What is Eucalyptus?
Eucalyptus is the name for a group of plants that includes over 600 species of trees and shrubs in the plant family Myrtaceae, or the Myrtle Family. Today, Eucalyptus is native to Australia and some surrounding islands, including islands in the countries of Papua New Guinea and Indonesia. Its most northerly occurrence is on the Philippine island of Mindanao. Eucalyptus is a very ecologically important group of plants in Australia, and it plays a dominant role in the vegetation of this very dry continent.
Characteristics of Eucalyptus
Eucalyptus plants are usually trees or have tree-like growth forms that can sometimes reach a massive size. Generally, these plants are characterized by sickle-shape leaves with visible oil glands that look like tiny dots on the leaf surface. The plants bear their flowers in groups, or inflorescences, that always include an odd number of flowers (1, 3, 5, 7 and 11). Each flower bud has a cap made up of the perianth (petals + sepals), which is the showy or colorful part of flower in most flowering plants. In Eucalyptus, however, this cap is lost when the flower opens and the stamens (pollen-producing structures) are the colorful, showy parts of flower. The stamens are numerous and are arranged in four bunches or in a continuous ring. The fruit is a woody capsule, a type of dry fruit that opens when mature to release the seeds. The capsule opens by valves. Characteristics of the bark, leaves, flower buds, flowers, and capsules are important for identifying species of Eucalyptus.
Eucalyptus globulus. Tree habit, leaves, closer view of oil glands on leaf, flower losing its perianth cap, flower, capsules.
All photos were taken by J. R. Hendricks at Natural Bridges State Park, California.
More images of extant Eucalyptus can be found at www.plantsystematics.org.
Eucalyptus in Patagonia
Joaquín Frenguelli (1883-1958) was the first to describe a fossil Eucalyptus from Patagonia. In 1953, he published the paper “Restos del género «Eucalyptus» en el Mioceno del Neuquén” (“Remains of the genus Eucalyptus in the Miocene of Neuquén”), in which he illustrated a single fossil of three Eucalyptus-like capsules collected in Neuquén Province, Argentina. This fossil was thought at the time to be Miocene in age, or about 5.3-23 million years old. It may, however, be older.
Following publication of this fossil, little else was reported on possible fossil Eucalyptus from Patagonia until Eucalyptus was discovered in the Laguna del Hunco paleoflora. In 2006 and 2007, Dr. Maria A. Gandolfo and collaborators first presented material thought to represent Eucalyptus from Laguna del Hunco at several scientific conferences. At that time, leaves, infructescences, and individual capsules thought to belong to Eucalyptus had been found associated with one another in the fossil flora.
In 2009, members of the Patagonian paleofloras project revisited some of the Laguna del Hunco localities. At a new fossil quarry, LH-27, they found three Eucalyptus flower buds and one open flower. These new finds provided critical information for understanding the relationships of the Patagonian fossils to living Eucalyptus plants.
In 2011, Drs. Gandolfo, Hermsen, and collaborators first published their findings on the Eucalyptus fossils from Laguna del Hunco paleoflora in the journal PLoS One. These fossils display all the traits typical of Eucalyptus plants. The leaves are sickle-shaped and are covered with small dots representing oil glands. The flower buds are topped by a cap, which, in extant buds, falls off when the flower is open. The fruits are capsules that are very similar in appearance to those of modern Eucalyptus species. Even the arrangement of the capsules in the infructescences is the same.
Following initial publication of the fossils, they were featured in an article for the Chronicle Online (Cornell’s daily news release) by Krishna Ramanujan. They have since been presented by Dr. Gandolfo at the XVIII International Botanical Congress in Melbourne, Australia. Work continues on the taxonomy of the fossils.
Importance of the Laguna del Hunco Eucalyptus
The Eucalyptus of Laguna del Hunco are important for several reasons. One is that the Eucalyptus fossils inform us of the past distribution of Eucalyptus. Prior to this discovery, it was not fully accepted that Eucalyptus may have once occurred naturally in South America. Our team of researchers can now definitively state that Eucalyptus was once native to the continent. It has long been known that South America once had a connection to Australia through Antarctica which has since been severed by continental drift. It is thus not entirely surprising that Eucalyptus was once found in South America. We predict it was once found in Antarctica as well.
These fossils also inform us about the ecology of ancient Eucalyptus. The Laguna del Hunco flora is interpreted as having been deposited in a caldera lake, or a lake that formed in the depression at the top of a volcano. Today, some species of Eucalyptus grow in rainforest habitats on volcanoes. The volcanoes are important to the ecology of the plants because Eucalyptus requires light to germinate and establish itself. Lava flows and other volcano-related activity thus create openings in the forest that Eucalyptus can invade. We believe that the Eucalyptus of Laguna del Hunco relied on openings created by volcanic activity to gain a foothold in its forest habitat, like some living Eucalyptus species.
Finally, this discovery tells us about the evolution of the genus Eucalyptus itself. The fossil flower buds from Laguna del Hunco exhibit an obvious horizontal scar. This scar is important, because it is characteristic of one particular subgroup within Eucalyptus known as Symphyomyrtus. This is the largest group within Eucalyptus at about 450 species. Discovery of these fossils suggests that this group or its ancestors may be older than previously thought.
Fossil Eucalyptus. Leaf, detail of leaf showing oil glands (dots), bud with horizontal scar, flower, grouping of fruits.
Photographs taken by M. A. Gandolfo (images 1,2, 5) and E. J. Hermsen.
Gandolfo, M. A., E. J. Hermsen, M. C. Zamaloa, K. C. Nixon, C. C. González, P. Wilf, N. R. Cúneo, and K. R. Johnson. 2011. Oldest known Eucalyptus macrofossils are from South America. PLoS One 6 (6): e21084. doi:10.1371/journal.pone.0021084.
Hermsen, E. J., M. A. Gandolfo, and M. C. Zamaloa. 2012. The fossil record of Eucalyptus in Patagonia. American Journal of Botany 99 (8): 1356–1374.
Gandolfo, M., C. Gonzalez, M. Zamaloa, N. Cuneo, and P. Wilf. 2006. Eucalyptus (Myrtaceae) macrofossils from the early Eocene of Patagonia, Argentina. Botany 2006, 28 July-2 August 2006, Chico, California, USA.
Gandolfo, M., C. C. Gonzalez, M. C. Zamaloa, N. R. Cuneo, and P. Wilf. 2007. Eucalyptus (Myrtaceae) macrofossils from the early Eocene of Patagonia, Argentina. 5th International Southern Connection Conference, 21-25 January 2007, Adelaide, Australia.
Hermsen, E. J., M. A. Gandolfo, K. R. Johnson, M. C. Zamaloa, K. C. Nixon, P. Wilf, and N. R. Cúneo. 2011. Eucalyptus from the early Eocene of Patagonia, Argentina: phylogenetic, biogeographic, and ecological implications for understanding eucalypt evolution. In Symposium 121: Patterns and Processes of Eucalyptus Evolution. 23rd International Botanical Congress, 23-30 July 2011, Melbourne, Australia, Abstracts: 317.
Selected Press ArticlesHamaker, P. 2011. Oldest Eucalyptus macrofossils found in South America. Examiner.com, June 29, 2011.
Ramanujan, K. 2011. Oldest known Eucalyptus fossils found in South America. Chronicle Online, July 18, 2011.
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Frenguelli, J. 1953. Restos del género «Eucalyptus» en el Mioceno del Neuquén. Universidad Nacional de Eva Perón, Facultad de Ciencias Naturales y Museo, Notas del Museo 16: 209-213.
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