Paleobotany

Paleobotany, also spelled as palaeobotany, is the branch of botany dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments (paleogeography), and the evolutionary history of plants, with a bearing upon the evolution of life in general. A synonym is paleophytology. It is a component of paleontology and paleobiology. The prefix palaeo- means "ancient, old",[1] and is derived from the Greek adjective παλαιός, palaios.[2] Paleobotany includes the study of terrestrial plant fossils, as well as the study of prehistoric marine photoautotrophs, such as photosynthetic algae, seaweeds or kelp. A closely related field is palynology, which is the study of fossilized and extant spores and pollen.

Paleobotany is important in the reconstruction of ancient ecological systems and climate, known as paleoecology and paleoclimatology respectively; and is fundamental to the study of green plant development and evolution. Paleobotany has also become important to the field of archaeology, primarily for the use of phytoliths in relative dating and in paleoethnobotany.

The emergence of paleobotany as a scientific discipline can be seen in the early 19th century, especially in the works of the German palaeontologist Ernst Friedrich von Schlotheim, the Czech (Bohemian) nobleman and scholar Kaspar Maria von Sternberg, and the French botanist Adolphe-Théodore Brongniart.[3][4]

Fagus sylvatica pliocenica MHNT.PAL.VEG.2002.31
A leaf fossil of the European beech (Fagus sylvatica) from the late Pliocene of France, approximately three million years ago

Overview of the paleobotanical record

Macroscopic remains of true vascular plants are first found in the fossil record during the Silurian Period of the Paleozoic era. Some dispersed, fragmentary fossils of disputed affinity, primarily spores and cuticles, have been found in rocks from the Ordovician Period in Oman, and are thought to derive from liverwort- or moss-grade fossil plants (Wellman, Osterloff & Mohiuddin 2003).

Rhynie chert
An unpolished hand sample of the Lower Devonian Rhynie Chert from Scotland

An important early land plant fossil locality is the Rhynie Chert, found outside the village of Rhynie in Scotland. The Rhynie chert is an Early Devonian sinter (hot spring) deposit composed primarily of silica. It is exceptional due to its preservation of several different clades of plants, from mosses and lycopods to more unusual, problematic forms. Many fossil animals, including arthropods and arachnids, are also found in the Rhynie Chert, and it offers a unique window on the history of early terrestrial life.

Plant-derived macrofossils become abundant in the Late Devonian and include tree trunks, fronds, and roots. The earliest tree was thought to be Archaeopteris, which bears simple, fern-like leaves spirally arranged on branches atop a conifer-like trunk (Meyer-Berthaud, Scheckler & Wendt 1999), though it is now known to be the recently discovered Wattieza.[5]

Widespread coal swamp deposits across North America and Europe during the Carboniferous Period contain a wealth of fossils containing arborescent lycopods up to 30 meters tall, abundant seed plants, such as conifers and seed ferns, and countless smaller, herbaceous plants.

Angiosperms (flowering plants) evolved during the Mesozoic, and flowering plant pollen and leaves first appear during the Early Cretaceous, approximately 130 million years ago.

Plant fossils

A plant fossil is any preserved part of a plant that has long since died. Such fossils may be prehistoric impressions that are many millions of years old, or bits of charcoal that are only a few hundred years old. Prehistoric plants are various groups of plants that lived before recorded history (before about 3500 BC).

Preservation of plant fossils

Gingkoites huttoni 1
Ginkgoites huttonii, Middle Jurassic, Yorkshire, UK. Leaves preserved as compressions. Specimen in Munich Palaeontological Museum, Germany.

Plant fossils can be preserved in a variety of ways, each of which can give different types of information about the original parent plant. These modes of preservation are discussed in the general pages on fossils but may be summarised in a palaeobotanical context as follows.

