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The Rhynie Chert beds in Aberdeenshire in the north of Scotland are importantfossil sites that reveal much about the evolution of life from the EarlyDevonian, approximately 408-360 millionyears ago. At this time, the earth's geographywas an aggregate mass of land in the Southern Hemisphere with smallercontinents in the equatorial region. Europe resided near the equator, so theRhynie Chert beds were in a tropical to subtropical climate and consistedmostly of flatlands and short-lived shallow pools of fresh water.
Rhynie fossils were preserved when mineral sediments settled around and ontop of the organic matter and compressed them. Over time, the fossils wereformed through silica replacement of the organic material. The silica depositsmay have originated from hot springs and volcanic activity common in theDevonian and around the Rhynie fossil beds. The result is a preservation inchert, a kind of finely crystalline quartz, which occurs in veinsthroughout the rest of the surrounding rocks.
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The fossilized organisms preserved in the chert are studied using thin sectionsmade through the rock in which they are embedded. Replacement of the planttissues and other organic material is so fine that plant cells and fungalfilaments are preserved in anatomical detail. One new method of thin sectionanalysis uses light-transmitting microscope-aidedstereophotomicrographyto discern details in the form or structure of the organic matter."The stereo image is created by juxtaposing two photographs of the samethree-dimensional object, taken from slightly different angles, by movingeither the camera or the object," (Winkelmann, 1995).
The kinds of organisms found in the Rhynie Chert include vascular plants,fungi, algae, and marine and terrestrialinvertebrates. The Devonian was an exceptional time for the evolution ofplants, and the Rhynie Chert provides fossilrecords of several genera from extinct groups of primitive plants.One of these,Aglaophyton, is often considered the earliest and simplestof vascular plants. More recent examination, however, has shown that theplant does not contain true vascular tissue but has conducting cells more likethose found in somemosses.Like most of the other early plants,Aglaophyton had no leaves or roots.Additional fossils indicate that Aglaophyton grew in abundance with anothersomewhat more developed relative,Rhynia, as well as the bizarreHorneophyton, and thelycophyteAsteroxylon.
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These groups of early vascular land plants have been discovered in excellentcondition in specimens where silicification of the plants occurred while theywere still alive and growing. Extremely minute detail has been preserved insome of these fossils, allowing a clear view of the fragile root-like rhizoids,the spores and reproductive structures, the vascular systems, gametophytegenerations, and even the seasonal growth of the plants. Such discoveriesare highly prized for allowing an insight into the development of thefirst vascular plants, especially in terms of their vascular transportsystem and the evolution of life histories.
Other significant discoveries from the fossils of the Rhynie Chert are thediverse fossils offungi and record of theirinteractions with the surrounding plants. The relationship between the algaPalaeonitella, and the aquatic fungusSorodiscus thatinvaded its cells, is one early example of parasitism found preserved inthe fossil record. It is quite difficult to establish a parasitic ora symbiotic relationship in paleontology, but this alga's swollen cells wherethe fungi was present reflects a phenomenon very similar to that seen today.Detailed Rhynie Chert fossils ofmycorrhizal fungi of the familyGlomaceae have also been found in association with the early vascular plants,having been discovered in the plants' preserved stems. Fossils of tinychytrids, simple fungi, also have beenfound in the tissue ofHorneophyton and even in the spores ofAglaophyton. The discovery from the fossil record that there wereplant-fungi relationships is important because it shows that by the EarlyDevonian complex interactions between terrestrial organisms were occurring.
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Other organisms found in the Rhynie Chert include marine and terrestrialinvertebrates. Crustaceans of the order Lipostraca as well astrigonotarbids(spider relatives) that might be some of the earliest members of their groupshave been discovered. The Lipostraca specimen,Lepidocaris rhyniensis,was preserved so well due to its hard outerbody shell that the fine details ofits limbs can be discerned. This is possibly one of the best-preservedcrustacean fossils ever found.
Principal figures in Devonian Rhynie Chert field research are Robert Kidstonand Henry Lang. Working in the 1920s, they were the first to describe manyof the algae, fungi, and plants from the Devonian period. Kidston, in hislater studies of the Rhynie fossils was mainly concerned with the organisms'morphology (the structure or body formations of organisms).Kidston and Lang's work is still considered to be a standard for theRhynie Chert, though many other scientists have also made contributions,and continue to excavate and study this rich locality.
For more pictures and explanatory text, readabout theRhynie Flora at UCMP'sVirtual Paleobotany Lab.
Those interested in current work in the Rhynie chert should visit theRhynie Chert Resource Page,featuring images of arthropod cuticle and links to related information.You definitely shouldn't missDie Rhynie Chert Floraat the University of Münster. The page is in German, but the pictures and information are valuable; it is a part of the largerHistory of Paleozoic Forests site.
