants.

Bark is the outermost layer of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost layer of the periderm. The outer bark on older stems includes the dead tissue on the surface of the stems, along with parts of the outermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the living periderm is also called the rhytidome.

Cycads are seed plants that typically have a stout and woody (ligneous) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow very slowly and live very long. Because of their superficial resemblance, they are sometimes mistaken for palms or ferns, but they are not closely related to either group.

Calamites is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights of 30–50 meters. They were components of the understories of coal swamps of the Carboniferous Period.

<span class="mw-page-title-main">Equisetidae</span> Subclass of ferns

Equisetidae is one of the four subclasses of Polypodiopsida (ferns), a group of vascular plants with a fossil record going back to the Devonian. They are commonly known as horsetails. They typically grow in wet areas, with whorls of needle-like branches radiating at regular intervals from a single vertical stem.

The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosterophyllophyta or Zosterophyta and the class or plesion Zosterophyllopsida or Zosteropsida. They were among the first vascular plants in the fossil record, and had a world-wide distribution. They were probably stem-group lycophytes, forming a sister group to the ancestors of the living lycophytes. By the late Silurian a diverse assemblage of species existed, examples of which have been found fossilised in what is now Bathurst Island in Arctic Canada.

Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant.

Sigillaria is a genus of extinct, spore-bearing, arborescent lycophyte, known from the Carboniferous and Permian periods. It is related to the more famous Lepidodendron, and more distantly to modern quillworts.

In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of stems and roots, causing them to elongate, and gives rise to primary tissue. Secondary growth occurs in most seed plants, but monocots usually lack secondary growth. If they do have secondary growth, it differs from the typical pattern of other seed plants.

Coal forests were the vast swathes of swamps and riparian forests that covered much of the land on Earth's tropical regions during the late Carboniferous (Pennsylvanian) and Permian periods. As plant matter from these forests decayed, enormous deposits of peat accumulated, which later became buried and converted into coal over the subsequent eras.

Stigmaria is a form taxon for common fossils found in Carboniferous rocks. They represent the underground rooting structures of arborescent lycophytes such as Sigillaria and Lepidodendron under the order Lepidodendrales.

The unifacial cambium produces cells to the interior of its cylinder. These cells differentiate into xylem tissue. Unlike the more common bifacial cambium found in later woody plants, the unifacial cambium does not produce phloem to its exterior. Also in contrast to the bifacial cambium, the unifacial cambium is unable to expand its circumference with anticlinal cell division. Cell elongation provides a limited amount of expansion.

The Fossil Grove is a group of plant fossils located within Victoria Park, Glasgow, Scotland. It was discovered in 1887 and contains the fossilised stumps and the stigmarian system of eleven extinct Lepidodendron lycopsids, which are sometimes described as "giant club mosses" but are more closely related to quillworts. The Fossil Grove is managed as a museum and has been a popular tourist attraction since it opened for public viewing in 1890.

<span class="mw-page-title-main">Lepidodendrales</span> Extinct order of vascular tree-like plants

Lepidodendrales or arborescent lycophytes are an extinct order of primitive, vascular, heterosporous, arborescent (tree-like) plants belonging to Lycopodiopsida. Members of Lepidodendrales are the best understood of the fossil lycopsids due to the vast diversity of Lepidodendrales specimens and the diversity in which they were preserved; the extensive distribution of Lepidodendrales specimens as well as their well-preservedness lends paleobotanists exceptionally detailed knowledge of the coal-swamp giants’ reproductive biology, vegetative development, and role in their paleoecosystem. The defining characteristics of the Lepidodendrales are their secondary xylem, extensive periderm development, three-zoned cortex, rootlike appendages known as stigmarian rootlets arranged in a spiralling pattern, and megasporangium each containing a single functional megaspore that germinates inside the sporangium. Many of these different plant organs have been assigned both generic and specific names as relatively few have been found organically attached to each other. Some specimens have been discovered which indicate heights of 40 and even 50 meters and diameters of over 2 meters at the base. The massive trunks of some species branched profusely, producing large crowns of leafy twigs; though some leaves were up to 1 meter long, most were much shorter, and when leaves dropped from branches their conspicuous leaf bases remained on the surface of branches. Strobili could be found at the tips of distal branches or in an area at the top of the main trunk. The underground organs of Lepidodendrales typically consisted of dichotomizing axes bearing helically arranged, lateral appendages serving an equivalent function to roots. Sometimes called "giant club mosses", they are believed to be more closely related to extant quillworts based on xylem, although fossil specimens of extinct Selaginellales from the Late Carboniferous also had secondary xylem.

This article attempts to place key plant innovations in a geological context. It concerns itself only with novel adaptations and events that had a major ecological significance, not those that are of solely anthropological interest. The timeline displays a graphical representation of the adaptations; the text attempts to explain the nature and robustness of the evidence.

Psaronius is an extinct genus marattialean tree fern which grew to 10m in height, and is associated with leaves of the organ genus Pecopteris and other extinct tree ferns. Originally, Psaronius was a name for the petrified stems, but today the genus is used for the entire tree fern. Psaronius tree fern fossils are found from the Carboniferous through the Permian.

A woody plant is a plant that produces wood as its structural tissue and thus has a hard stem. In cold climates, woody plants further survive winter or dry season above ground, as opposed to herbaceous plants that die back to the ground until spring.

A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers and fruits, transports water and dissolved substances between the roots and the shoots in the xylem and phloem, photosynthesis takes place here, stores nutrients, and produces new living tissue. The stem can also be called halm or haulm or culms.

Leptophloeum is an extinct genus of vascular plants in the lycophyte clade. It is widely distributed being, known from the Laurasian and Gondwanan settings between the Devonian and Early Carboniferous periods.

References

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↑ Hetherington, A.J.; Berry, C.M.; Dolan, Liam (2016). "Networks of highly branched stigmarian rootlets developed on the first giant trees". PNAS. 113 (24): 6695–6700. doi: 10.1073/pnas.1514427113 . PMC 4914198 .
↑ Strullu-Derrien, Christine; Strullu, Désiré-Georges (November 2007). "Mycorrhization of fossil and living plants". Comptes Rendus Palevol. 6 (6–7): 483–494. doi:10.1016/j.crpv.2007.09.006.
↑ Thomas, B.A. and Watson, Joan (1976). "A rediscovered 114-foot Lepidodendron from Bolton, Lancashire". Geological Journal. 11 (1). Wiley Online Library: 15–20. doi:10.1002/gj.3350110102.{{cite journal}}: CS1 maint: multiple names: authors list (link)
1 2 Thomas, Barry A.; Cleal, Christopher J. (May 2018). "Arborescent lycophyte growth in the late Carboniferous coal swamps". New Phytologist. 218 (3): 885–890. doi: 10.1111/nph.14903 . PMID 29282734.
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↑ Lucas, Spencer G.; DiMichele, William A.; Opluštil, Stanislav; Wang, Xiangdong (2023-06-14). "An introduction to ice ages, climate dynamics and biotic events: the Late Pennsylvanian world". Geological Society, London, Special Publications. 535 (1): 1–15. doi:10.1144/SP535-2022-334. ISSN 0305-8719.
↑ Xu, Zhen; Hilton, Jason; Yu, Jianxin; Wignall, Paul B.; Yin, Hongfu; Xue, Qing; Ran, Weiju; Li, Hui; Shen, Jun; Meng, Fansong (September 2022). "End Permian to Middle Triassic plant species richness and abundance patterns in South China: Coevolution of plants and the environment through the Permian–Triassic transition". Earth-Science Reviews. 232: 104136. doi:10.1016/j.earscirev.2022.104136.