While many of us recognise eucalypts as an emblematic Australian icon, a recent fossil discovery may provide a linkage for the Eucalyptus genus with Patagonia, South America.

The eucalypt fossil record is relatively poor due to a number of factors, including the size of Australia, the small number of people looking for these fossils and the age of suitable sedimentary deposits. Due to this paucity of records, paleobotanists have had difficulty determining the exact age and movement of the genus from fossils, with few published records of fossils of reproductive organs thought to exist within Australia. From these records and other sources, there has been speculation that the genus dates to the Late Cretaceous, some 100.5-66 million years ago (MYA). A recent paper published by researchers from Cornell University and the University of Buenos Aires further speculates that Eucalyptus distribution may have established more recently.

A fossil of Eucalyptus caldericola showing leaf venation and capsules

The paper, published in 2012, introduced an important chapter to the eucalypt story, by providing evidence for the existence of a eucalypt within the Chubut Province of Patagonia (Argentina). The Eucalyptus fossils upon which this discovery is based are around 52 million years old, putting them in the early Eocene Epoch (56- 33.9 MYA). The fossils include leaves and reproductive material that are similar to Eucalyptus, although in some cases also similar to the closely related genus Corymbia.

This fossil material is significant on many levels. Not only does it add precious reproductive material evidence to the scarce fossil record, but it includes the oldest eucalypt macrofossils currently known. They also represent credible evidence of Eucalyptus fossils occurring outside of Australasia and suggest a broader geographic distribution. They do this by showing that eucalypts once inhabited the far western locations within the continents associated with the supercontinent, Gondwanaland.

A fossil of Eucalyptus caldericola showing leaf venation and capsules that are typical of living eucalypt species today. Photo by Elizabeth Hermsen.

While Gondwana began breaking up in the mid-Mesozoic Era (160-80 MYA), South America, Antarctica, and Australia remained connected until more recently. Although these continents have since drifted apart, the palaeontological evidence suggests that organisms were able to cross amongst these continents into the Late Cretaceous and probably the earlier part of the Paleogene Period (66- 23 MYA). This connection was apparently maintained through the Antarctic Peninsula on the South America-Antarctica side of Gondwana. The biotic connection among these three landmasses was eventually broken by them moving away from one another as well as by Antarctic cooling.

Whatever the case, eucalypts form a key part of Australia’s native forests and these new fossils highlight the journey that this remarkable group of trees has made. These fossils also fill an important knowledge gap in the natural history of the environment we rely on everyday.

eucalypt-history

mapping
Estimated extent of Gondwana (the large southern land mass) during the Late Cretaceous Period (100.5-66 MYA) – a time when Eucalyptus may have dispersed to South America from Australia via Gondwana. Imagery from Blakey Paleogeography Mapping.
eucalypt-map
Estimated extent of Gondwana during the Eocene Epoch (56 – 33.9 MYA) – a time when the Patagonian fossils were deposited. The discovery of these fossils assumes that Eucalyptus had successfully reached South America via Gondwana, but for some unknown reason, they went regionally extinct at a later date.

Acknowledgements

Special thanks goes to John Moss for drawing my attention to the article and Elizabeth Hermsen (Assistant Professor at Ohio University) who undertook the original study. For those keen to visualise global paleogeography reconstructions, there are two websites that may help:

• Blakey Paleogeography Mapping – www2.nau.edu/rcb7
• The PALEOMAP Project – www.scotese.com

References

Hermsen EJ, Gandolfo MA, Zamaloa Mdel C (2012) The fossil record of Eucalyptus in Patagonia. American Journal of Botany 99(8): 1356–1374.

Article by Doug Mohr Land for Wildlife member Eden’s Landing, Logan

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