PLANT FOSSILS – PALEOBOTANY
Plant fossils are classified into two groups: (1) macrofossils, which are the larger plant members, such as stems and leaves, usually detached and often fragmented; and (2) microfossils, which are the shells or resistant outer walls of minute plants, such as diatoms, or of pollen grains and spores. Fruits and seeds vary considerably in size and overlap each group.
For several reasons pollen and spores have a particular importance in present-day geological research in New Zealand. (1) Being produced in large numbers, readily dispersed, and highly resistant, they are more often preserved than macrofossils. (2) They occur in sediments of terrestrial origin where no marine fossils are available, often in sufficient numbers for a comparison to be made of the relative abundance of different types. (3) Since they are also deposited in coastal waters and may be found associated with marine organisms they help to “date” sedimentary rocks in relation to marine fossil sequences.
Evidence of Climatic Change
Much of the evidence of climatic change depends on the former occurrence of lowland trees north or south of their present limits, the number of degrees in latitude giving some indication of the magnitude of the climatic difference. Wood and resin, indistinguishable from that of the living kauri, occur in Tertiary lignites in Otago. These show that kauri forests once grew much further south than their present range, and indicate a warmer climate in the Tertiary. Kauri pollen occurs in South Island Upper Miocene to Pliocene deposits, and well-preserved leaves of Oligocene age have been found at Landslip Hill, so that the southern extension of the group to which the kauri belongs is shown by these different fossils – wood, resin, leaves, and pollen. Cone scales from Shag Point (Upper Cretaceous) and Mokoia and Waikawa (Jurassic) show also that the kauri family has had a long history in New Zealand.
Seasonal variation in climate may be shown by differences in growth rings. These are clearly shown in Upper Jurassic fossil woods from Waikawa, Curio Bay, and Waikato Heads. Coniferous wood, which seems to have predominated in the formation of the Tertiary lignites of the South Island, also shows growth rings. (The Tertiary fossil woods, as well as spores and leaf cuticles, are described by W. P. Evans in the N.Z. Journal of Science and Technology, Vol. 9–19, 1928–37.)
Apart from numerous, but small and mostly unclassifiable fragments of Mid-Devonian to Carboniferous age, Paleozoic plant life is represented in New Zealand by plant fossils from the Gore district. The fossils identified are of Permian age and link New Zealand with the supposed ancient southern continent, Gondwanaland, and in particular with Queensland through the occurrence there and in New Zealand of Cladophlebis roylei Arb. (a fern-like plant), Sphenopteris lobifolia Morris (seed-fern), and Neoggerathiopsis hislopii (Bunbury) (a cone-bearing plant).
Links With Modern Ferns
One of the oldest plant microfossils so far named from New Zealand deposits is Osmundacidites wellmani Couper. This Jurassic spore is considered to be related to the living family Osmundaceae, which is represented in New Zealand by three species of Todea. The apparent link between this ancient spore and the modern ferns is supported by other evidence. Fossil stems known as Osmunda dunlopi Kidston and Gwynne-Vaughan, from Waikawa and Kawhia (Jurassic) are evidently closely related. These are of interest on account of their anatomical similarity to modern forms, and they show the extreme slowness with which the family structure has been modified. According to Sinnott (Annals of Botany, 1914) “there is perhaps no other case among vascular plants where there has been so little change from Mesozoic time to the present”. This stability over a period of more than 135 million years is well exemplified by our living Todea barbara (L.) Moore, not only in stem anatomy but also in frond and spore type.
Cladophlebis australis (Morris) is presumed also to be the foliage of an osmundaceous fern. Hence there is a plausible linkage between micro - and macrofossils which has a bearing on the ancestry of certain present-day ferns. The history of the Osmundaceae in New Zealand is extended by the presence of Cladophlebis roylei among the Permian plant fossils mentioned above. Of similar antiquity is Cyathidites, a fern spore which may be ancestral to the modern tree-fern genus Cyathea, the spores of which are common in Quaternary deposits. The black tree fern Cyathea medullaris (Forst. f.) Swartz, the finest of our present-day tree-ferns, appears to be a comparative newcomer in this ancient lineage, since the spores have not been found in deposits older than Upper Miocene, though common in younger deposits.
