Wednesday, 22 February 2012

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 positions. Fossil forests are well documented in many parts of the world, and some give excellent records of the trees that formed them, but most of these show poor preservation, and are made up of either trunks or stumps without evidence of the rest of the plants.

In a paper published in the Proceedings of the National Academy of Sciences on 21 February 2012, a team of scientists lead by Jun Wang of the State Key Laboratory of Palaeobiology and Stratigraphy at Nanjing Institute of Geology and Palaeontology describe the preservation of an area of Early Permian forest, roughly 298 million years old, preserved largely intact by volcanic ash fall; what the researchers refer to as a vegetational Pompeii. The site covers over 1000 m² in the Wuda Coal Field in Inner Mongolia.

An artists impression of part of the Wuda Forest. The tall, slim trees on the right are Sigillaria, which is related to modern Club Mosses. The taller trees on the left are Cordaites, an early form of Conifer. The lower story comprises Marattialean Tree Ferns and Noeggerathiales, a spore bearing plant of uncertain affinities. The vine is a form of Sphenopteris, a seed fern (plants with foliage resembling ferns that produced seeds; not actually closely related to ferns). There is a herbaceous layer, mostly of the fern Nemejcopteris fermaeformis, and Sphenophyllum, a relative of modern Horsetails. From Wang et al. (2012).

The Wuda Forrest was made up of several distinct layers of vegetation, as with modern forests.

The tallest trees were the Lycopsid Sigillaria and the primitive conifer Cordaites, both of which could reach heights of 25 m. These seem to have occurred as individual trees standing above the canopy, rather than as a true layer of the forest.

Leaves of Sigillaria. Scale bar 1 cm. From Wang et al. (2012) Supplementary material.

Stem/trunk of Sigillaria. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Strobilius (soft cone) of Sigillaria. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Leaves of Cordaites. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Reproductive structure of Cordaites. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

The main tree canopy was at a height of 10-15 m, and was made up largely of Marattialean Tree Ferns, with some early Cycads and Noeggerathiales, early vascular plants of uncertain affinities. A vine of the genus Sphenopteris grew in this canopy. This was a seed fern, a plant with leaves resembling a fern, but which produced seeds, and which was not closely related to ferns.

Leaves (a & c) and (b & d) sporangia of Pecopteris, a Marattialean Tree Fern. Scale bar 2 mm in (a & c), 500 μm in (b & d). From Wang et al. (2012) Supplementary material.

Leaves of Pecopteris, a Marattialean Tree Fern. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Leaves of Pecopteris, a Marattialean Tree Fern. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Leaves of Pecopteris, a Marattialean Tree Fern. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Leaves of Pecopteris, a Marattialean Tree Fern. Scale bar 3 cm. From Wang et al. (2012) Supplementary material.

Noeggerathiales. (a-d) Tingia unita: (a) a crown with strobili and once pinnate compound leaves attached to the stem, (b) isolated strobilus, (c) leaf with only large pinnules exposed, and (d) leaf with both large and small pinnules exposed; (e-h) Paratingia wudensis: (e) a crown with strobili and once pinnate compound leaves attached to the stem, (f) leaf with only large pinnules exposed, and (g) with small pinnules exposed after degagement, (h) a number of leaves likely attached to a common stem; (i and j) Paratingia sp.: (i) a crown with strobili and once pinnate compound leaves attached to the stem, (j) a leaf with both large and small pinnules visible. (Scale bars, 3 cm in a and h; 1 cm b–d; 2 cm e–j.) From Wang et al. (2012) Supplementary material.


Seeds of the Cycad Samaropsis. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

The vine Sigillaria. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

Finally there was a herbaceous ground cover layer, made up of the fern Nemejcopteris fermaeformis, and the Sphenopsids Sphenophyllum and Asterophyllites.

The Sphenopsid (Horsetail) Asterophyllites. Scale bar 2 cm. From Wang et al. (2012) Supplementary material.

At the time when the Wuda Forest was growing it would have been located on the Northwest of the North China Block, a small continental landmass in the tropical Paleotethys Ocean, unattached to any other land. The flora of Wuda, and other East Asian Permian plant assemblages, resemble, but are not identical too, the flora of Europe and North America, and are distinct from Gondwanan floras of the same period. Carboniferous forests, which are better studied and understood than Permian forests, are not thought to have the distinct floral layering seen in Wuda (and indeed modern forests) suggesting this was a Permian development.

An artists impression of part of the Wuda Forest, at the far end to the top illustration. From Wang et al. (2012).

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