Rhynie Chert
From Palaeos
| Devonian |
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One of the most famous of the Devonian Lagerstätten is the Rhynie Chert from Scotland. This is one of the earliest more-or-less terrestrial sites known. This deposit is dated as from the Pragian Age of the early Devonian. Paleogeographic reconstructions and other evidence suggest the environment was tropical to subtropical. This deposit is a petrified peat bog preserving the plants in exquisite anatomical detail in the place where they grew and died.
Rhynie is located in Aberdeenshire, Scotland, about 50 km west of Aberdeen. It was first found and its plant fossils studied by Robert Kidston and William Lang in 1917.
The Rhynie Chert represents a swampy peat bog from the Early Devonian with the peculiar addition of mineral springs. The plant tissues were preserved by silicate diagenesis, in which the plant tissues and even individual cells were replaced by silicates from the springs almost immediately after burial.
The peat species include Aglaophyton (formerly Rhynia major), Horneophyton, Nothia and the proto-lycopod Asteroxylon, but the only plant preserved exactly in its growth position is Rhynia gwynne-vaughanii.
The preservation of all these plants is so fine that the structure of individual cells can be seen, extending almost to subcellular organelles. The detail of preservation shows, for example, that the stomata of Rhynia were connected to an extensive intercellular system of air spaces, essential for the ventilation of a land plant, and that groundwater was absorbed through unicellular hairs on the horizontal stems. The plant assemblage itself is interesting for the Early Devonian in that its members are not recognized or recorded elsewhere in Euramerica.
It is impossible to determine how typical the Rhynie Chert flora was of the wetter areas of Euramerica. Other Early Devonian assemblages contain plants with far greater amounts of thick-walled structural tissues, and are thus thought to have lived in places subjected to much drier periods.
As well as a number of types of land-plants, Fungi, including mycorrhizal fungi, have been recovered from the Rhynie Chert. Wefts of fine, sparingly septate hyphae, some terminating in vesicles, which occur within degraded tissue of vascular plants, are usually identified as a saprotrophic fungus (Phycomycetes), but thick-walled spore-like bodies superficially similar to those of endomycorrhiza (Endogone) suggest that the fungal hyphae lived in symbiotic association with the vascular plants even at that early stage of terrestrial evolution, just as they do today.
Also found are algae, including mats of filamentous blue-green algae, a charophyte green alga called Palaconitella, and filamentous green algae.
Small arthropods are exquisitely preserved between the plant stems and within sporangia. They include crustaceans, a springtail (Subphylum Hexapoda, Class Collembola), several small mites, the first spider and numerous larger extinct mite-like arachnids called trigonotarbids. The trigonotarbids probably preyed on other arthropods while the insects and mites ate spores, leaf-litter, and microorganisms or sucked plant sap, as the associated wounded plant stems suggest.
The Rhynie Chert has allowed us to study early terrestrial plants in considerable detail. However, more than this, it shows how the entire community was structured. The community in question was, no doubt, aberrant for that time or any other. Nevertheless, we see every component from predatory spiders down to fungal saprophytes. No other site from a comparable time period gives us such a detailed look at an entire terrestrial community.
- Palaeos com - Devonian
Credits
credits: MAK021023; ATW041123 public domain. Palaeos org MAK061001, MAK061003