Character of Earlier Forests
Branchlets from Cretaceous and Eocene beds in the South Island were identified as belonging to the genus Casuarina, which contains the modern sheoaks of Australia and Polynesia. Also believed to represent this genus, and first appearing in the Upper Cretaceous, is Triorites harrisii Couper, a pollen type which is common in most Tertiary assemblages, but became extinct with the onset of the Pleistocene glaciations. The presence of Casuarina suggests that a forest of more open type than today's dense rain forest was prevalent in the warmer epochs of the Tertiary. Important groups in New Zealand Tertiary vegetation were beeches, conifers, and casuarinas.
The oldest dated deposits with the remains of angiosperms (flowering plants with enclosed seeds) are the Mid-Cretaceous beds in the Clarence valley, near Coverham. The pollen grains are of an unspecialised type, but with the three furrows which distinguish them as dicotyledonous. Of the three more specialised pollen types of the southern beeches, two appear during the Cretaceous and the third in the early Tertiary. The first to appear, and the one which became dominant during the Tertiary, represents a section of the genus Nothofagus with living species in New Guinea and New Caledonia, but now extinct in New Zealand. Fossil Fagus (European beech) pollen has not been identified in New Zealand as yet and, as it is of an unspecialised type, confident identification is hardly to be expected. Thus the linkage of the southern beeches, genus Nothofagus, with the Northern Hemisphere beeches, genus Fagus, remains obscure. As a possible “missing link” may be mentioned the much discussed leaf fossils known as Artocarpidium arberi Laurent of Upper Cretaceous age from Pakawau. Oliver, in an unpublished manuscript, points out the resemblance of the venation to that of Fagus sylvatica and proposes a new genus for them, Protofagus. Oliver also suggests an affinity of the New Zealand leaf fossil Nothofagus ulmifolia Ett. (Cretaceous) with the South American beeches, in particular N. procera. Most of the fossil southern beeches had larger leaves than the living New Zealand species. The leaf fossil Nothofagus ninnisiana Unger from the Upper Cretaceous (Shag Point) and Lower Tertiary (Waikato and Ohai) is a broad-leaf type and fossil wood, believed to be that of large-leaved beeches, was described by Evans in the articles mentioned above. In the fossil record, as compared with the four or five species now living, 23 species have been named from macrofossils and 12 or 13 from microfossils, indicating the variety and importance of the southern beeches in the former vegetation of New Zealand and the closeness of present-day New Zealand to the centre of evolution and dispersal within this group.
Fruits and Seeds
Fossil fruits and seeds as yet yield a meagre record. At Coopers Beach, Mangonui, and at one or two other localities along the east coast, fossil coconuts of probable Miocene age have been found. These were named Cocos zealandica by Berry. The coconut fruits may have been brought by oceanic drift, but in favour of a more local origin are the facts that New Zealand may have consisted at the time of an archipelago of small islands, and that large palm fronds have been found at the same and other localities. Berry also referred to another fruit resembling that of a species of the Australian Hakea. Hakea-like pollen is known from the Eocene of New Zealand. Seeds are more commonly preserved in Quaternary sediments and may make a valuable contribution to our knowledge of the Quaternary history of the flora. In 1953 D. R. McQueen described an assemblage of fossil seeds from the Rangitikei Valley (Quaternary), of which 24 belonged to living species. A concise summary of the history of the flora of New Zealand was published by W. R. B. Oliver in a Swedish journal (Svensk Botanisk Tidskrift, 1955), and Oliver also contributed a similar article to Tuatara (1950). A useful work on the conditions under which the New Zealand flora evolved is New Zealand Biogeography, by C. A. Fleming, in Tuatara (1962). In the Transactions of the Royal Society of New Zealand, Vol. 83, there are papers by Shona Bell, H. J. Harrington and I. C. McKellar, and D. R. McQueen, and these give further useful references to the literature on New Zealand plant fossils. For plant micro-fossils the reader is referred to N.Z. Geological Survey Paleontological Bulletin 32 (1960), by R. A. Couper.
by William Francis Harris, D.SC., New Zealand Geological Survey, Department of Scientific and Industrial Research, Lower Hutt.