Origins of Icehouse Vegetation in Southern South America: Climate Change and Turnover in Eocene and Oligocene Patagonian Fossil Floras
Restricted (Penn State Only)
- Author:
- Harris, Gabriella
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 03, 2023
- Committee Members:
- Donald Fisher, Program Head/Chair
Mark Patzkowsky, Major Field Member
Peter Wilf, Chair & Dissertation Advisor
Sarah Ivory, Major Field Member
Jesse Lasky, Outside Unit & Field Member - Keywords:
- Argentina
Patagonia
Río Pichileufú
Laguna del Hunco
fossil leaves
leaf architecture
Araucaria
Austrocedrus
Oligocene
Eocene
climate change
Araucariaceae
Cupressaceae
conifers
EECO
total evidence phylogeny
rainforest
Gondwana
South America
Adobe Bridge
diversity
turnover
mosaic evolution
paleoecology
morphology
vegetation
paleobotany
seasonality
Splendid Isolation - Abstract:
- How did vegetation respond to climatic cooling after the elevated global temperatures of the early Eocene and the tectonic isolation of South America during the final fragmentation of the ancient supercontinent Gondwana? Little is known about the shifts in plant communities for the Eocene to Oligocene period in southern South America (Patagonia), where early Eocene temperate rainforests likely fragmented with South American isolation and declined as seasonality increased. The Río Pichileufú (RP) macroflora in Río Negro, Argentina, is known for its high diversity and is the only well-preserved assemblage in South America that is well-dated to the earliest middle Eocene (47.7 Ma). However, the RP flora has not been comprehensively studied since the 1930’s by E.W. Berry and needs modern revision. I reevaluated the historical material from RP and incorporated substantial new fossil collections from the past 20 years. I developed a keywording method to apply leaf architectural character data to a large digital image library of the fossils in Adobe Bridge. Paired with the exhaustive in-person study of the collections in the USA and Argentina, this approach facilitated the identification of 158 informal species-like entities from 1962 total specimens, referred to as leaf morphotypes, within the RP collections. The results indicate that although Berry overestimated the number of species from the original collection (131 species, which I reduce here to 82), the total site diversity remains high when also considering the new collections. Previous comparisons with the early Eocene (52 Ma) Laguna del Hunco (LH) flora from the same lithologic unit, which is well sampled and known for very high plant diversity, suggested the same flora was present at both sites. Here, I found that the RP flora was more diverse and showed high compositional turnover, with only 11% of RP species shared with LH. Despite global and regional evidence of cooling, the leaf size at RP was 27% smaller than at LH, and species with entire margins increased (46.6% vs. 61.3%), suggesting that the climate was drier at RP but still warm. Furthermore, the lower abundance or extinction of several podocarp conifers, with well-demonstrated physiological drought vulnerability in living analogs, supports the conclusion that the moisture availability at RP was more seasonal than at LH. Finally, I looked at the earliest true icehouse vegetation in the same area by describing new conifer species from Prem, a new earliest Oligocene (33.5 Ma) site just 1.5 km from the RP site. I describe leafy sprays and attached ovulate cones of the first South American fossils of Austrocedrus (Cupressaceae) and report mosaic evolution of a stem lineage of Araucaria Section Araucaria (Araucariaceae) from isolated leaves and ovuliferous complexes that retain wings, unlike the wingless living species. These results are significant because the living Ciprés de la Cordillera, Austrocedrus chilensis, and the Pehuén, Araucaria araucana, inhabit the region today. Combined with the dominance of Nothofagus at the Prem site, these findings provide the first evidence for the emergence of the present-day southern Andean forest biome at the precise time of the earliest Oligocene (Oi-1) glaciation.