Australian Fossil Cycads

From Pacsoa
Jump to: navigation, search
Welcome to the PACSOA Palms and Cycads wiki !

If you have any information about this topic, please help by updating this article. Once you are registered you can contribute, change, or correct the text, and even add photos on this page. Click on the edit tab above and play around. Any mistake can be easily corrected, so don't be afraid.

Contents

Introduction:

Cycads are often described by botanists and horticulturists alike as living fossils, and after the release of films such as Jurassic Park and its sequel, potted seedling cycads were even sold in major plant nurseries labelled as dinosaur food. This reputation is based in large part on the long fossil record of the cycads and their relatives. The Daintree rainforest, in north east Queensland, was granted World Heritage status in part because these rainforests contained a rich representation of cycads and other living fossils. The earliest known cycads are known from Upper Carboniferous rocks (305-286 million years ago), although cycads reached their greatest diversity during the Jurassic Period (208-144 mya). Cycads are often confused with palms, but cycads are more closely related to conifers than they are to palms or other flowering plants.

The most common macroscopic cycad fossils found are leaves (pollen, being microscopic, is less visible but far more common as a fossil). Cycad cones and trunks are quite rare as fossils, although spectacular fossil trunks are known. This poses problems for palaeobotanists as superficial comparisons can be confusing as many quite unrelated plants can have similar appearing leaves (e.g. palms and cycads!). Cycad leaf fossils are often identified by comparing the shapes and arrangements of the cells of the epidermis of the leaf, based on the waxy coating on all leaves, the cuticle. The leaf cuticle is like a plant fingerprint, and every plant family, and most genera and species have a unique set of characteristics. All modern cycads have haplocheilic stomata which are typically irregularly arranged, and the epidermal cells are not in well-delineated rows. A final complication, however, is that many more species are extinct (and only known as fossils) than are alive today. This means that from time to time a fossil leaf may be recognised (mistakenly) as being from a cycad, only later to be found to have come from a plant that combined cycad-like leaves with quite un-cycad like reproductive structures. For this reason most cycad fossils are grouped as cycadophytes. The cycadophytes include the true cycads (Cycadales) and the extinct cycadeoids (Bennettitales).

Australia has a relatively diverse cycad flora today, with both the family Zamiaceae (Bowenia , Lepidozamia and Macrozamia) and Cycadaceae (Cycas) represented, and so you might expect a rich and diverse fossil record for these ancient plants. Oddly, this is not the case. The oldest known cycad fossils from Australia are from Late Permian (255-245 mya) to Triassic Period rocks (245-208 mya), particularly from the Sydney region. [paleogeog] One Triassic species, Cycadopteris scolopendrina, is known from a small collection made in a brick pit in the 1880s at St Peters in Sydney, and has not been collected anywhere else. More common are cycad leaves given the name Taeniopteris , which translates from the Latin as tapeworm leaf, a reference to the long narrow leaves which presumably reminded the author of the name of the tapeworm Taenia! Very large leaves of this type (up to 1 metre in length) are often given the name Macrotaeniopteris. The anatomy of these leaves is well known, with typical cycad cuticle characteristics, but little is known of their seed and pollen cones, so exact relationships are uncertain.

Cycadophytes are perhaps best known from rocks of the Jurassic and Cretaceous Periods (144-65 mya), the time periods during which dinosaurs came to dominate the world. [paleogeog] Cycads were important components of world vegetation during these times, and cycadophyte cuticle has been found in fossilised dinosaur dung, and so cycad leaves likely formed part of the diet of herbivorous dinosaurs. Jurassic and Cretaceous cycadophytes include many groups now extinct, including the cycadeioids or Bennettitalean cycadophytes. Typical cycadophyte foliage from Australian Jurassic and Cretaceous rocks include the Bennettitaleans Otozamites and Pterophyllum, and the cycads Ptilophyllum, Taeniopteris and Zamites. Bennettitalean cycadophytes are thought to have had flower like structures (named Williamsonia as a fossil) borne on their squat large trunks, rather than the typical seed and pollen cones of true cycads. One Jurassic leaf found in Australia, Taeniopteris spatulata, was once considered a cycad (or at least a cycadophyte), but is now known to be part of a quite different plant, the slender-stemmed Pentoxylon, a representative of an extinct group of plants, the seed ferns. Jurassic and Cretaceous cycadophytes are best known from Chinchilla in Queensland, Koonwarra in Victoria, and Bauhinia Downs in the Northern Territory. These cycads grew in forests rich in conifers (including the Wollemi pine) and maidenhair trees (Ginkgo), and were likely understory plants. Early flowering plants (i.e. angiosperms) would have grown alongside cycads in the understory of the Cretaceous forests of Victoria and Queensland.

