Deborah Painter (USA)
Plant fossils are available in a greater variety than this creator used to consider. This text will talk about a few of the totally different ways in which plant fossils are preserved.
Casts
The fossil in Figs. 1 and a pair of is a cast of Stigmaria. It is comprised of greyish sandstone. Sandy sediments initially filled the empty space left by the decaying plant; in this case, a root decayed and was replaced by shale that did not preserve the cell structure. Stigmaria is a form genera name for the roots of Carboniferous lycopod trees. Form genera are genera defined for a part of an organism or plant rather than for the entire plant, in all its different parts (leaves, roots, trunk, spores and so on). The Stigmaria is a root and it may be the root of Sigillaria or Lepidodendron.
The fossil dates from the Mississippian period (Upper Carboniferous) and is part of the Price Formation. The Price Formation is sandstone, conglomerate, quartzarenite, limestone, coal, and shale. The sandstone is feldspathic and slightly micaceous, with a few greyish red beds. The Price Formation is a westward, thinning clastic wedge that is equivalent to part of the Grainger Formation in the south-westernmost part of Virginia and is 44m thick in Pulaski County. This specimen was collected by a family friend years ago at Cloyds Mountain, at a roadcut near Poplar Hill in Pulaski County, Virginia (Fig. 3).
Some casts are formed in an uncommon way, such as the chalcedony (agate) tubes sold as “fossil bamboo”. These are erroneously named. They were formed during the late Quaternary period (Pleistocene and Anthropocene epochs), when original remains of horsetails (genus Equisetum) and other marsh plants were covered in siliceous ash that resulted from volcanic eruptions in the Mount Merapi area of Java, in Indonesia (Fig. 4).
These are known from a single locality in Java, where locals gather them, and there has been limited availability to foreign collectors. However, I managed to find some at a gem and fossil show. The actual location is not revealed due to concerns about potential theft. Grooves and striations along the inner walls of these fossils suggest a cast. The agate tubes come in a variety of colours, from yellow to white, red, purple and pink. Permineralization occurred with brown or black plant parts inside many tubes.
The specimen in Figs. 5 and 6 is approximately 15cm in length and approximately 2cm in diameter. Some “fossil bamboo” specimens are more globular in shape than tube-shaped. These suggest that these fossilised plant parts were once fruits of an unknown species.
Palaeobotanist, William Walton Wright, visited the site and posits a hypothesis to explain their formation. Upon deposition of the ash on the marsh, the ash weathered and oxidised. This released silica, iron, manganese and other ions into the marsh. As a result, the water turned acidic and there was an increase in colloidal suspension of clays from feldspars that decomposed. Acidity fluctuated due to the varying heights of the water column during alternating rainy and dry seasons. Pieces of the stems of buried rushes, horsetails and grasses broke and became buried deeper in the marsh.
Living Equisetum plants have a high percentage of silica in their stems, which made these plants ideal nuclei for the process of deposition of cryptocrystalline quartz (agate). Slow decomposition of the organic plants occurred while buried under increasingly thick layers of ash from repeated eruptions. Suspended minerals and clays became deposited in the hollow cores in a sort of “bog iron”, which became filled within by siderite, while agate rings formed around one another. Some fossils have tiny barite crystals around their exteriors. Wright surmises that this is due to temporary silica depletion and replacement by carbonates or barium.
Moulds
A mould is the depression in which a cast formed. Fig. 7 is a photograph of a cast and mould of Alethopteris in the collection of the University of Michigan’s Museum of Natural History in Ann Arbor, Michigan. Alethopteris is a Lower Carboniferous (Mississippian period) seed fern found in Will County, Illinois. The two specimens are actually the same fossilisation event. The cast is on the left-hand side of the photo and the mould is on the right-hand side of the photo.
Permineralization
During diagenesis (the process by which sediments are altered after deposition but before they are lithified), pores of plants are protected from decomposition, while being slowly impregnated by minerals from the groundwater or in ponds, lakes and oceans. The fibres and the cellulose completely dissolve and are replaced by minerals. When viewed under a microscope, thinly cut specimens of most permineralized plant fossils reveal the original configuration down to the last detail at the cellular level.
