Australia

Introduction

Late Jurassic deposits in Tasmania and Queensland Australia contain specimens of gymnosperms and ferns, which include the arborescent seed fern genus Donponoxylon and various representatives of Osmundaceae (Dernbach, 1996). Many of the Osmundaceae specimens in these areas are found as cobblestones. The ferns grew and died in Jurassic aged deposits. Later, they were eroded, transported, and weathered in Eocene aged rivers.

Osmundaceae, the Royal Fern family, makes its appearance in the Permian. Sixteen living species are recognized along with nearly a hundred fossil forms (Miller, 1971). Osmundaceae is the best-represented family of ferns in the fossil record and is known from foliage, stems, roots and reproductive structures. The family diversified and was widespread during the Mesozic era, but decreased in numbers and geographic range during the Tertiary (Tidwell, 2002). The ferns in this family have rhizomes that grow upright and produce closely spaced fronds (leaves). The stem is composed of persistent leaf bases and rootlets (Tidwell, 1998). It is the wonderful pattern of leaf traces, petiole cross-sections and rootlets surrounding a central stele (pith, xylem and phloem) in the permineralized stem, which attracts the interest of the fossil wood collector. The typical anatomy of the osmundaceous stem in cross-section starts with a centralized pith surrounded by a circle of horseshoe-shaped xylem strands. Phloem tissue is just outside the central xylem and may be just inside as well in some species. A mantle of material surrounds this central stele that is composed of leaf shoots that contain c-shaped xylem strands. As one moves outwards from the center a ring of supportive tissue surrounds the C-shaped xylem, denoting cross-sections of fronds petioles. Some of these petioles are outlined with stipular wings.

Mr. B.P. Srivastava first described Pentoxylon, found in Jurassic deposits of India in 1935, (Dernbach, 1996). Srivastava was a promising young paleobotanist who had the work of describing the Pentoxyleae family entrusted to him. Tragically, he died before he could complete this important work. Birbal Sahni, Drs. A.R. Rao and R.V Sitholey revised Srivastava’s unfinished manuscript, notes, and illustrations, so that the posthumous paper of 1945 could be published in Srivastava’s own name (Halle, ). The name reflects the structure of the stem as can be seen in cross-section. The wood is not made of a single mass, but of 5 or more petal-shaped masses each with their own growth zones. The Pentoxylon out of India is never bigger than a couple of inches. Paleobotanists and collectors have noted that pentoxylon-like wood can be found in the Jurassic deposits of Queensland Australia. Petal-shaped vascular segments make up the center of the stem as in Pentoxylon. As one moves out from the center the vascular segments seem to multiple outwards creating a wave-like pattern (Daniels and Dayvault, 2006). Many collectors speculated that this Pentoxylon-like wood was an arborescent seed fern, which has been confirmed by recent work.

Donponoxylon is a genus of large, arborescent seed ferns known from Middle to Late Jurassic deposits of Australia and New Zealand. Donponoxylon stems in cross-section consist of vascular segments often surrounding a pith. The vascular segments are oval, wedge or pear-shaped. The segments are variable in number and exhibit secondary growth that is mostly centrifugal (towards the exterior). The genus includes two species, Donponoxylon jacksonii and Donponoxylon bennettii. D. bennettii exhibits peripheral secondary segments that may form ring-like patterns, which D. jacksonii lacks. The central segments of D. jacksonii are larger and more regularly arranged around the pith than D. bennettii. The permineralized axes of Donponoxylon are not associated with any leaf or reproductive structures. Broader relationships with other seed ferns are unknown. Thus the new genus and species are classified as an incertae sedis spermatophyte or a seed fern of uncertain placement. Donponoxylon grew in moist, temperate to cool climates along the Eastern margin of Gondwana. An ocean to continental convergent boundary produced volcanic mountain arcs. Material from these volcanic moutains buried and preserved Jurassic forests in floodplain deposits (Tidwell, Britt & Wright, 2013, p. 49).

From the fossil and geologic evidence we may surmise that many forested areas of the Jurassic had arborescent gymnosperms and tree ferns as a lower canopy with large fern families, such as Osmundaceae, making up the ground cover. These gymnosperm/fern forests grew in humid areas (Kenrick and Davis, 2004).

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Bibliography

Daniels, F.J. and Dayvault, R.D. (2006). Ancient Forests: A Closer Look at Fossil Wood. Western Colorado Publishing Company: Canada.

Dernbach, U. and Tidwell, W.D. (2002). Secrets of Petrified Plants: Fascination from Millions of Years. D’ORO: Germany.

Dernbach, U. (1996). Petrified Forests: The World’s 31 Most Beautiful Petrified Forests. D’ORO: Germany.

Halle, T.G. Professor Sahni’s Paleobotanical Work. Paleobotanist, 1:23-41. See http://www.bsip.res.in/Founder/contribution-%20Fossil%20plants.htm

Kenrick P. and Davis, P. (2004). Fossil Plants. Smithsonian Books: Washington.

Miller, C.N.Jr. (1971). Evolution of the Fern Family Osmundaceae Based on Anatomical Studies. Contributions From the Museum of Paleontology The University of Michigan, Vol 23, No.8, pg 105-169.

Sahni, B. (1948). The Pentoxyleae: a new group of Jurassic gymnosperms from the Rajmahal Hills of India. Botanical Gazette 110: 47-80.

Tidwell, W.D. (1998). Common Fossil Plants of Western North America. [2nd Edition]. Smithsonian Institution Press: Washington, pgs 214-215.

Tidwell, W.D. (2002). The Osmundaceae: A Very Ancient Group of Ferns. In Dernbach, U. & Tidwell, W.D. Secrets of Petrified Plants: Fascination from Millions of Years (pp. 135-147). Germany: D’ORO Publishers.

Tidwell, W.D., Britt, B.B. & Wright, W. (2013). Donponoxylon gen. nov., a new spermatophyte axis from the Middle to Late Jurassic of Australia and New Zealand. Review of Palaeobotany and Palynology. 196, pp. 36-50.

Birbal Sahni Institute: http://www.bsip.res.in/