Rhynie chert
57°20′12″N 002°50′29″W / 57.33667°N 2.84139°W
The Rhynie chert is a
This fossil bed is remarkable for two reasons. First, the age of the site (
Location
The bed is under at least 1 metre of overburden, in a small field near the village of Rhynie, so is effectively inaccessible to collectors; besides which, the site is a Site of Special Scientific Interest. A second unit, the Windyfield chert, is some 700 m from the Rhynie. The Rhynie chert extends for at least 80 m along strike and 90 m down-dip.[5]
History of research
The chert was discovered by William Mackie while mapping the western margin of the Rhynie basin in 1910–1913.
Until recently, the Rhynie chert was the only such deposit[clarification needed] known from the geological record, although recent work has turned up other localities from different time periods and continents.[7]
Conditions of formation
The chert was formed when silica-rich water from volcanic springs rose rapidly and petrified the early terrestrial ecosystem, in situ and almost instantaneously,[1] in much the same fashion that organisms are petrified by hot springs today[8] - although the astounding fidelity of preservation has not been found in recent deposits.[9] Hot springs, with temperatures between 90 and 120 °C (194 and 248 °F),[8] were active in a number of episodes; the water had probably cooled to under 30 °C (86 °F) before it reached the fossilised organisms.[5] Their activity is preserved in 53 beds, 80 mm (3 in) thick on average, over a 35.41 m (116.17 ft) sequence,[10] interbedded with sands, shales and tuffs - which speak of local volcanic activity.[11] Deposition was very rapid.[12] The fluids originated from a shallowly dipping extensional fault system to the west, which bounded an extensional half-graben.[11]
Fossils were formed as silica formed in the hot springs themselves;
Sedimentary textures which appear to have formed in the hydrothermal vents themselves are preserved with a brecciated texture;[8] "geyserite", a sediment with a botryoidal form reminiscent of modern vent margins, is also found.[8] Spores collected from within surrounding rocks had been heated to different degrees, implying a complex history of local heating by volcanic processes.[12]
Preservation
Plants
The preservation of plants varies from perfect three-dimensional cellular
Plants were only found on the land - none lived in the water of lakes or hot springs.[10] Rhynia typically grew on sandy surfaces, and is often preserved there in life position; Horneophyton grew on sinter, the sediment formed by the hot springs. These two colonisers were subsequently joined by other genera.[10] The time between sinter deposition events was too short to allow the populations to develop to climax communities, and correspondingly early colonisers appear most frequently, pseudo-randomly, in logged sequences.[5]
Plants demonstrate best the great value of the exceptional preservation of the Rhynie chert. The presence of soft tissue, including
The preservation of spores attached to
Analysis of spores shows that the flora was lacking in some elements common elsewhere at this time, likely due to its setting in a mountainous region, rather than in a lowland flood plain like most other fossil deposits.[17] However, the spores, which are distinctive enough to permit their producing organism to be identified - are identical to those found elsewhere in "normal" environments.[17] There is no clear-cut evidence that the plants of the Rhynie assemblage were specifically adapted to stressed environments,[10] and it is likely that the flora in fact represents those members of the global fauna that happened to be capable of colonising and surviving a hot spring environment by virtue of fortuitous preadaptations.[17]
Seven land plant taxa have been identified in the Rhynie and Windyfield cherts:[18]
Another group,
Algae
Several putative chlorophytes have been discovered in the Rhynie assemblage (Mackiella and Rhynchertia). A well-preserved charophyte, Palaeonitella,[9] which inhabited the alkaline freshwater pools towards the end of the sinter apron, has been characterized.[19]
Arthropods
As a result of its exquisite preservation, the Rhynie chert boasts the most diverse non-marine fauna of its time,
The oldest known
Fungi
Fungi known from the Rhynie chert include the
The Chytridiomycetes, or Chytrids, are a basal group of fungi, closely related to the true fungi.