Calcite, silica, iron and pyrite are common minerals involved in the formation of these fossils (Fig. 8).
The famed Petrified Forest National Park between Apache County and Navajo County, Arizona provides visitors with the chance to see scores of logs of permineralized conifer tree fossils of the Triassic Chinle Formation, approximately 225 to 207 million years old (Figs. 9 and 10). Most are specimens of Araucarioxylon arizonicum, the state fossil. There are also specimens of Woodworthia arizonica and Schilderia adamanica represented, as well as other conifers, all extinct. The conifers are not the only plants represented. Cycads, ginkgoes, lycophytes and ferns lived alongside them. In the Triassic, there were no flowering plants.
Less famous examples of permineralization are the fossil palm wood specimens found at Toledo Bend, Texas and Louisiana. Toledo Bend is an impoundment of the Sabine River forming the border between Texas to the west and Louisiana to the east (Fig. 11).
This area is well known among fossil enthusiasts for the Oligocene Catahoula Formation specimens of Palmoxylon, the official state fossil of Louisiana. The cellular structure of the palm trunk is beautifully preserved, as seen in the 12.7cm-long, broken specimen in Figs. 12 and 13. The rod-like structures are fossilised sclerenchyma bundles that provided support for the tree.
Plant pollen (Fig. 14) can date to the Palaeozoic, Mesozoic and Cenozoic eras. The oldest permineralized multicellular plant cells discovered to date are spores from the Ordovician period.
Impressions
A leaf may fall into silt and when the silt becomes siltstone or clay through lithification, and the leaf may remain as an impression. Many Eocene epoch plants from the Cenozoic era have been fossilized in this way as impressions in clay pits that became shale (Fig. 15).
Carbonisation
This process results in a preservation of the outline of a fossil. A carbonaceous film or carbon film is a residue formed by compression. When an organism is buried under sediment, heat and pressure increase during diagenesis and this pressure and heat forces gases and liquids from the plant body. What is left is a thin film that is rather like a silhouette.
The carbonised Lepidodendron terminal branches in shale in Fig. 16 were collected at Cloyds Mountain, on the eastern flank of Route 100, near Poplar Hill, Pulaski County, Virginia, in the same locality as the Stigmaria of a lycopod tree seen in Figs. 1 and 2. It may also be from the same genus.
Original remains
In any situation when plants or plant parts are protected from the atmosphere by rapid burial in ice, thick sediments, asphaltum or amber, preserved original remains result. Some examples are set out below.
For example, diatomaceous earth consists of the accumulated original remains of the silica-rich shells of marine and freshwater phytoplankton known as diatoms (Fig. 17), mixed with some silt. Diatoms (that is algae) have existed since the Jurassic period. Few have been found that were older.
Finding plant fossils in sediment beneath the Greenland ice sheet was not the reason behind the arduous construction of Camp Century by the US Army Corps of Engineers beginning in 1959. Camp Century is located approximately 240km east of the Thule, Greenland air base, now renamed the Pituffik Space Base. However, the plant fossils are one of the former base’s lasting legacies.
Camp Century was promoted as a scientific research facility but was actually a strategic base with hidden ballistic missiles. The camp would be warned early of an attack and the missiles could be launched in the event of an attempted strike by the Soviet Union. The base was built into the continental glacier’s snow using steel arches, and ice and snow to support its walls and ceilings (Fig. 18). A nuclear reactor was assembled offsite and installed to power the camp, which had accommodation for 250 men.
Unfortunately, 50 had to be on rotational duty at all times to shore up the walls and ceilings with ice. Why? The ice and snow were melting from the heat from cooking, the laundry, and the operation of the reactor. Furthermore, the ice sheet was not staying in place as expected, but was migrating. The entire facility was abandoned by 1967.
