EOCENE FOSSILS OF ARAUCARIA SECT. EUTACTA FROM PATAGONIA AND THEIR IMPLICATIONS FOR FLORAL TURNOVER DURING THE INITIAL ISOLATION OF SOUTH AMERICA
Open Access
- Author:
- Harris, Gabriella
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 04, 2019
- Committee Members:
- Peter Daniel Wilf, Thesis Advisor/Co-Advisor
Mark E Patzkowsky, Committee Member
Elizabeth Ann Hajek, Committee Member - Keywords:
- Araucaria
Araucariaceae
conifers
EECO
Eocene
Laguna del Hunco
Patagonia
rainforest
Río Pichileufú
total evidence phylogeny - Abstract:
- Premise of the study — The iconic conifer genus Araucaria, found worldwide during the Mesozoic, now has a relict, disjunct distribution between South America (two species) and Australasia (18 species). Australasian Araucaria Section Eutacta is the most diverse clade with 16 species, all but two of them endemic to New Caledonia. Fossils with affinities to Sect. Eutacta, with first appearances in the Jurassic, usually are represented by single dispersed organs, making it difficult to diagnose the section and test the generally post-Gondwanan (~20-25 Ma) molecular estimates of its crown age. Araucaria fossils thought to belong to Sect. Eutacta are abundant in early and middle Eocene Argentine caldera-lake deposits from Laguna del Hunco (~52.2 Ma) and Río Pichileufú (~47.7 Ma). The Laguna del Hunco flora was deposited during the Early Eocene Climatic Optimum, and the Río Pichileufú flora is from the earliest middle Eocene, deposited during the initial opening of the Drake Passage and start of climatic cooling. Araucaria pichileufensis Berry 1938 from Río Pichileufú was described as a member of Sect. Eutacta and later reported from Laguna del Hunco. Although there is increasing evidence of angiosperm species turnover between these floras, apparently related to decreasing rainfall, the diverse conifers found at Laguna del Hunco and Río Pichileufú are thought to represent the same set of species. However, the relationship of A. pichileufensis to Sect. Eutacta and the conspecificity of the Araucaria material among these floras have not been tested using modern methods. Methods — We describe the type material of A. pichileufensis alongside large (n = 192) new fossil collections of Araucaria from Laguna del Hunco and Río Pichileufú, including the multi-organ preservation of Araucaria leafy branches, cuticle, ovuliferous complexes, and pollen cones with in-situ pollen. We use a total evidence phylogenetic analysis to test for the nearest living relatives of the fossils. Key Results — We improve the whole-plant concept for Araucaria pichileufensis at Río Pichileufú and recognize a new Araucaria species from Laguna del Hunco. Taxonomic analysis of characters, including those of an attached terminal pollen cone discovered from Río Pichileufú, establishes a relationship of both species to Sect. Eutacta. Araucaria pichileufensis resolves in the stem of Sect. Eutacta, and A. new species resolves in the crown of Sect. Eutacta, within the New Caledonian clade. The phylogenetic placement of both Eocene species within Sect. Eutacta confirms the taxonomic treatment, indicates the presence and survival of this group in Patagonia during initial separation from Antarctica, and adds to the Gondwanan connection of Patagonian fossil floras to Australasia. Conclusions — The Araucaria fossils described here comprise one of the most complete representations of fossil Eutacta in the world, and they predate the molecular age estimates for the crown of the clade by ~30 million years. The differentiation of two Araucaria species is the first direct evidence of a change in the abundant conifer species between Laguna del Hunco and Río Pichileufú, adding to the signal of turnover between the two floras during the climate change and movement of landmasses that occurred from the Early Eocene Climatic Optimum to the earliest middle Eocene.