Jurassic Vegetation of Australia

Australia: The Land Where Time Began

Jurassic Vegetation of Australia

The Jurassic vegetation of Australia was predominantly made up of abundant southern conifers including Kauri Pines (Agathis), Monkey-puzzles, similar to Hoop, Bunya and Norfolk Island Pines. There were also Podocarps resembling living species of Podocarp. It is believed that the common Pentoxylon Cycadophytes of the Jurassic may be ancestral to the modern Screw Pine (Pandanus), a monocot Angiosperm. A group of seed-ferns were present that are thought may possibly be ancestral to other flowering plants. A rich flora from this time occurs in the Talbragar Fish Bed deposits in western New South Wales.

At some places in Queensland there are refuge areas where the Jurassic vegetation is almost unchanged, dominated by conifer forests with species such as Araucaria.

Outcrops of Jurassic rocks are often beneath younger rocks (Kear et al., 2011)¹.

Plant macrofossils of the Jurassic²

In Australia, though there are Jurassic macrofloras that are widely distributed that have been poorly studied. There are 3 major modification of the Jurassic floras of Australasia that have been recognised based on the palynological record that is more extensive. The first of these appeared some time after the beginning of the period; the second is from the Toarcian; and the third in the early part of the Late Jurassic (McKellar, 1996 in press) Grant-Mackie et al., 2000).

In Australia there was a relatively abrupt decline of the Dicroidium flora, that was corystosperm dominated, at the transition from the Triassic to the Jurassic, the rise of conifer and bennettite dominated floras characterising the Jurassic. In the eastern Clarence-Moreton Basin, and possibly also the Nullarbor Basin, the Hettangian strata display a transitional palynoflora indicating relationships significantly closer to the Alisporites/Falcisporites Microflora (The palynological representation of the Dicroidium Flora) than to the subsequent palynofloras of the Sinemurian-Pliensbachian-earliest Toarcian) rich in Classopollis. In the mid-Early Jurassic the dominance of the Classopollis in the palynofloras suggests the cheirolepidiacean conifers were a significant component of the vegetation, though so far no microfossils of this group have been found in Australian deposits of this age that can be identified with confidence. The Clarence-Moreton and Nambour basins are where the best-described of the floras of the Early Jurassic that are generally associated with the later part of the epoch have been found (e,g, Gould, 1968,1971; Rozenfelds & Sobbe, 1983; Grant-Mackie et al., 2000; Pattemore & Rigby, 2005; Jansson et al., 2008b). Among these assemblages from the Pleinsbachian-Toarcian age there is a range of lycophytes, equisetaleans, osmundaceous dipteridaceous dickinsoniaceae, and matoniaceous ferns, the pteridosperms Komploteris and Rintoulia, caytonialeans [Sagenopteris and Caytonia], bennettitaleans (Otozamites spp.), putative gingkoaleans, small-leafed conifers (Plagiophyllum and Allocladus), and Palissya fructifications.

At Ida Bay and Lune River in Tasmania the fossil floras include a wide array of conifers that have been permineralised exquisitely. There are bennettites, and especially osmundaceous and cyathealean ferns (Gould, 1972; Tidwell, 1987, 1992; Tidwell & Jones, 1987; Tidwell et al., 1987; Tidwell et al., 1989; Tidwell et al., 1991; White, 1990, 1991; Tidwell & Pigg, 1993). Radiometric dating has shown that these assemblages have an age of 175 ± 6.4 Ma (Toarcian-Bajocian), the age of basalts associated with the sediments (Hergt et al., 1989; Bromfield et al., 2007), though they have traditionally been ascribed to the Late Jurassic.

Through a short phase during the late Early Jurassic floras of Australasia and Gondwana were generally modified. This has allowed the plant assemblages of this time to used as indicators of palaeoclimate (McKellar, 1996; Hallam, 1996). At this time the most notable change was the decline of cheirolepidiacean conifer pollen and an increasing relative abundance of araucarian and podocarp conifers. In the sedimentary record of several basins in eastern Australia at this time of transition there are 1 or more beds of oolitic ironstone (e.g. Grant-Mackie et al., 2000) suggesting there was a significant perturbation of the terrestrial environment (Cranfield et al., 1994). The authors² suggest it is significant that these terrestrial changes were roughly coeval with the global sea level rise that peaked at about this time, early Toarcian (see above and McKellar in press), and the Tethys Ocean anoxic event that occurred in the early Toarcian.