  1. Adpressions (compressions – impressions). These are the most commonly found type of plant fossil. They provide good morphological detail, especially of dorsiventral (flattened) plant parts such as leaves. If the cuticle is preserved, they can also yield fine anatomical detail of the epidermis. Little other detail of cellular anatomy is normally preserved.
    Rhynia stem
    Rhynia, Lower Devonian Rhynie Chert, Scotland, UK. Transverse section through a stem preserved as a silica petrifaction, showing preservation of cellular structure.
  2. Petrifactions (permineralisations or anatomically preserved fossils). These provide fine detail of the cell anatomy of the plant tissue. Morphological detail can also be determined by serial sectioning, but this is both time consuming and difficult.
  3. Moulds and casts. These only tend to preserve the more robust plant parts such as seeds or woody stems. They can provide information about the three-dimensional form of the plant, and in the case of casts of tree stumps can provide evidence of the density of the original vegetation. However, they rarely preserve any fine morphological detail or cell anatomy. A subset of such fossils are pith casts, where the centre of a stem is either hollow or has delicate pith. After death, sediment enters and forms a cast of the central cavity of the stem. The best known examples of pith casts are in the Carboniferous Sphenophyta (Calamites) and cordaites (Artisia).
    Crossotheca nodule
    Crossotheca hughesiana Kidston, Middle Pennsylvanian, Coseley, near Dudley, UK. A lyginopteridalean pollen organ preserved as an authigenic mineralization (mineralized in situ). Specimen in Sedgwick Museum, Cambridge, UK.
  4. Authigenic mineralisations. These can provide very fine, three-dimensional morphological detail, and have proved especially important in the study of reproductive structures that can be severely distorted in adpressions. However, as they are formed in mineral nodules, such fossils can rarely be of large size.
  5. Fusain. Fire normally destroys plant tissue but sometimes charcoalified remains can preserve fine morphological detail that is lost in other modes of preservation; some of the best evidence of early flowers has been preserved in fusain. Fusain fossils are delicate and often small, but because of their buoyancy can often drift for long distances and can thus provide evidence of vegetation away from areas of sedimentation.
Rhynia stem
Rhynia, Lower Devonian Rhynie Chert, Scotland, UK. Transverse section through a stem preserved as a silica petrifaction, showing preservation of cellular structure.
Crossotheca nodule
Crossotheca hughesiana Kidston, Middle Pennsylvanian, Coseley, near Dudley, UK. A lyginopteridalean pollen organ preserved as an authigenic mineralization (mineralized in situ). Specimen in Sedgwick Museum, Cambridge, UK.

Fossil-taxa

Plant fossils almost always represent disarticulated parts of plants; even small herbaceous plants are rarely preserved whole. Those few examples of plant fossils that appear to be the remains of whole plants in fact are incomplete as the internal cellular tissue and fine micromorphological detail is normally lost during fossilisation. Plant remains can be preserved in a variety of ways, each revealing different features of the original parent plant.

Because of these difficulties, palaeobotanists usually assign different taxonomic names to different parts of the plant in different modes of preservation. For instance, in the subarborescent Palaeozoic sphenophytes, an impression of a leaf might be assigned to the genus Annularia, a compression of a cone assigned to Palaeostachya, and the stem assigned to either Calamites or Arthroxylon depending on whether it is preserved as a cast or a petrifaction. All of these fossils may have originated from the same parent plant but they are each given their own taxonomic name. This approach to naming plant fossils originated with the work of Alexandre Brongniart[6] and has stood the test of time.

For many years this approach to naming plant fossils was accepted by palaeobotanists but not formalised within the International Rules of Botanical Nomenclature.[7] Eventually, Thomas (1935) and Jongmans, Halle & Gothan (1935) proposed a set of formal provisions, the essence of which was introduced into the 1952 International Code of Botanical Nomenclature.[8] These early provisions allowed fossils representing particular parts of plants in a particular state of preservation to be referred to organ-genera. In addition, a small subset of organ-genera, to be known as form-genera, were recognised based on the artificial taxa introduced by Brongniart (1822) mainly for foliage fossils. Over the years, the concepts and regulations surrounding organ- and form-genera became modified within successive codes of nomenclature, reflecting a failure of the palaeobotanical community to agree on how this aspect of plant taxonomic nomenclature should work (a history reviewed by Cleal & Thomas (2010)). The use of organ- and fossil-genera was abandoned with the St Louis Code (Greuter et al. 2000), replaced by "morphotaxa".

The situation in the Vienna Code of 2005[9] was that any plant taxon whose type is a fossil, except Diatoms, can be described as a morphotaxon, a particular part of a plant preserved in a particular way. Although the name is always fixed to the type specimen, the circumscription (i.e. range of specimens that may be included within the taxon) is defined by the taxonomist who uses the name. Such a change in circumscription could result in an expansion of the range of plant parts and/or preservation states that can be incorporated within the taxon. For instance, a fossil-genus originally based on compressions of ovules could be used to include the multi-ovulate cupules within which the ovules were originally borne. A complication can arise if, in this case, there was an already named fossil-genus for these cupules. If palaeobotanists were confident that the type of the ovule fossil-genus and of the cupule fossil-genus could be included in the same genus, then the two names would compete as to being the correct one for the newly emended genus.

