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. 2011;6(6):e21084.
doi: 10.1371/journal.pone.0021084. Epub 2011 Jun 28.

Oldest known Eucalyptus macrofossils are from South America

María A Gandolfo  1 , Elizabeth J Hermsen, María C Zamaloa, Kevin C Nixon, Cynthia C González, Peter Wilf, N Rubén Cúneo, Kirk R Johnson
Affiliations

Oldest known Eucalyptus macrofossils are from South America

María A Gandolfo et al. PLoS One. 2011.

Abstract

The evolutionary history of Eucalyptus and the eucalypts, the larger clade of seven genera including Eucalyptus that today have a natural distribution almost exclusively in Australasia, is poorly documented from the fossil record. Little physical evidence exists bearing on the ancient geographical distributions or morphologies of plants within the clade. Herein, we introduce fossil material of Eucalyptus from the early Eocene (ca. 51.9 Ma) Laguna del Hunco paleoflora of Chubut Province, Argentina; specimens include multiple leaves, infructescences, and dispersed capsules, several flower buds, and a single flower. Morphological similarities that relate the fossils to extant eucalypts include leaf shape, venation, and epidermal oil glands; infructescence structure; valvate capsulate fruits; and operculate flower buds. The presence of a staminophore scar on the fruits links them to Eucalyptus, and the presence of a transverse scar on the flower buds indicates a relationship to Eucalyptus subgenus Symphyomyrtus. Phylogenetic analyses of morphological data alone and combined with aligned sequence data from a prior study including 16 extant eucalypts, one outgroup, and a terminal representing the fossils indicate that the fossils are nested within Eucalyptus. These are the only illustrated Eucalyptus fossils that are definitively Eocene in age, and the only conclusively identified extant or fossil eucalypts naturally occurring outside of Australasia and adjacent Mindanao. Thus, these fossils indicate that the evolution of the eucalypt group is not constrained to a single region. Moreover, they strengthen the taxonomic connections between the Laguna del Hunco paleoflora and extant subtropical and tropical Australasia, one of the three major ecologic-geographic elements of the Laguna del Hunco paleoflora. The age and affinities of the fossils also indicate that Eucalyptus subgenus Symphyomyrtus is older than previously supposed. Paleoecological data indicate that the Patagonian Eucalyptus dominated volcanically disturbed areas adjacent to standing rainforest surrounding an Eocene caldera lake.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Distribution of extant eucalypts after Ladiges et al. (shaded regions in Australasia and adjacent island of Mindanao) and present position of the early Eocene Laguna del Hunco localities, Chubut Province, Argentina (star).
Figure 2
Figure 2. Eucalyptus leaves.
A–B. MPEF-Pb 2329, A. Overall view showing the linear to lanceolate shape, the acute and acuminate apex, and the acute and decurrent base. B. Detail of the venation pattern; note the intramarginal, secondary, and intersecondary veins. C. MPEF-Pb 3729, detail of lamina showing island oil glands and higher order venation pattern. D. E. bridgesiana R.T. Baker, BH 37791, detail of the higher order venation pattern. E. E. camaldulensis Dehnh, detail of the lamina showing the island oil glands. F. E. tereticornis Sm., PBP-1003, overall view of a cleared leaf; note the general morphology of the lamina and the venation pattern. Scale bars: A, F, 1 cm; B–C, 1 mm; D–E, 5 mm.
Figure 3
Figure 3. Eucalyptus flower buds and flower.
A. MPEF-Pb 3735, overall view of flower bud showing corolline operculum and transverse scar left after loss of the calyx. B. MPEF-Pb 3733, flower bud with hemispherical hypanthium and corolline operculum of coherent petals (arrows show petal margins). C. MPEF-Pb 3734, calycine scar (arrow) denotes the separation between the hypanthium and corolline operculum. D. E. microcorys, BH 37596, flower buds and open flower; note the hypanthium, and corolline operculum of coherent petals with visible margins (arrow) on the flower buds. E. MPEF-Pb 3738, bisexual flower with single style and stigma and numerous stamens. Scale bars: A–C, E, 2 mm; D, 3 mm.
Figure 4
Figure 4. Eucalyptus fossil infructescences.
A. MPEF-Pb 3750, infructescence composed of two umbellasters. B. MPEF-Pb 981, capsules in unbellaster. C. MPEF-Pb 3740, infructescence showing the morphology and orientation of the fruit disks. D. MPEF-Pb 3739, infructescence composed of at least three fruits showing valves and disks. E–F. MPEF-Pb 2374. E. Top view of a capsule showing corolline operculum scar (c), staminophore scar (s), valves (v), disk (d), and hypanthium (h). F. Side view of capsule showing the same features. Scale bars: A, B, D, 3 mm; C, 2 mm; E–F, 1 mm.
Figure 5
Figure 5. Strict consensus tree.
Strict consensus tree of five most parsimonious trees (length 381steps, CI 0.66, RI 0.78) found during simultaneous analysis of morphological and molecular sequence data. Terminal names for Corymbia subgenera and sections are after Parra-O. et al. ; the subgeneric terminal name for Eucalyptus is after Brooker . For a full list of exemplar taxa used in the analysis, see Table S1.
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