The Walloon Coal Measures are believed to be of Bajocian-Bathonian age, as are their equivalents in the Clarence-Moreton, Mulgildie and Surat basins based on macrofloral affinity with the flora in the Clent Hills Group in New Zealand (Grant-Mackie et al., 2000), or based on evidence from palynology, of Bathonian-Callovian age. The latest Temaikan marine succession at Awakino (North Island, New Zealand; de Jersey & Raine,2002), that contain the oldest record of Contignisporites glebulentus, are encompassed by the latter, and also the oldest records of Murospora florida, a species found in the Temaikan of New Caledonia (Grant-Mackie et al., 2000), though it is not been reported in New Zealand. Collectively, these occurrences of Contignisporites glebulentus and Murospora florida indicate Callovian age in terms of international stage divisions. Both species first appear in the order cited part way through the succession in the Walloon Coal Measures, the Surat Basin, though it has been suggested that the range of C. glebulentus could possibly extend to just above the formation base (McKellar, in press). These species are not present in the formation in the southern Queensland portion of the adjacent Clarence-Moreton Basin, the authors² suggesting the formation may be somewhat older (Bathonian). The unconformity between the Middle and Late Jurassic marks the top of the succession in eastern Australia.

A typical Middle Jurassic flora is present in the coal measures, that includes liverworts, selaginellalean lycophytes, equisetaleans, osmundaceous, marattiaceous, matoniaceous, dicksoniaceous and dipteridaceous ferns and araucarian, podocarpacean, and palissyacean conifer [Araucarites sp., Mataia sp., Allocladus sp., Elatocladus spp., Pagiophyllum sp., Brachyphyllum crassum, Masculostrobus sp., Palissya ovalis, bennettitaleans (Ptilophyllum pecten, Otozamites feistmanteli, Williamsonia sp.), Pentoxylaleans (Taenopteris spatulata, as well as some gymnosperms of unknown affinity (Pachypteris and Pachydermophyllum spp., (Tenison-Woods, 1882, 1883; Walkom, 1917a, 1917b, 1919; Day, 1964; Hill eet al., 1966; Townrow 1967a, 1967b, Gould 1968, 1975, 1980; Grant-Mackie et al., 2000). Gingkoaleans have not been found in the Australian Middle Jurassic, though they are abundant in most parts of the middle to high latitudes in both hemispheres at this time, the suggestion being that the Australian climate was too warm for this group to proliferate in lowland settings, though they did in the Late Triassic and Early Jurassic and Early Cretaceous of the region (McKellar, 1996).

In the Walloon Coal Measures there is a rich macroflora that is included in the Otozamites-Ptilophyllum Flora concept (Gould & Shibaoka, 1980), though in some of the assemblages are bennettitaleans abundant (McLoughlin & Drinnan, 1995). A mesothermal climate is generally favoured by large dipteridaceous ferns and araucarian conifers with large leaves.

Volcanic sediments from the Middle Jurassic of eastern Australia commonly contain gymnosperm wood that has been permineralised, though this fossils wood hasn't been studied extensively. A taxonomic revision of the global wood records from the Jurassic-Cretaceous has been carried out (Bamford & Philippe, 2001), and it has been suggested that as a result of this revision this fossil wood may be more may be used more in terrestrial biostratigraphy and palaeogeography (Philippe et al., 2004). At Miles in the Surat Basin in situ stumps have been found but at the time of writing remain unstudied. Nearby deposits have produced a range of osmundaceous and dicksoniaceous fern axes that have been permineralised that have been described (Tidwell & Rozenfelds, 1990, 1991; Tidwell & Clifford, 1995). The climate is inferred to have been seasonal by the presence of pronounced growth rings in conifer and the abundant matted bennettitalean-pentoxylalean leaf horizons, some that plants in the community were deciduous. It has been suggested that the cause of the formation of these growth rings will probably be difficult, as different environmental factors can result in these features, and can be found at low altitudes at low latitudes (Fritts, 1976).

