The evolution of Trees is
considered to be a key development in the colonization of land by
life, creating a range of new habittats as well as raising oxygen
levels and stabilizing soils for the first time. The earliest known
fossil forest comes from the late Middle Devonian of Gilboa in New York State, and comprises a
diverse assemblage of trees including Archaeopteridaleans (trees with
woody trunks and leafy branches probably related to living Conifers),
Pseudosporochnaleans (small to medium trees showing some
morphological similarity to living Tree Ferns and Palms), and
Lycopsids (Giant Club Mosses). Lycopsid Trees are known from a variety of other
fossil forest sites, as well as individual floated Trees in marine
sediments through the Late Devonian, and were a major part of the
flora of the Carboniferous Coal forests.
In a paper published in the journal Geology on 24 November 2015, Christopher Berry of the School of Earth and Ocean Sciences at Cardiff University and John Marshall of the National Oceanography Centre at the University of Southampton discuss a fossil Lycopsid forest from the Plantekløfta Formation at Munindalen on Svalbard Island in the Norwegian Arctic.
The Plantekløfta Formation has previously been described as being Latest Devonian in age, but examination of palynological evidence (preserved pollen and spores) by Berry and Marshall leads them to conclude that the deposits in fact date from the earliest Late Devonian, making these deposits only a little younger than those at Gilboa.
The formation is exposed at three localities, each showing numerous Lycopsid stumps preserved in situ, many having extensive root systems as well as variable amounts of upright trunk, with the largest trunks being about 100 mm across. These have previously been described as Archaeosigillaria, but which Berry and Marshall consider to belong to the species Protolepidodendropsis pulchra, which has previously only been described from isolated specimens from marine deposits identified as floating logs.
In-situ Lycopsid fossils. (H) Locality AF3, partial trunk in situ, base sheared by small fault (arrow), showing variation of level of preservation in cortex from oval leaf base parenchyma at base to diamond-shaped leaf bases at top. (I) Locality AF1, upright trunk with slightly flared base. (J,K) Locality AF2 Sandstone cast base removed from loose shale, showing flaring of extreme basal portion and separation of diamond leaf bases by secondary expansion. Scale bars are 50 mm. Berry & Marshall (2015).
Although it is located within the Arctic Circle today, during the Devonian Svalbad was close to the equator, so that the forests of Plantekløfta would have been tropical in nature. The roots of the trees are preserved in a palaeosol (fossil soil), but the upper parts are preserved in a conglomerate; a sedimentary deposit comprising a mixture of rock types, typically associated with landslides, floods or similar catastrophic events. The upper layers of this contain amorphous organic matter, thought to have been formed in an anoxic lake. Berry and Marshall interpret this as being a rapidly subsiding lake basin with aluvial sediments being deposited by a river or river entering from the west and forming a fan or delta.
The trees apparently lived in monospecific stands (stands of a single species of tree), quite unlike the forests of Gilboa, which were diverse in nature with a diverse range of trees living alongside one another. Since the Gilboan forest is slightly older, this cannot imply that the Plantekløfta forest predates the evolution of a wider range of trees, suggesting that this a reflection of the forests ecology. This is contrary to modern expectations, with a diverse tree assemblage in Gilboa, thought to have had a dry temperate climate, but a low diversity forest in the wet tropics at Plantekløfta.
See also...
The first leaves; leafy plant fossils from the Early Devonian of South China. Plant leaves are split into two groups by botanists; microphylls, which
are simple plates of undifferentiated cells, as found in Mosses and
Liverworts, and megaphylls, or true leaves, which have cellular
differentiation and veins; such structures...
Two new species of Moss from the Permian of Brazil. Mosses (Bryophytes) are simple plants which lack vascular systems to
pump water and nutrients from a root system, instead relying on what
they can absorb through their leaves, and generally only reaching a few
cm in height. This means that they are at their...
A Permian forest preserved in volcanic ash. Plants are an important part of all
terrestrial ecosystems on Earth, and are abundant in the fossil record,
but the relationship between plants in ancient environments is often
unclear, since most plant fossils represent disarticulated specimens,
removed from their life...
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