Journal Number 112
May 2009
ORIGINAL PAPERS
Earliest Fossil Dendrobium and Earina from Early Miocene NZ
John G. Conran -
Australian Centre for Evolutionary Biology and Biodiversity, Benham Bldg DP312,
School of Earth and Environmental Sciences, University of Adelaide, SA 5005 Australia.
Jennifer M. Bannister -
Department of Botany, University of Otago, PO Box 56, Dunedin, New Zealand.
Daphne E. Lee -
Department of Geology, University of Otago, PO Box 56, Dunedin, New Zealand.
The orchids are the largest family of flowering plants with some 25,000 species, but paradoxically, virtually no fossil record [1]. There are several ambiguous vegetative impressions from the Eocene [2], and an alleged Miocene fossil flower impression [3], but the only unequivocal organic fossil is pollinia preserved on a bee in amber from the Early to Middle Miocene Caribbean [4].
Molecular phylogenies suggest divergence of Orchidaceae from the Asparagalean "lilies" between the Early and Late Cretaceous (c. 110-80 Ma) [4, 5], but the lack of good fossils makes accurate dating difficult.
Several monocot families have been recorded as macrofossils from New Zealand [6], but there are no orchid fossils reported from the Southern Hemisphere.
Early Miocene plant fossils from maar lake sediments near Middlemarch, South Island indicate a diverse subtropical rainforest [7], containing numerous ferns, conifers, angiosperm leaves, flowers, fruits, seeds and including leaves of at least 10 different monocots. Some of these leaves were found to represent two new species of extant New Zealand orchid genera from subfamily Epidendroideae: Earina fouldenensis and Dendrobium winikaphyllum, the latter so named because it is apparently close to the sometimes segregated genus Winika [8].
This makes them the oldest definite organically-preserved orchid fossils, the first records for the largely epiphytic subfamily Epidendroideae, as well as the first fossil orchids for the Southern Hemisphere.
The new orchid fossils are close matches on leaf form and cuticle anatomy to their modern NZ and Australasian relatives in Dendrobium (Fig.17) and Earina (Fig.18). In particular, the shape of the stomata and presence of 2celled papillae on the D. winikaphyllum help to place it into the Australasian clade (lineage) within the genus.
Similarly, the very distinctive raised ring of cells surrounding the stomata in the E. fouldenensis places it closest to that genus. The leaf of E. fouldenensis is wider than either living New Zealand species and has more secondary veins, but the width is well within the size ranges for the Pacific Island Earina species [9].
The specimens were collected from diatomaceous sediments deposited originally in a deep maar lake created by explosive volcanic activity in the Early Miocene (23-20 Ma) [10].
The ecology of extant New Zealand Dendrobium and Earina species makes them ideal candidates for preservation in such an environment. Both orchids are abundant in a wide range of habitats, including as epiphytes on exposed branch ends and lithophytes around freshwater lakes. Their leaves have thick cuticles, disarticulate readily from the stems at senescence, and occur in modern lake bed sediments some distance from the shore, indicating the ability to survive transport and become fossilised.
The endemic New Zealand D. cunninghamii (=Winika cunninghamii sits at the base of the Australasian Dendrobium alliance [11]. The presence of Earina and a Winika-like Dendrobium in Early Miocene New Zealand indicates early radiation by these genera into the now largely submerged subcontinent of Zealandia [12] and helps to place the diversification of the crown Epidendroideae at least as early as the Middle Eocene c. 40 Ma.
When the extant distributions of Earina and the members of Dendrobium related to D. cunninghamii are examined, all are found to be more or less restricted to former continental fragments of Zealandia or Australia, with local endemism in New Caledonia, Vanuatu and New Zealand, although E. valida also extends to oceanically-derived Samoa [9] although unpublished evidence suggests that Earina is a New Zealand endemic, with the Pacific Island species a separate genus [13].
The restriction of most of the Earina complex (which is an early segregate from the large and diverse Vandeae / Cymbideae lineage), as well as the basally divergent Australasian Dendrobium lineages (=Winika and Cannaeorchis) to New Zealand, New Caledonia and continentally-derived Pacific Islands indicates an early radiation into Zealandia prior to this subcontinent becoming largely submerged during the middle Cenozoic [12].
Nevertheless, the presence of a diverse and complex subtropical rainforest with epiphytic orchids surrounding a freshwater lake in Early Miocene Otago supports the assertion that there was land in the New Zealand region throughout the Cenozoic [14], with theories arguing for post-drowning recolonization by New Zealand endemic taxa [15, 16] being much less plausible.
The position of Earina (as part of Agrostophyllinae) relative to Vandeae / Cymbideae and Epidendreae, as well as the relationships within and between Dendrobium lineages are still unresolved making their origins and biogeography areas for future study.
There is also a need to determine the relationship between New Zealand and Pacific Earina, as well as between the relatives of D. cunninghamii, and exploration of their apparently low rates of dispersal away from continentally-derived land masses. Nevertheless, the presence of Dendrobium and Earina in Early Miocene New Zealand and their present distributions suggest early diversification and radiation, leading to remnant distribution patterns stemming from the subsidence of previously more extensive land masses.
Acknowledgements
The landowners, Mr N. and Mrs Y. McRae, and Dr A. Walker of Featherston Resources are thanked for kindly allowing us access to the site.
The Departments of Geology and Botany, University of Otago and the School of Earth and Environmental Sciences, The University of Adelaide are thanked for resources to undertake this research.
Funds for this study were provided by an Otago Research Grant and a Fleming Award from the Royal Society of New Zealand.
Jon Lindqvist, Ian Raine and Liz Kennedy provided valuable field assistance.
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