Palaeozoic Terrestrial Ecosystems
Early Terrestrial Ecosystems
Terrestrialisation - the colonisation of the land surface by life - completely transformed the history of our planet. Unlike some of the other major events in life history, terrestrialisation has been a protracted and continuing process with no clearly definable beginning. The first organisms to inhabit the continents were probably cyanobacterial mats and communities of microbes during the Precambrian which have left signs of palaeosols (fossil soils) in Proterozoic rocks. At the beginning of the Palaeozoic the land appears to have differed little from the environments of the Precambrian. By the Middle Cambrian and Ordovician periods however, a variety of simple spores appear in terrestrial sediments which are generally referred to as 'cryptospores' since the affinity of the plants that produced them is unclear. The producers of these early spores are most likely to have been non-vascular plants such as liverworts, hornworts and a group collectively known to botanists as ‘Bryophytes’. Traditionally the liverworts and hornworts have also been included under the bryophyte umbrella, however, more recent evidence reveals that these plants fall into related groups of their own. The first evidence for vascular plants is found in Silurian rocks in the form of simple leafless plants with vascular tissue such as Cooksonia. These diminutive plants would have formed a short carpet of vegetation in wetter areas of the land surface. During the Early Devonian there was a major diversification of plant forms in tandem with both fungi and early terrestrial animals. This radiation of terrestrial life led to the development of the first complex ecosystems of detritivores, herbivores, saprotrophs and fungal symbionts. Near to the small village of Rhynie in Aberdeenshire, Scotland, a set of extremely fortuitous geological circumstances have conspired to create one of palaeontology’s wonders of the world; the Rhynie Chert. Around 410 million years ago sediments were being formed in an environment not dissimilar to the bubbling volcanic pools that can be seen at Yellowstone National Park today in Wyoming. In the Devonian, this landscape would have been filled with hydrothermally charged springs, geysers and shallow mineral rich ponds known as sinter pools. Sinter is the name given to mineral deposits of amorphous opaline silica derived from silica saturated hydrothermal waters. When a sinter pool burst or flooded the mineral-charged hydrothermal waters would have spilled into the surrounding vegetation and soils. This would have instantly killed any animal life swimming in cooler fresh waters and entombed the stems of vegetation in silica, and therefore each of these layers provides a true snapshot in time of this Devonian landscape. Over millions of years under burial pressure the opal crystals rearrange and eventually form the rock geologists refer to as chert, hence the name Rhynie Chert. Chert is a tremendously hard glassy rock that is extremely resistant to compaction, the perfect time capsule to preserve fossils for over 400 million years. The Rhynie Chert is invaluable for two main reasons; firstly its age is key to understanding a crucial period in the development of land based ecosystems. Secondly the quality of preservation of the organisms entombed in the chert is second to none. Plants, animals and fungi have their delicate tissues preserved in microscopic cellular detail and in three dimensions telling us an enormous amount about the composition and interactions of early terrestrial ecosystems. |
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Late Palaeozoic Terrestrial Ecosystems
As the ecosystems of the land became increasingly complex, communities of plants, animals and fungi developed intricate relationships and diversified. The first forests began to produce enormous volumes of biomass and altered the weathering cycles of the continents. The spread of forests sequestered atmospheric carbon dioxide whilst increasing the oxygen content of the atmosphere possibly up to 35% (compared with 21% today). Given such high oxygen levels it is unsurprising that fire played an important role in these ecosystems, as evidenced by the abundant quantities of charcoal from rocks of this age. Fossils from these early forests show that many of the animal inhabitants were arthropods, a group which also dominates modern forest ecosystems. During the Carboniferous period, several groups of arthropods achieved an impressively large size. Fossils of large dragonfly-like insects such as Meganeura have been found from rocks of this age with a wingspan of over 65 centimetres. Large trace fossil trackways have also been found produced by millipede-like Arthropleurids. These tracks occur in many locations around the world from the late Silurian to the early Permian, reaching peak abundance and size in the Carboniferous. The tracks consist of parallel lines of millipede-like prints that extend along rock bedding planes and often veer around the fossilised trunks of plants. The track ways reach up to 50 centimetres in width, and based on calculations extrapolated from the number of legs, some species such as Arthropleura reached up