OFR 151.pdf - CRC LEME

1. Palaeontological evidence
Like plant tissues, animal remains may be found ±chemically unaltered, replaced by other
minerals, or occurring as impressions and casts. Two of the more commonly found remains
preserved in terrestrial deposits are mollusc shells and skeletal remains of land vertebrates.
The majority of faunal remains have been transported by water, wind and/or dismembered by
scavengers prior to burial. Taphonomic biases are similar to plant macrofossils in that the
body parts that are most likely to be preserved are the more robust bones and teeth of animals
living by or in streams and lakes. Examples are herbivores, particularly those that grazed in
herds, burrowing rodents, fish, crocodiles, turtles and water birds. Empirical evidence
confirms that carnivores and other life-forms that are rare in living faunas, are equally rare in
the fossil record. Under-utilised sources of proxy-climatic data are vertebrate tracks (Lockey
1998) and insects (cf. Rozefelds 1988, Wilf and Labandeira 1999).
Australian fossil mammal assemblages extend back into Early Cretaceous time, such as at
Lightning Ridge (Archer et al. 1994), and many of these marsupial clades underwent adaptive
radiation following the diversification of flowering plants in the mid Cretaceous and
aridification of the continent in the Late Tertiary. However, only rarely can the faunal
assemblages (Local Faunas) be directly dated using geochronometric techniques, or indirectly
by plant microfossils (Macphail 1996a, 1996b, 1996c). Lateral facies variation, and the
geographically restricted extent of most distinctive strata usually restrict the use of
lithostratigraphy to correlate faunal assemblages with distant, more easily dated rock
formations.
At present, most Tertiary faunas are assigned to Local Faunas, which are correlated using
superposition and stage-of-evolution criteria (Megirian 1994). This approach assumes that
relative position within a given clade is evidence of geological age. Turnover, used to define
the boundaries of these faunal biochrons, occurs mostly at the species level. At present
marsupials, which appear to have evolved rapidly in their morphology and also dispersed
rapidly over large geographic areas, provide the highest resolution. However ‘Mammal
(strictly speaking Marsupial) Ages’ have not been defined for Australia due to the highly
fragmented nature of the faunal record and also because of minimal immigration due to
prolonged isolation of the continent during the Tertiary.
A study of Late Paleocene faunas in Wyoming indicates that climatically-forced changes in
the vegetation can have abrupt and long-lasting effects on the evolution of mammalian
communities (Clyde and Gingerich 1998). Similarly, many aspects of the history and
structure of Australian herbivores are adaptations to the harvesting and consumption of
particular plant groups (Archer et al. 1994). For this reason, it is possible to use anatomical
features such as dentition to infer some aspects of their diet and therefore past vegetation and
climates.
2. Isotopes
Stable isotopes of carbon ( 12 C/ 13 C) and oxygen ( 16 O/ 18 O) are widely used to reconstruct
palaeotemperatures, in particular sea surface temperatures (SSTs), and can be used to infer
palaeosalinities. 16 O/ 18 O (δ 18 O) can be used for terrestrial sediments as well as for any
calcareous fossils. For example Bird and Chivas (1993) have used the oxygen-isotope
composition of clay minerals to develop a broad-brush weathering chronology and
circumscribe the conditions under which deep weathering of the regolith occurred during the
Mesozoic and Tertiary across Australia.
The use of carbon isotopes to reconstruct terrestrial palaeotemperatures depends on the
observation that the three types of photosynthetic pathways found in the higher plants,
designated as C3, C4 and CAM, provide different competitive advantages under different
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