A year earlier, a core was obtained from a depth of almost 1.4km of ice, at the interface with sediment. Soon afterward, it would be impossible to do any more scientific exploration of the secrets hidden under the ice sheet. At the bottom of the core, sediment contained well-preserved plants and biomolecules from at least one ice-free period during the Pleistocene epoch.
Decades later, using in situ cosmogenic nuclides, infrared-stimulated luminescence, geochemistry, optical microscopy, and scanning electron microscopy and energy dispersive X-ray spectroscopy analyses, Andrew J Christ, Paul R Bierman, Joerg M Schaefer and John Southon of the University of Vermont re-examined the core and found that enriched stable isotopes in pore ice indicated precipitation at lower elevations than the present day.
This implies the lack of an ice sheet. Therefore, the continental glacier in this high latitude in Greenland has melted and then recovered at least once during the past million years. Original remains of fungi, freshwater diatoms, pollen, and the stems of a species of moss known as Polytrichum juniperinum suggest a tundra ecosystem.
Original remains caught in asphaltum (late Quaternary, including the Anthropocene epoch) can be preserved. Rancho La Brea – the famed La Brea Tar Pits of Los Angeles, California – is an example of this mode of preservation (Fig. 19). Many pollen and plant stem specimens have been recovered along with the skeletons of camels, dire wolves, sabre-toothed cats, mastodons and other animals.
Original remains can be preserved in amber as well. In 1872, a German palaeontologist found a nearly perfect flower 28mm in width in Baltic amber dating from the Eocene epoch. In the twenty-first century, some pollen was extracted from the flower and it was determined that it was from the genus Symplocos (Symplocos kowaleski)and was related to living Asian species of a tree genus found in both Asia and North and South America, known as horse-sugar or sapphire berry.
And no, the scientists did not decide to do any DNA experiments using living horse-sugar tree DNA and this newly discovered species’ genetic material to bring the approximately 33.9 to 38-million-year-old plant back from the dead like the characters did with DNA in Jurassic Park.
About the author
Deborah Painter is an ecologist and general environmental scientist. She lives in the United States.
References
Andruss, Gary. Fossil Bambu page: https://www.indoagate.com/bamboo.html
Christ, Andrew J., Paul R. Bierman, Joerg M. Schaefer and John Southon. January 27, 2021. A multimillion-year-old record of Greenland vegetation and glacial history preserved in sediment beneath 1.4 km of ice at Camp Century.
Proceedings of the National Academy of Sciences 118 (13), University of California San Diego and La Jolla.
Foster, Clinton. Oldest Fossilized Land Plant Spores Have Scientists Rethinking How Plants Evolved. Nature. August 14, 2021: https://www.sciencealert.com/480-million-year-old-fossil-spores-has-scientists-rethinking-how-plants-evolved-on-land
Funnell, Rachael. January 12, 2023. The Biggest Fossil Flower Ever Found In Amber Has A New Name: Blooming Heck. IFL Science: https://www.iflscience.com/the-biggest-fossil-flower-ever-found-in-amber-has-a-new-name-67049
Kentucky Geological Survey. Fossil of the Month: Stigmaria: http://www.uky.edu/KGS/fossils/fossil-month-Stigmaria.php
Kosanke, Robert M. A long-leaved specimen of Lepidodendron. Geological Society of America Bulletin (1979) 90 (5), 431-434.
McElwain, Jennifer, Marlene Hill Donnelly and Ian Glasspool. 2021. Tropical Arctic: Lost Plants, Future Climates, and the Discovery of Ancient Greenland. University of Chicago Press, 144 pages.
Milsome, Clare and Sue Rigby. 2009. Fossils at a Glance, Wiley-Blackwell; 2nd edition. 170 pages.
Olcott, A.N., Downen, M.R., Schiffbauer, J.D. et al. The exceptional preservation of Aix-en-Provence spider fossils could have been facilitated by diatoms. Communications Earth & Environ 3, 94 (2022): https://doi.org/10.1038/s43247-022-00424-7
Sweetman, Steven C. 2009. Sieving out the big picture. Deposits Issue 18, Spring 2009, pp, 41-45.
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