to two metres in length. Fossils of the actual Arthropleurid animals have been recovered from rocks complete with fossilised gut contents including plant fragments and spores, showing that they either fed directly on living plants or on the decaying plant matter. The increase in abundance of large woody vegetation by the Late Devonian and Carboniferous also had a dramatic effect on the flow of rivers across the landscape; the spread of large rooting structures stabilised the banks and promoted the development of meandering channel systems as opposed to the braided fluvial systems that dominate the pre-Devonian sedimentary record. The dramatic increase in plant biomass associated with forest ecosystems in the late Palaeozoic led to the formation of much of the coal that has been exploited since the industrial revolution. Much of the European and North American coals are composed of material derived from the wetland forests of the Carboniferous period. Vast forests of peat-forming plants were using photosynthesis to capture the sun’s energy just as they do today. The bodies of these plants accumulated over time into thick mats of peat, as occurs in modern wetland environments. Over time this amounted to an enormous volume of carbon locked up as dead plant matter. A significant proportion of this peat material was subsequently buried and subject to heat and enormous pressures which eventually turned it to coal. The plants that formed these lowland forests of equatorial Euramerica were somewhat different to the types that populate today's forests. Gigantic lycopsids such as Lepidodendron were among the most common tree-sized plants to inhabit the Carboniferous wetlands. The forests of the Palaeozoic were completely devoid of flowers, angiosperms had yet to evolve and so the structure of interactions between insects and plants was markedly different. The Carboniferous wetlands of tropical Euramerica declined in extent towards the close of the Carboniferous, primarily because the lowland habitat they occupied was gradually destroyed by mountain uplift coupled with a changing climate. Elsewhere these rainforests continued to flourish in Cathaysia (landmasses that would later become China) well into the Permian. Throughout the Permian the climate became increasingly arid, reducing the extent of the Southern Hemisphere ice cap. During the Permian the continents of the Southern Hemisphere were clustered into a great landmass known as Gondwana, composed of South America, Africa, Madagascar, India, Antarctica, Australia and New Zealand as well as a number of other peripheral landmasses. My research has primarily focused on the floras of Gondwana during the Permian period. The Permian was an interesting time for terrestrial life as the typically lycopsid-dominated forests of the Carboniferous gave way to floras largely dominated by gymnosperms (seed plants) and pteridosperms (seed ferns). The Permian also saw a diversification of insects, including the Coleoptera (beetles), a group which were later to become incredibly diverse. Much of my work has centred around a remarkable fossil site from Antarctica in the Prince Charles Mountains known as the Toploje Member chert. I have been researching the plants, fungi and animals preserved inside the chert as well as the factors that led to its preservation. The dominant plant of many of these Permian coal-forming ecosystems was the Glossopteris tree. Glossopteris is the name given to the genus of tongue-shaped leaf fossils common in many terrestrial Permian sediments in the Southern Hemisphere and in India. Glossopteris is a fossil famed for its role in the development of early theories of continental drift. The distribution of Glossopteris fossils across the now widely dispersed southern continents gave clues to the fact that these landmasses were once joined: Glossopteris fossils were mentioned by Alfred Wegener in 1912 in his writings on continental drift, prior to the eventual discovery of the plate tectonics mechanism driving this phenomenon. The end of the Permian Period, 251 million years ago, marks the close of the Palaeozoic era on the geological time scale and coincides with the largest mass extinction known. Up to a possible 95% of all species on Earth perished according to extrapolations based on the marine shelly record. In Gondwana, the glossopterid-dominated wetland forests were among the major terrestrial casualties of this largest of all mass extinctions. |
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Further Reading
Anderson, J.M., Anderson, H.M., Archangelsky, S., Bamford,
M., Chandra, S., Dettmann, M., Hill, R., McLoughlin, S., Rösler, O., 1999.
Patterns of Gondwana plant colonisation and diversification. Journal of African
Earth Sciences 28, 145–167.
Anderson, L.I., Trewin, N.H., 2003. An early Devonian arthropod fauna from the Windyfield cherts, Aberdeenshire, Scotland. Palaeontology 46, 467–509.
Benton, M.J., Newell, A.J., 2013. Impacts of global warming on Permo-Triassic terrestrial ecosystems. Gondwana Research. http://dx.doi.org/10.1016/j.gr.2012.12.010. 1180
Benton, M.J., Twitchett, R.J., 2003. How to kill (almost) all life: the end-Permian extinction event. Trends in Ecology and Evolution 18, 358–365.
