OFR 151.pdf - CRC LEME

6.4.2 Palaeobotany
Cenomanian floras represent the early stages in the transition from gymnosperm- to
angiosperm-dominated terrestrial ecosystems. For example, Austral Conifer Forest
communities lacked sphenopsids and some of the gingkophyte species found in the
understorey of Early Cretaceous forests and woodlands. Since most, if not all, early
angiosperms were subcanopy shrubs or herbs or grew in low or open vegetation types such as
heath, it is unlikely that angiosperms greatly altered the physiognomy of the forests, or
indicate any fundamental shift in canopy-climate relationships. Niche disruption at ground
level and altered conditions at the forest margins, however, may have enhanced rates of
ecological speciation (cf. Brown 1999).
The palaeobotanical record for the Cenomanian is strongly biased towards coastal and
riparian taxa whose miospores were transported by wind or water into offshore sediments.
Two distinct types of vegetation are represented: (1) coastal heath communities dominated by
ferns and increasing numbers of angiosperms, and (2) Austral Conifer Forest dominated by
pteridosperms, araucarians and/or podocarps. The latter communities are presumed to have
formed the regional/upland/hinterland vegetation.
6.4.3 Palaeoclimates
The composition and structure of the vegetation in palaeo-southern (Tasmania to southeastern
New South Wales) and palaeo-central Australia are likely to reflect winter darkness as well as
strong seasonal contrast in temperature and possibly precipitation. Low photoperiods are less
likely to have influenced plant growth in palaeo-northern Australia (North West Shelf).
Palaeo-northern Australia
Norvick and Burger (1975) have proposed that climates along the palaeo-northern margin
(including present-day north-west Australia) were warm and possibly dry. The latter is
inconsistent with the high diversity of cryptogams, including schizaeaceous ferns, although
very warm (upper mesotherm) temperatures are supported by the microfossil data. For
example, araucarians were prominent in Austral Conifer Forest. Re-examination of Norvick
and Burger’s (1975) data indicates humidity was overall high (perhumid) although it is
unclear whether there was a pronounced dry season. The presence of palms (and virtual
absence of Sphagnum) hint that mean annual temperatures were in the upper mesotherm range
in present-day north-west Australia.
Palaeo-central Australia
Gymnosperm and cryptogam communities occupying central areas of the Eromanga Basin
appear to be floristically impoverished compared to communities in eastern areas of this basin
or on the palaeo-northern margin: Austral Conifer Forest continued to dominate elevated
regions in the hinterland. Conditions appear to have been humid-perhumid and cool-cold
(microtherm range) and seasonal contrasts were higher than in regions closer to the coast.
Palaeo-southern Australia
In spite of the considerable difference in latitude (~20 0 ), plant communities on the palaeosouthwestern
margin of the continent are similar to those found on the palaeo-northern
margin, except that palms are not recorded. Climates appear to have been as humid but cooler
(upper microtherm to possibly lower mesotherm), apart from present-day south-west Western
Australia where subhumid conditions may have prevailed.
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