Morphotaxa were introduced to try to overcome the issue of competing names that represented different plant parts and/or preservation states. What would you do if the species-name of a pollen-organ was pre-dated by the species name of the type of pollen produced by that pollen organ. It was argued that palaeobotanists would be unhappy if the pollen organs were named using the taxonomic name whose type specimen is a pollen grain. As pointed out by Cleal & Thomas (2010), however, the risk of the name of a pollen grain supplanting the name of a pollen organ is most unlikely. Palaeobotanists would have to be totally confident that the type specimen of the pollen species, which would normally be a dispersed grain, definitely came from the same plant that produced the pollen organ. We know from modern plants that closely related but distinct species can produce virtually indistinguishable pollen. It would seem that morphotaxa offer no real advantage to palaeobotanists over normal fossil-taxa and the concept was abandoned with the 2011 botanical congress and the 2012 International Code of Nomenclature for algae, fungi, and plants.

Fossil groups of plants

StigmariaOhioPennsylvanian
Stigmaria, a common fossil tree root. Upper Carboniferous of northeastern Ohio.
LepidodendronOhio
External mold of Lepidodendron from the Upper Carboniferous of Ohio.

Some plants have remained almost unchanged throughout earth's geological time scale. Horsetails had evolved by the Late Devonian,[10] early ferns had evolved by the Mississippian, conifers by the Pennsylvanian. Some plants of prehistory are the same ones around today and are thus living fossils, such as Ginkgo biloba and Sciadopitys verticillata. Other plants have changed radically, or became extinct.

Examples of prehistoric plants are:

Notable paleobotanists

See also

References

  1. ^ Stearn, W.T. (2004). Botanical Latin (4th (p/b) ed.). Portland, Oregon: Timber Press. p. 460. ISBN 978-0-7153-1643-6.
  2. ^ Liddell, Henry George & Scott, Robert (1940). "παλαιός". A Greek-English Lexicon. Oxford: Clarendon Press. Retrieved 2019-07-16.
  3. ^ "Brongniart, Adolphe-Théodore". www.encyclopedia.com. Encyclopedia.com: FREE online dictionary. Retrieved 22 February 2017.
  4. ^ Cleal, Christopher J.; Lazarus, Maureen; Townsend, Annette (2005). "Illustrations and illustrators during the 'Golden Age' of palaeobotany: 1800–1840". In Bowden, A. J.; Burek, C. V.; Wilding, R. (eds.). History of palaeobotany : selected essays. London: Geological Society of London. p. 41. ISBN 9781862391741.
  5. ^ Speer, Brian R. (10 June 1995), The Devonian Period, retrieved 12 May 2012
  6. ^ Brongniart (1822)
  7. ^ Briquet, J. (1906), Règles internationales de la nomenclature botanique adoptées par le Congrès International de Botanique de Vienne 1905, Jena: Fischer, OCLC 153969885
  8. ^ Lanjouw et al. 1952
  9. ^ McNeill 2006
  10. ^ Elgorriaga, A.; Escapa, I.H.; Rothwell, G.W.; Tomescu, A.M.F.; Cúneo, N.R. (2018). "Origin of Equisetum: Evolution of horsetails (Equisetales) within the major euphyllophyte clade Sphenopsida". American Journal of Botany. 105 (8): 1286–1303. doi:10.1002/ajb2.1125. PMID 30025163.