Cleal, C.J., Thomas, B.A., 1994. Plant Fossils of the British Coal Measures. Dorchester: The Palaeontological Association. ISBN 0901702536
Cleal, C.J., Thomas, B.A., 2009. An Introduction to Plant Fossils. Cambridge: Cambridge University Press. ISBN 978052188715.1
Davies, N.S., Gibling, M.R., 2010. Cambrian to Devonian evolution of alluvial systems: the sedimentological impact of the earliest land plants. Earth Science Reviews 98, 171–200.
Edwards, D., 1996. New insights into early land ecosystems: a glimpse of a lilliputan world. Review of Palaeobotany and Palynology 90, 159–174.
Knoll, A.H., 1984. Patterns of extinction in the fossil record of vascular plants. In: Nitecki, M. (Ed.), Extinctions. University of Chicago Press, Chicago, IL, pp. 21–65.
Hilton, J., Cleal, C.J., 2007. The relationship between Euramerican and Cathaysian tropical floras in the Late Palaeozoic: palaeobiogeographical and palaeogeographical implications. Earth-Science Reviews 85, 85–116.
Labandeira, C.C., 2005. Invasion of the continents: cyanobacterial crusts to tree-inhabiting arthropods. Trends in Ecology and Evolution 20, 253–262.
McLoughlin, S., 2001. The breakup history of Gondwana and its impact on pre-Cenozoic floristic provincialism. Australian Journal of Botany 49, 271–300.
McLoughlin, S., 2011b. Glossopteris— insights into the architecture and relationships of an iconic Permian Gondwanan plant. Journal of the Botanical Society of Bengal 65, 93–106.
Rees, P.M., 2002. Land plant diversity and the end-Permian mass extinction. Geology 30, 827–830.
Slater, B.J., McLoughlin, S., Hilton, J., 2014. A high-latitude Gondwanan lagerstätte: The Permian permineralised peat biota of the Prince Charles Mountains, Antarctica. Gondwana Research http://dx.doi.org/10.1016/j.gr.2014.01.004.
Anderson, L.I., Trewin, N.H., 2003. An early Devonian arthropod fauna from the Windyfield cherts, Aberdeenshire, Scotland. Palaeontology 46, 467–509.
Benton, M.J., Newell, A.J., 2013. Impacts of global warming on Permo-Triassic terrestrial ecosystems. Gondwana Research. http://dx.doi.org/10.1016/j.gr.2012.12.010. 1180
Benton, M.J., Twitchett, R.J., 2003. How to kill (almost) all life: the end-Permian extinction event. Trends in Ecology and Evolution 18, 358–365.
Cleal, C.J., Thomas, B.A., 1994. Plant Fossils of the British Coal Measures. Dorchester: The Palaeontological Association. ISBN 0901702536
Cleal, C.J., Thomas, B.A., 2009. An Introduction to Plant Fossils. Cambridge: Cambridge University Press. ISBN 978052188715.1
Davies, N.S., Gibling, M.R., 2010. Cambrian to Devonian evolution of alluvial systems: the sedimentological impact of the earliest land plants. Earth Science Reviews 98, 171–200.
Edwards, D., 1996. New insights into early land ecosystems: a glimpse of a lilliputan world. Review of Palaeobotany and Palynology 90, 159–174.
Knoll, A.H., 1984. Patterns of extinction in the fossil record of vascular plants. In: Nitecki, M. (Ed.), Extinctions. University of Chicago Press, Chicago, IL, pp. 21–65.
Hilton, J., Cleal, C.J., 2007. The relationship between Euramerican and Cathaysian tropical floras in the Late Palaeozoic: palaeobiogeographical and palaeogeographical implications. Earth-Science Reviews 85, 85–116.
Labandeira, C.C., 2005. Invasion of the continents: cyanobacterial crusts to tree-inhabiting arthropods. Trends in Ecology and Evolution 20, 253–262.
McLoughlin, S., 2001. The breakup history of Gondwana and its impact on pre-Cenozoic floristic provincialism. Australian Journal of Botany 49, 271–300.
McLoughlin, S., 2011b. Glossopteris— insights into the architecture and relationships of an iconic Permian Gondwanan plant. Journal of the Botanical Society of Bengal 65, 93–106.
Rees, P.M., 2002. Land plant diversity and the end-Permian mass extinction. Geology 30, 827–830.
Slater, B.J., McLoughlin, S., Hilton, J., 2014. A high-latitude Gondwanan lagerstätte: The Permian permineralised peat biota of the Prince Charles Mountains, Antarctica. Gondwana Research http://dx.doi.org/10.1016/j.gr.2014.01.004.