By the Tertiary (65-2 mya) cycads can be differentiated on the basis of their foliage into Cycas -type (i.e. pinnate leaves with the pinnae continuous with the rachis) and zamioid types (i.e. pinnate leaves with pinnae narrowing at the point of attachment to the rachis). Australian Tertiary cycads are only known from leaf fragments, or rarely whole fronds, and all represent the family Zamiaceae. The lack of Cycadaceae fossils may indicate that these cycads only entered Australia from south-east Asia relatively recently, however it is more likely that as most Cycas species grow in drier forests (often subject to fire), and their fronds generally shrivel prior to being shed that their leaves have not been preserved as fossils. The known fossils of Zamiaceae closely resembles their living relatives, and so it is highly likely that they were quite modern in appearance. During the Tertiary Australia broke free from Antarctica and began its northward drift towards south-east Asia and drier climates. For much of the early Tertiary (65-21 mya) Australia experienced a wet climate over much of the continent, but experienced warm and cold periods. Rainforests clothed much of the continent, but drier climates in the interior were seeing the evolution of the more typical modern day Australian vegetation.

Leaflets of Bowenia have been described from several Eocene (57-36 mya) sites in southern Australia, including Anglesea in Victoria, sites near Adelaide, southern New South Wales, and on the west coast of Tasmania. All of these fossil leaves have serrate margins, and so these plants may have been quite similar in appearance to the living species, B. serrulata, which occurs in tropical rainforests in northern Queensland today. As all living species of Bowenia have underground stems, it seems reasonable that the Eocene species did also. In all likelihood, fronds of Bowenia sprouted from the ground as part of the undergrowth in the Eocene rainforests of southern Australia, in much the same way as they do today in tropical rainforests in Queensland today.

Leaf fragments of Lepidozamia (most similar to Lepidozamia peroffskyana) have been described from Eocene rocks in southern New South Wales, and the Miocene (23-5 mya) brown coal seams at Bacchus Marsh (most similar to Lepidozamia hopei) and the LaTrobe Valley, in Victoria. The living species, L. hopei , grows to a great height, towering above the undergrowth and even breaching the canopy in some situations. While pure speculation, it is possible that the rainforests and swamps from which the Bacchus Marsh coals formed contained giant Lepidozamia trees amongst the kauri pines (Agathis species) and other rainforest plants. The extinct cycad, Pterostoma, is known from several sites in southern Australia, and also from New Zealand which today has no native cycads.

Pterostoma is thought to belong in the Zamiaceae, showing some similarities to Dioon, and is even considered by some authors to be synonymous with Macrozamia. Robert Hill of the University of Tasmania points to the presence of an abcission zone at the base of the frond, suggesting that this plant may have been winter deciduous, a trait found in very few living species of cycad. This cycad likely had a trunk (from which the fronds were shed), and either was deciduous in response to winter drought (as found in Cycas armstrongii in the Northern Territory), or more strangely, as suggested by Hill, may have been deciduous in response to short days in winter when southern Australia lay much closer to the Antarctic Circle in the Eocene and earlier.

Macrozamia leaves have been described from Cethana in Tasmania from Oligocene rocks (36-23 mya). Many living Macrozamia species (e.g. budawang) grow as understory plants in eucalypt forests. The Cethana forest contains a mixture of rainforest plants and the type of sclerophyllous (hard leaved) plants typical of the dry country eucalypt forests of eastern Australia.

The mainland fossil sites where the fossil Bowenia, Lepidozamia and Pterostoma are known, appear to have been formed from ancient rainforests containing the types of flowering trees, conifers, shrubs, and ferns found today in the dense species rich tropical rainforests in the Daintree of north eastern Queensland. These ancient rainforests were dominated by trees of the avocado family (Lauraceae), as they are today, and included ancient rainforest relatives of Banksia, Grevillea, and other members of the Proteaceae, kurrajongs and flame trees (Elaeocarpus species and Brachychiton species), lillypilly (Syzygium species), coachwood and relatives (Cunoniaceae), illawarra plumpine (Podocarpus species), kauri pine (Agathis species), and the climbing fern Lygodium. All of these plants are today only found together in the rainforests of the Daintree region. Interestingly, the extinct cycad, Pterostoma has always been found in association with Gymnostoma, an ancient rainforest relative of the she oaks (Casuarina species). Gymnostoma is an attractive tree shaped like a candelabra and today is found solely in Australia along rivers draining off the slopes of Thornton Peak, in the Daintree rainforest. Perhaps a specimen of the enigmatic extinct cycad, Ptersostoma, will one day be found in the Daintree.