The chytrids display a range of behaviour in the Rhynie chert. Eucarpic and holocarpic forms are known - i.e. some forms grew specialised fruiting bodies while others did not show specialisation in this fashion.[24] Saprotrophy may be present, and parasitism is common; one individual has even been found parasitising a germinating gametophyte.[24] The fungi were aquatic, and grew in both plants and algae; they are also found preserved "loose" in the chert matrix.[24] Their flagellate spores are preserved.[24]
The largest organism present in Rhynie was probably a fungus, the enigmatic Prototaxites, growing as a mound a metre or more taller than anything in the community, whose isotopic composition varied like a saprotroph and whose septate pores resemble those of fungi.
Cyanobacteria
In the rare instances that cyanobacteria are found in the fossil record, their presence is usually the subject of much controversy, for their simple form is difficult to distinguish from inorganic structures such as bubbles.
However, bona fide cyanobacteria are preserved in the Rhynie chert. The aquatic organisms are thought to belong to the Oscillatoriales section on the basis of biomarker absence.[28] The fossils are filamentous, around 3 μm in diameter, and grew on plants and the sediment itself. They occasionally form structured colonies which go on to create microbial mats.[28]
Lichens
A new genus of lichen, Winfrenatia, has been recovered from the Rhynie chert. The lichen comprises a thallus, made of layered, aseptate hyphae; a number of depressions are formed on its top surface. Each depression contains a net of hyphae holding a sheathed cyanobacterium. The fungus appears to be related to the Zygomycetes, and the photobiont resembles the coccoid Gloeocapsa and Chroococcidiopsis.[29]
Interactions
The Rhynie chert, by preserving a snapshot of an ecosystem in situ in high fidelity, gives a unique opportunity to observe interactions between species and kingdoms.[1] There is evidence of parasitic behaviour by fungi on algae Palaeonitella, provoking a hypertrophic response.[24] Herbivory is also evident, judging by boring and piercing[30] wounds in various states of repair, and the mouthparts of arthropods.[31]
Plants responded to fungal colonisation in different ways, depending on the fungus. The rhizoids of
Fungal interactions are known to promote speciation in modern plants, and presumably also affected Devonian diversity by providing a selection pressure.[26]
See also
References
- ^ S2CID 182210855.
- ^ Nunn, Elizabeth. "The Rhynie Chert". Fossil Lagerstätten. Department of Earth Sciences, University of Bristol. Archived from the original on 1 December 2017. Retrieved 23 November 2017.
- S2CID 128977213.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - S2CID 128679831.
- ^ S2CID 128937466.
- ^ S2CID 128424299.
- S2CID 53975045.
- ^ PMID 9243014.
- ^ doi:10.1139/e03-065. Archived from the originalon 2012-12-16. Retrieved 2008-05-15.
- ^ S2CID 129847888.
- ^ S2CID 55042118. Retrieved 2008-05-15.
- ^ S2CID 128754463.
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- Contains useful reconstructions of both the plant associations, and the regional setting.
- ^ S2CID 52103830.
- S2CID 128501945.
- PMID 15809414.
- ^ PMID 15255055.
- ^ University of Aberdeen, The Biota of Early Terrestrial Ecosystems: The Rhynie Chert.
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- ISSN 1936-6426.
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- S2CID 4339420.
- S2CID 4431205.
- ^ JSTOR 2445050.
- PMID 16389979.
- ^ PMID 17447919.
- ^ PMID 17447903.
- ^ .
- S2CID 25684294.
- .
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- ^ S2CID 129545961.
- S2CID 131202472.
- PMID 29254958.
- PMID 11607500.
Further reading
- Taylor, T.N.; Taylor, E.L. (2000). The Rhynie chert ecosystem: a model for understanding fungal interactions (Free access @ Google books). CRC Press. )
External links
- "Rhynie Chert Learning Resource". University of Aberdeen.