Abstract

Tertiary Climatic changes are inferred from changes in the plant micro-fossils described by Couper (1960). Critical forms are: the Nothofagus ‘fusca’ group for temperate conditions, the Nothofagus ‘brassi’ group for warm temperate or equivalent subtropical montane conditions, and some other forms of present day subtropical distribution.

The Basis of Palaeoclimatic Interpretation by Plant Fossils

Many Plants are sensitive climatic indicators and their fossils can give a good idea of past climates. There are several types of plant fossils but for only three has there been much published in New Zealand: leaves; spores and pollens; and fruits.

Leaves are variable and have few diagnostic features related to the taxonomy of living plants, so for most of the Tertiary fossil leaves it is impossible to determine them even to a generic level.

Taxonomic determination is more feasible for spores and pollens than for leaves, and the spores and pollens of the New Zealand Tertiary have been described in considerable detail by Couper (1960), whose work forms the basis of the palaeoclimatic interpretations in this paper.

The small size of spores and pollens gives them an additional advantage over leaves. Fossil spores and pollens are mostly whole, whereas fossil leaves, particularly in New Zealand, are rarely found page 50 whole. This lack of entire leaves is largely the result of the strong deformation of most of New Zealand's Tertiary rocks.

Fruits can often be identified with precision, but in New Zealand they are rare, except in Quaternary deposits.

The usefulness of plants as climatic indicators is offset to some degree by transport before fossilization. Leaves that have been washed from mountains, where the climate is appreciably cooler, have been found in the process of being buried in lake beds some 2,000 feet lower. (McQueen, unpubl). Spores and pollens, being much smaller, are transported further and not necessarily downhill (Moar, 1967), as with leaves.

In interpreting past climates, greater weight must be placed on plant assemblages than on individual species. If a particular plant assemblage is restricted to a particular climatic range today, then the same climatic range is inferred for the past, if the same plant assemblage were to be found as fossils.

Of the forest trees of New Zealand, Nothofagus spp. (southern beeches) are one of the most important elements at present and have probably been equally important for most of the Tertiary. It is fortunate that the three groups within Nothofagus, distinguishable by their pollens, are favoured by different climates.

Nothofagus menziesii grows only in the cooler parts of New Zealand: at sea level in Southland and above 2,000 feet in the Wellington district. The ‘jusca' group of three species with indistinguishable pollens, occupies a broader climatic belt than N. menziesii, but does not grow at sea level north of latitude 39°S. Nothofagus, with pollens referable to the N. ‘fusca' and N. “menziesii’ groups, also grows in South America, Australia and Tasmania, in about the same climatic conditions as in New Zealand.

Nothofagus ‘brassi’ is used here for the pollen group representing the large-leaved species of Nothofagus, now extinct in New Zealand. Species of this group still grow in New Caledonia above 2-3,000 feet (Dawson, 1966) and in New Guinea above 8,000 feet (Brass, 1956). Judged by their distribution in New Caledonia and New Guinea, species of the N. “brassi’ group indicate a climate of constant humidity and one warmer than that occupied by the N. “fusca’ group.

Couper (1960) records pollen of the group N. “brassi’ for the whole length of New Zealand from the middle Eocene until the end of the Pliocene. Species of the Proteaceae were much more numerous in the New Zealand Tertiary than they are today and it may be significant that the great majority of proteaceous species are now found in areas of seasonal rainfall (South and West Australia, Cape Province and Central Africa).

Another group of plants found in New Zealand forests today, and well represented in the Tertiary, are the conifers of the family Podocarpaceae. This family has a distribution embracing that of all three Nothofagus groups, and can not be used with confidence in climatic interpretation, unless pollen of present-day species is found.

The present-day distribution of New Zealand plants is partly controlled by temperature differences related to latitude zones, and these zones probably existed during the Tertiary. One such example is Bombacacidites, which was common in New Zealand, north of latitude 40°S in much of the Tertiary, but is not known to the south. Bombacacidites is identified from its pollen (Couper, 1960) and considered to be related to Bombax, the kapok tree of the tropical zones.

It is almost certain that the distribution of several other species could be shown to be related to latitude if the unpublished lists based on pollens and spores were to be analysed with latitudinal zonation in mind.

Tertiary Climate

Dannevirke Series:

Two pollen species from the Dannevirke Series indicate a climate considerably warmer than at present. Cupanieidites is not uncommon throughout and is probably related to the living genus Cupania which lives in the tropical and subtropical zones, and is not known south of latitude 34°S. Anacolosidites, not uncommon in the early Dannevirke, but absent from the late Dannevirke, is probably related to Anacolosa, a tropical genus that now lives in India.

Nothofagus menziesii appeared in the Mata Series at the end of the Cretaceous and continued in small numbers until the end of the Tertiary and then in much larger numbers in the Pleistocene and Recent.

One species of the Nothofagus “brassi’ group was abundant at the end of the Cretaceous but the group is not known from the Dannevirke, although it reappeared immediately after in Arnold times. The place of the Nothofagus “brassi’ group appears to be taken by podocarps which could indicate humid, subtropical conditions.

Nothofagus species are particularly slowly migrating and the absence of N. “brassi’ groups from the Dannevirke Series is page 52

Fig. 1: Fluctuations in plant fossil content and inferred climates from rocks of Tertiary and early Pleistocene age in New Zealand. All records, except for the leaves in the Hautawan, are of pollen grains from Couper (1960). Climatic zones below are based on Zotov (1938).
(Black triangle represents pollen of Nothofagus “fusca’ group and N. menziesii with leaves of N. menziesii and N. solandri var. cliffortioides.).

probably due to the group having been eliminated from the New Zealand area by climatic or other reasons at the end of the Cretaceous and not having had sufficient time to migrate back.

Conclusions

Pollens of extinct species and particularly of groups of species indicate both the terrestrial climatic fluctuations in the New Zealand Tertiary and the relief of the land masses of those times. The present interpretation is based only on those pollen forms that have been assigned to taxa known in New Zealand and elsewhere. There remains a large proportion of pollen forms not as yet assigned at the generic, or even family level. It is to be hoped that increasing knowledge of the morphology of plants of the Pacific region will, in the future, enable even more similarities to be discovered between floras of the New Zealand Tertiary and those still surviving to the north of New Zealand.

Acknowledgements

The authors wish to thank Drs. W. F. Harris and J. W. Dawson for their help in assembling the material for this publication and Professor H. W. Wellman for his invaluable criticism in editing this manuscript.

Discussion

Mr. I. Devereux. Your pattern of absences in the Dannevirke seems rather similar to that of Dr. Jenkins. He has postulated some page 55 calamitous event to wipe out the foraminifera and then some delay before they repopulated. Could the same sort of explanation be held for your case?

Dr. D. R. McQueen. Possibly, but it does seem a long time delay.

Dr. C. A. Fleming. It there a complete absence of the “brassi’ group or just a lessening of its occurrence?

Dr. D. R. McQueen. From the published results we have examined there is a complete lack of the group.

Professor H. GORDON. Dr. McQueen quite rightly stressed the need to study plants as groups. There are some species that live here today that are outliers of tropical and sub-tropical families and so you would find today quite appreciable pollen from a group that is considered tropical. He also mentioned that the Proteaceae are indicative of a dry climate but there are species living in rather wet climates in Tasmania.

Dr. N. deB. Hornibrook. In the Dannevirke Series there are some very fine sediments so there was presumably very low relief. It is possible that these alpine species retreated to very small pockets until another period of mountain building began? Dr. D. R. McQueen. The question of relief is very important. When the two groups are “mirror images’ of each other everything seems normal but when not a change of relief is suspected.