Descriptions of some fossil cycad leaf types:

Pterostoma - Pinnate leaves with the pinnae lanceolate and narrowing towards the point of attachment to the rachis. Occasional anastomoses (junctions between adjacent veins) occur. Stomata are scattered with random orientation in broad bands between veins, with the guard cells sunken in a pit. Quite characteristic are the strongly sinuous epidermal cell walls.

Ptilophyllum - Pinnate leaves with the pinnae attached to the upper surface of the rachis with variable angles of attachment. The points of attachment to the rachis are rounded and occasionally overlap.

Taeniopteris - Simple undivided leaves, 3-30 cm in length with entire to delicately toothed margins, with secondary veins arising at near right angles from the midrib.

Zamites - Pinnate leaves with linear or lanceolate pinnae attached to the upper surface of the rachis. The pinnae bases are greatly constricted, with parallel or slightly divergent veins.


Examples of Australian fossil cycads and cycadeoids:

Species

Age

Localities

References

Dunedoonia reticulata

Permian Dunedoo, NSW Holmes (1977)

Cycadopteris scolopendrina

Triassic St Peters, NSW White (1986)

Otozamites bechei

Jurassic to Cretaceous Yarragadee, WA; Bauhinia Downs, NT White (1986)

Otozamites bengalensis

Jurassic to Cretaceous Bauhinia Downs, NT White (1986)

Pterophyllum fissum

Cretaceous Bauhinia Downs, NT White (1986)

Taeniopteris daintreei

Cretaceous Koonwarra, VIC Drinnan & Chambers (1986)

Bowenia eocenica

Eocene Anglesea, VIC Hill (1978)

Bowenia papillosa

Eocene Nerriga, NSW Hill (1978)

Pterostoma anastomosans

Eocene Nerriga, NSW Hill (1980)

Pterostoma hirsutus

Eocene Regatta Point, TAS Hill & Pole (1994)

Pterostoma zamioides

Eocene Anglesea, VIC Hill (1980)

Lepidozamia foveolata

Eocene Nerriga, NSW Hill (1980)

Macrozamia australis

Oligocene Cethana, TAS Carpenter (1991)

Lepidozamia hopeites

Miocene Bacchus Marsh, VIC Cookson (1953); Johnson (1959)


Sources of information on fossil cycads:

Carpenter, R.J. 1991. Macrozamia from the Early Tertiary of Tasmania and a Study of the Cuticles of Extant Species. Australian Systematic Botany v. 4, pp. 433-444.

Cookson, I.C. 1953. On Macrozamia hopeites an Early Tertiary cycad from Australia. Phytomorphology v. 3, pp. 306-312.

Douglas, J.G. 1969. The Mesozoic Floras of Victoria, Part 1. Geological Survey of Victoria Memoir v. 28.

Drinnan, A.N. and Chambers. T.C. 1986. Flora of the Lower Cretaceous Koonwarra Fossil Bed (Korumburra Group), South Gippsland, Victoria. Memoirs of the Association of Australasian Palaeontologists v. 3, pp. 1-77.

Hill, R.S. 1978. Two new species of Bowenia Hook. ex Hook. f. from the Eocene of Eastern Australia. Australian Journal of Botany v. 26, pp. 837 - 846.

Hill, R.S. 1980. Three new Eocene cycads from Eastern Australia. Australian Journal of Botany v. 28, pp. 105- 122.

Hill, R.S. 1998. The fossil record of cycads in Australia. Flora of Australia v. x, 539-544. AGPS, Melbourne.

Hill, R.S. and Pole, M.S. 1994. Two new species of Pterostoma R.S. Hill from Cenozoic sediments in Australia. Review of Palaeobotany and Palynology v. 80, pp. 123-130.

Holmes, W.B.K. 1977. A pinnate leaf with reticulate venation from the Permian of New South Wales. Proceedings of the Linnean Society of New South Wales v. 102, pp. 52-57.

Johnson, L.A.S. 1959. The families of cycads and the Zamiaceae of Australia. Proceedings of the Linnean Society of New South Wales v. 84, pp. 64-117.

Taylor, T.N. and Taylor, E.L. 1993. The Biology and Evolution of Fossil Plants. Prentice Hall.

White, M.E. 1986. The Greening of Gondwana. Reed Books.


Contributed by:

David R. Greenwood,
Senior Lecturer in Ecology and Palaeoecology at Victoria University of Technology, Melbourne.

External Links:

VCE, North American and European fossil cycads

Google, Google Images, PACSOA Forums