Sixty-five million years ago the Central American Seaway separated South America from North America, but the Drake Passage was not yet open. Plant populations were likely continuous across southern South America and Antarctica - the remains of super-continent Gondwana. Shallow seas reached far inland to central Patagonia. The Southern Andes were still young, with many more mountain-building phases ahead. They did not cast a rain-shadow over Patagonia the way the mountains do today, but they had already created basins to the east where sediments and fossils could accumulate. Southern South America was covered in wet, warm forests that extended to Antarctica and across to Australia. The region may have served as a refuge for many plant species during the extinction event kicked off by the Chicxulub impactor in Mexico. Fossil leaves and their insect damage from these forests indicate that biodiversity was high in Southern South America compared to similar latitudes in North America, despite the greater land area in the north.
Plant fossils are abundant in the lower Paleocene Salamanca Formation in Chubut, Argentina, but one feature that really makes the Salamanca Formation exceptional is the preservation of delicate fossilized flowers. Fossilized flowers are often especially useful to paleontologists because they preserve suites of features that permit high confidence in their identification.
In a recent paper published in PLOS ONE, we described fossil flowers and leaves that belong to the family Rhamnaceae, which includes buckthorns and jujube. Apart from dispersed pollen grains that have been attributed to the family, this is the southern-most fossil occurrence of the Rhamnaceae. They are globally distributed and species diversity is highest in Mediterranean climates. However, fossil evidence and the relationships among the living species indicate that the association with Mediterranean environments is a recent development. These fossils are associated with megathermal to warm-temperate climates and pre-date the diversification of the family in Mediterranean habitats.
Through careful examination of the features preserved in several fossils we showed that the new genus, Notiantha, is most similar to Ziziphus, Sarcomphalus, and several other living genera of the family that belong to the ziziphoid group within the Rhamnaceae. The associated fossil leaves are also similar to those living members of the group that includes Ziziphus and Sarcomphalus, the tribe Paliureae.
These plants were among those that grew in the early Paleocene forests of Southern South America, along with palms, legumes, Akaniaceae, Lauraceae, a variety of southern conifers, and several others currently under examination. By comparing the early Paleocene Salamanca flora with that preserved in the uppermost Cretaceous rocks, we can piece together a list of southern survivors and determine which kinds of plants made it through one of earth's greatest mass extinction events.