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LOCALITIES OF THE DEVONIAN:


RHYNIE CHERT, SCOTLAND



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The Rhynie Chert beds in Aberdeenshire in the north of Scotland are important
fossil sites that reveal much about the evolution of life from the Early
Devonian, approximately 408-360 million years ago. At this time, the earth's
geography was an aggregate mass of land in the Southern Hemisphere with smaller
continents in the equatorial region. Europe resided near the equator, so the
Rhynie Chert beds were in a tropical to subtropical climate and consisted mostly
of flatlands and short-lived shallow pools of fresh water.

Rhynie fossils were preserved when mineral sediments settled around and on top
of the organic matter and compressed them. Over time, the fossils were formed
through silica replacement of the organic material. The silica deposits may have
originated from hot springs and volcanic activity common in the Devonian and
around the Rhynie fossil beds. The result is a preservation in chert, a kind of
finely crystalline quartz, which occurs in veins throughout the rest of the
surrounding rocks.



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> Preserved in rock : At left, a piece of the Rhynie chert; the black structures
> woven among the rock crystals are fossilized plant stems. At right, a thin
> section through a fossilized stem of Aglaophyton major, showing the exquisite
> detail of preservation; cell walls and tissue structure is preserved at the
> anatomical level.

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The fossilized organisms preserved in the chert are studied using thin sections
made through the rock in which they are embedded. Replacement of the plant
tissues and other organic material is so fine that plant cells and fungal
filaments are preserved in anatomical detail. One new method of thin section
analysis uses light-transmitting microscope-aided stereophotomicrography to
discern details in the form or structure of the organic matter. "The stereo
image is created by juxtaposing two photographs of the same three-dimensional
object, taken from slightly different angles, by moving either the camera or the
object," (Winkelmann, 1995).

The kinds of organisms found in the Rhynie Chert include vascular plants, fungi,
algae, and marine and terrestrial invertebrates. The Devonian was an exceptional
time for the evolution of plants, and the Rhynie Chert provides fossil records
of several genera from extinct groups of primitive plants. One of these,
Aglaophyton, is often considered the earliest and simplest of vascular plants.
More recent examination, however, has shown that the plant does not contain true
vascular tissue but has conducting cells more like those found in some mosses.
Like most of the other early plants, Aglaophyton had no leaves or roots.
Additional fossils indicate that Aglaophyton grew in abundance with another
somewhat more developed relative, Rhynia, as well as the bizarre Horneophyton,
and the lycophyte Asteroxylon.



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> Early plants : At left, Aglaophyton major, one of the most structurally simple
> plants found in the Rhynie flora. The small circle to the left of the plant
> diagrams a cross-section throught the stem with false vascular tissue. At
> center, Horneophyton, a bizzare vascular plant; the sporangia were forked, and
> vascular tissue extended only partway up the stems. At right, Rhynia, one of
> the more complex plants found here, though like Aglaophyton and Horneophyton,
> it had no leaves or roots.

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These groups of early vascular land plants have been discovered in excellent
condition in specimens where silicification of the plants occurred while they
were still alive and growing. Extremely minute detail has been preserved in some
of these fossils, allowing a clear view of the fragile root-like rhizoids, the
spores and reproductive structures, the vascular systems, gametophyte
generations, and even the seasonal growth of the plants. Such discoveries are
highly prized for allowing an insight into the development of the first vascular
plants, especially in terms of their vascular transport system and the evolution
of life histories.

Other significant discoveries from the fossils of the Rhynie Chert are the
diverse fossils of fungi and record of their interactions with the surrounding
plants. The relationship between the alga Palaeonitella, and the aquatic fungus
Sorodiscus that invaded its cells, is one early example of parasitism found
preserved in the fossil record. It is quite difficult to establish a parasitic
or a symbiotic relationship in paleontology, but this alga's swollen cells where
the fungi was present reflects a phenomenon very similar to that seen today.
Detailed Rhynie Chert fossils of mycorrhizal fungi of the family Glomaceae have
also been found in association with the early vascular plants, having been
discovered in the plants' preserved stems. Fossils of tiny chytrids, simple
fungi, also have been found in the tissue of Horneophyton and even in the spores
of Aglaophyton. The discovery from the fossil record that there were plant-fungi
relationships is important because it shows that by the Early Devonian complex
interactions between terrestrial organisms were occurring.



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> More than plants : On the left, a thin section through plant tissue preserved
> in the Rhynie chert reveals fungal filaments growing among the cells. On the
> right, Eophrynus, a trigonotarbid similar to those found at Rhynie, Scotland.

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Other organisms found in the Rhynie Chert include marine and terrestrial
invertebrates. Crustaceans of the order Lipostraca as well as trigonotarbids
(spider relatives) that might be some of the earliest members of their groups
have been discovered. The Lipostraca specimen, Lepidocaris rhyniensis, was
preserved so well due to its hard outerbody shell that the fine details of its
limbs can be discerned. This is possibly one of the best-preserved crustacean
fossils ever found.

Principal figures in Devonian Rhynie Chert field research are Robert Kidston and
Henry Lang. Working in the 1920s, they were the first to describe many of the
algae, fungi, and plants from the Devonian period. Kidston, in his later 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 the Rhynie Chert, though many other scientists
have also made contributions, and continue to excavate and study this rich
locality.



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For more pictures and explanatory text, read about the Rhynie Flora at UCMP's
Virtual Paleobotany Lab.

Those interested in current work in the Rhynie chert should visit the Rhynie
Chert Resource Page, featuring images of arthropod cuticle and links to related
information. You definitely shouldn't miss Die Rhynie Chert Flora at the
University of Münster. The page is in German, but the pictures and information
are valuable; it is a part of the larger History of Paleozoic Forests site.



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Sources:
 * Gray, Jane. 1993. Major Paleozoic land plant evolutionary events.
   Palaeogeography, Palaeoclimatology, & Palaeoecology.
 * "Kidston, Robert". Britannica Online. [Accessed 15 March 1998].
 * Marshall, J E A. 1996. Rhabdosporites langii, Geminospora lemurata and
   Contagisporites optivus: an origin for heterospory within the Progymnosperms.
   Review of Palaeobotany and Palynology.
 * "Rhynie plants" Britannica Online. [Accessed 17 March 1998].
 * Taylor, T. N., W. Remy, and H. Hass. 1992. Parasitism in a
   400-million-year-old green alga. Nature.
 * Trewin, N. H. and C. M. Rice. 1992. Stratigraphy and sedimentology of the
   Devonian Rhynie chert locality. Scottish Journal of Geology.
 * Trewin, Nigel H. 1994. Depositional environment and preservation of biota in
   the Lower Devonian hot-springs of Rhynie, Aberdeenshire, Scotland.
   Transactions of the Royal Society of Edinburgh.
 * Winkelman, Heather; et al. 1995. Stereophotomicrography of thin sections
   under transmitted light. Journal of Paleontology.

Images of plants created by Caroline Stromberg.

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