In the Araucariacean Phase of the Middle Jurassic floras are similar in all localities in Australasia, as well as correlating with assemblages from Argentina, and comparable assemblages have been found in peninsula India, Madagascar and Antarctica (Grant-Mackie et al., 2000). The biostratigraphy of the Southern Hemisphere has been calibrated by U-Pb zircon ages which has allowed fresh insights into the age of plants from the Botany Bay Group, Antarctica. An absolute age of 168.9 ± 1.3 My - 167.1 ± 1.1 Ma (Bajocian to early Bathonian GTS 2004) for reference flora (Hunter et al., 2005) that had been previously assigned to the Early Jurassic (Gee, 1898; Rees, 1993; Rees & Cleal, 2004).

Plant microfossils from the Jurassic are poorly known in Australia (Young & Laurie, 1996; Hill et al., 1996). They are broadly similar to those from the Early and Middle Jurassic, though they are known from different parts of the continent, representing conifers, bennettitaleans, pentoxylaleans, pteridosperms, ferns, and equisetaleans. A rich impression flora, traditionally assigned to the early Late Jurassic or early Middle Jurassic, based on evidence from palynology (Morgan,1984) has been found in the Talbragar Fish Beds in southern part of the Surat Basin, western New South Wales. A Late Jurassic age (latest Oxfordian-Kimmeridgian-Tithonian) for these beds has been suggested by radiometric dating. Included among the flora, that has been most thoroughly studied by Walkom 1921b; White, 1981a), are conifers [Podzamites jurassica, ?Rissikia talbragarensis, "Brachyphyllum sp.", "Pagiophyllum peregrinum", Elatocladus australis, Allocladus cribbii, Allocladus milneanus, Elatocladus australis [?]], Pentoxylaleans (Taeniopteris, Carnoconites, and Sahnia spp., ?cycadophytes (Nilssonia compta), some enigmatic seed ferns (Rintoulia sp.) and fragments of ferns. The fossils of Podozamites jurassica from the Talbragar Fish Beds have been compared to the foliage of the extant wollemi pine (Wollemia nobilis) in a number of reports, which would suggest a lineage for the wollemi pine that was of great antiquity. It has been pointed out that the venation pattern of P. jurassica is not a close match, nor are the leaf shape or phyllotaxy of any extant araucariacean genera, including Wollemia (McLoughlin and Vajda, 2005). The Late Cretaceous is the most distant time that Wollemia can be traced with convincing evidence.

Jurassic age fossil sites where assemblages of small fossil plants have been found include the Carpentaria Basin, far northern Queensland (e.g. Gould, 1975), Tarlton Range, Eromanga Basin, Northern Territory, Gould, 1978) and the northern Perth Basin (e.g. Walcom1921a; Glauert, 1923; McLoughlin & Hill, 1996; McLoughlin & Pott, 2009) have also been considered to be of Late Jurassic age. Among these floras are putative lycophytes, dipteridacean, ?gleicheniacean, osmundacean and ?matoniacean ferns, bennettitaleans, pentoxylaleans, and small-leafed conifers (Pagiophyllum and Elatocladus spp.). It has been suggested they bear some resemblance to Neocomian floras of southeastern Australia, in terms of their generic composition and conifers with typically small leaves (McLoughlin et al., 2002), though similarities to floras from the Middle Jurassic of India, Patagonia and the Antarctic peninsula have been suggested based on other features such as the medium-leafed bennettitaleans (McLoughlin & Pott, 2009). In the Battle Camp Formation of the Laura Basin, north Queensland Pachypteris leaves of Late Jurassic to Early Cretaceous age are notable for the presence of lepidopteron leaf mines, among the oldest of the known phytopathology in the Australian fossil record (Rozenfelds, 1988). It has been suggested that there is great scope for further systematic studies of fossil floras, as well as their application to non-marine stratigraphy, because there is a near-complete record of continental sedimentation during the Jurassic in several basins of eastern Australia.

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