Further reading

  • Brongniart, A. (1822), "Sur la classification et la distribution des végétaux fossiles en général, et sur ceux des terrains de sediment supérieur en particulier", Mém. Mus. Natl. Hist. Nat., 8: 203–240, 297–348
  • Cleal, C.J. & Thomas, B.A. (2010), "Botanical nomenclature and plant fossils", Taxon, 59: 261–268
  • Greuter, W.; McNeill, J.; Barrie, F R.; Burdet, H.M.; Demoulin, V.; Filgueiras, T.S.; Nicolson, D.H.; Silva, P.C.; Skog, J.E.; Turland, N.J. & Hawksworth, D.L. (2000), International Code of Botanical Nomenclature (Saint Louis Code), Königstein.: Koeltz Scientific Books, ISBN 978-3-904144-22-3
  • Jongmans, W.J.; Halle, T.G. & Gothan, W. (1935), Proposed additions to the International Rules of Botanical Nomenclature adopted by the fifth International Botanical Congress Cambridge1930, Heerlen, OCLC 700752855
  • Lanjouw, J.; Baehni, C.; Merrill, E.D.; Rickett, H.W.; Robyns, W.; Sprague, T.A. & Stafleu, F.A. (1952), International Code of Botanical Nomenclature: Adopted by the Seventh International Botanical Congress; Stockholm, July 1950, Regnum Vegetabile 3, Utrecht: International Bureau for Plant Taxonomy of the International Association for Plant Taxonomy, OCLC 220069027
  • McNeill, J.; et al., eds. (2006), International code of botanical nomenclature (Vienna Code) adopted by the seventeenth International Botanical Congress, Vienna, Austria, July 2005 (electronic ed.), Vienna: International Association for Plant Taxonomy, archived from the original on 6 October 2012, retrieved 2011-02-20
  • Meyer-Berthaud, Brigitte; Scheckler, S.E. & Wendt, J. (1999), "Archaeopteris is the Earliest Modern Tree", Nature, 398 (6729): 700–701, Bibcode:1999Natur.398..700M, doi:10.1038/19516
  • Thomas, H.H. (1935), "Proposed additions to the International Rules of Botanical Nomenclature suggested by British palæobotanists" (PDF), Journal of Botany, 73: 111
  • Wellman, Charles H.; Osterloff, Peter L. & Mohiuddin, Uzma (2003), "Fragments of the Earliest Land Plants" (PDF), Nature, 425 (6955): 282–285, Bibcode:2003Natur.425..282W, doi:10.1038/nature01884, PMID 13679913
  • Wilson N. Stewart and Gar W. Rothwell. 2010. Paleobotany and the Evolution of Plants, Second edition. Cambridge University Press, Cambridge, UK. ISBN 978-0-521-38294-6.
  • Thomas N. Taylor, Edith L. Taylor, and Michael Krings. 2008. Paleobotany: The Biology and Evolution of Fossil Plants, 2nd edition. Academic Press (an imprint of Elsevier): Burlington, MA; New York, NY; San Diego, CA, USA, London, UK. 1252 pages. ISBN 978-0-12-373972-8.

External links

Alethopteris

Alethopteris is a prehistoric plant genus of fossil Pteridospermatophyta (seed ferns) that developed in the Carboniferous period (around 360 to 300 million years ago).It is in the family Alethopteridaceae.

Cordaites

Cordaites is an important genus of extinct gymnosperms which grew on wet ground similar to the Everglades in Florida. Brackish water mussels and crustacea are found frequently between the roots of these trees. The fossils are found in rock sections from the Upper Carboniferous (323 to 299 million years ago) of the Dutch - Belgian - German coal area. A number of many noteworthy types from this line are:

Cordaites principalis

Cordaites ludlowi (named after Ludlow, a coal area in England)

Cordaites hislopii. Found in Paleorrota geopark in Brazil.In contrast to many other plants, fossilized Cordaites seeds are not rare, because they are rather large (up to 10 mm); those seeds are named Cordaicarpus.

Dicroidium

Dicroidium is an extinct genus of fork-leaved seed ferns that were distributed over Gondwana during the Triassic (252 to 201 million years ago). Their fossils are known from South Africa, Australia, New Zealand, South America and Antarctica. They were first discovered in Triassic sediments of Tasmania by Morris in 1845. Fossils from the Umm Irna Formation in Jordan indicate that these plants already existed in Late Permian.

Eophyllophyton

Eophyllophyton bellum is the oldest known plant bearing megaphyllous leaves.

Form classification

Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of science is to move "form taxa" to biological taxa whose affinity is known.Form taxonomy is restricted to fossils that preserve too few characters for a conclusive taxonomic definition or assessment of their biological affinity, but whose study is made easier if a binomial name is available by which to identify them. The term "form classification" is preferred to "form taxonomy"; taxonomy suggests that the classification implies a biological affinity, whereas form classification is about giving a name to a group of morphologically-similar organisms that may not be related.A "parataxon" (not to be confused with parataxonomy), or "sciotaxon" (Gr. "shadow taxon"), is a classification based on incomplete data: for instance, the larval stage of an organism that cannot be matched up with an adult. It reflects a paucity of data that makes biological classification impossible. A sciotaxon is defined as a taxon thought to be equivalent to a true taxon (orthotaxon), but whose identity cannot be established because the two candidate taxa are preserved in different ways and thus cannot be compared directly.

Fossil wood

Fossil wood is wood that is preserved in the fossil record. Over time the wood will usually be the part of a plant that is best preserved (and most easily found). Fossil wood may or may not be petrified. The study of fossil wood is sometimes called palaeoxylology, with a "palaeoxylologist" somebody who studies fossil wood.

The fossil wood may be the only part of the plant that has been preserved, with the rest of the plant completely unknown: therefore such wood may get a special kind of botanical name. This will usually include "xylon" and a term indicating its presumed affinity, such as Araucarioxylon (wood of Araucaria or some related genus), Palmoxylon (wood of an indeterminate palm), or Castanoxylon (wood of an indeterminate chinkapin).

Neuropteris

Neuropteris is an extinct seed fern that existed in the Carboniferous period, known only from fossils.

Major species include Neuropteris loschi.

Pachytheca

Pachytheca are fossils of ancient plants, dating from the upper Silurian to the lower Devonian.

Pecopteris

Pecopteris is a very common form genus of leaves. Most Pecopteris leaves and fronds are associated with the marattialean tree fern Psaronius. However, Pecopteris-type foliage also is borne on several filicalean ferns, and at least one seed fern. Pecopteris first appeared in the Devonian period, but flourished in the Carboniferous, especially the Pennsylvanian. Plants bearing these leaves became extinct in the Permian period.

Progymnosperm

The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the trimerophytes, and eventually gave rise to the gymnosperms. They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida (as opposite). The stratigraphically oldest known examples belong to the Middle Devonian order the Aneurophytales, with forms such as Protopteridium, in which the vegetative organs consisted of relatively loose clusters of axes. Tetraxylopteris is another example of a genus lacking leaves. In more advanced aneurophytaleans such as Aneurophyton these vegetative organs started to look rather more like fronds, and eventually during Late Devonian times the aneurophytaleans are presumed to have given rise to the pteridosperm order, the Lyginopteridales. In Late Devonian times, another group of progymnosperms gave rise to the first really large trees known as Archaeopteris.

Other characteristics:

Vascular cambium with unlimited growth potential is present as well as xylem and phloem.

Ancestors of the earliest seed plants as well as the first true trees.

Strong monopodial growth is exhibited.

Some were heterosporous but others were homosporous.

Review of Palaeobotany and Palynology

Review of Palaeobotany and Palynology is a peer-reviewed scientific journal of palaeobotany and palynology established in 1967. It is published by Elsevier on a monthly basis. The journal is edited by H. Kerp (Westfälische Wilhelms-Universität Münster) and M. Stephenson (British Geological Survey).

Sagenopteris

Sagenopteris is a genus of extinct seed ferns from the Triassic to late Early Cretaceous.

Samaropsis

Samaropsis is a form genus named by Goeppert in 1864. Later Sewart (1917) redefined the taxon to refer only to the seeds.

Sphenopteris

Sphenopteris is a genus of seed ferns containing the foliage of various extinct plants, ranging from the Devonian to Late Cretaceous.

Stigmaria

Stigmaria is a form taxon for common fossils found in Carboniferous rocks. They represent the underground rooting structures of coal forest lycopsid trees such as Sigillaria and Lepidodendron. These swamp forest trees grew to 50 meters and were anchored by an extensive network of branching underground structures with "rootlets" attached to them. Analysis of the morphology and anatomy of these stigmarian systems suggests they were shoot-like and so they are called rhizomes or rhizophores. The stigmarian rhizomes are typically covered with a spiral pattern of circular scars where "rootlets" were attached. Since the stigmarian systems are shoot-like, these "rootlets" may be modified leaves, adapted to serve the function of roots. However, some paleontologists argue that the "rootlets" were true roots, with a complex branching structure and root hairs, comparable to the roots of the closest living relative of Lepidodendron, the quillworts (genus Isoetes).

Tarella

Tarella was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Tarella in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Thrinkophyton

Thrinkophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Thrinkophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Trimerophytopsida

Trimerophytopsida (or Trimeropsida) is a class of early vascular plants from the Devonian, informally called trimerophytes. It contains genera such as Psilophyton. This group is probably paraphyletic, and is believed to be the ancestral group from which both the ferns and seed plants evolved. Different authors have treated the group at different taxonomic ranks using the names Trimerophyta, Trimerophytophyta, Trimerophytina, Trimerophytophytina and Trimerophytales.

Zosterophyllum

Zosterophyllum was a genus of Silurian-Devonian vascular land plant with branching axes on which kidney-shaped sporangia were arranged in lateral positions.

Some species have been transferred to other genera:

Z. artesianum to Danziella artesiana

Z. contiguum to Demersatheca contigua

Z. subverticillatum to Adoketophyton subverticillatumA cladogram published in 2004 by Crane et al. places the species of Zosterophyllum in a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

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