The birth of the modern world:understanding the evolutionary

The transition between the Jurassic (201-145 million years ago) and Cretaceous (145-66 million years ago) represents a pivotal moment in the history of life on Earth. The rise of flowering plants (angiosperms) around 130 million years ago drastically changed terrestrial ecosystems, potentially leading to the rise in diversity of insects, mammals, and other animal groups. This led to the birth of our modern world. However, the causes and context for the rise of flowering plants are still hotly debated and mostly unknown. On the one hand, the rapid climatic changes during that period could have created new condition for the flowering plants to flourish. On the other, the decline of the pre-existing vegetation might have left some empty ecological possibilities that the flowering plant could then exploit. Another view sees the flowering plants outcompeting their naked seed (gymnosperm) relatives, thanks to their novel advantageous traits such as fruits or more efficient leaves. Moreover, these abiotic and biotic factors might have interacted in complex ways to lead to these outcomes. Our project plans to understand the context of the rise of the angiosperms by looking at the dynamics of the plants that were dominant before and during their period of diversification and rise, namely the gymnosperm groups cycads, conifers, and Gnetales. These groups are too often misunderstood as living fossils, with a long, static evolutionary history that saw them declining because of the competition of the more advanced flowering plants. Is this really the case? We plan to reconstruct the evolutionary history of these three groups to identify the relationships of fossil and extant plants. We will leverage the availability of DNA sequences from extant species and a careful investigation of the morphology of extant and fossil plants to generate phylogenetic trees using state-of the art statistical methodologies that integrate the uncertainty over ages and diversification histories. This is vital when dealing with the vagaries of the fossil record. These trees will then help us to understand how the rates of speciation and extinction through time, as well as the patterns of morphological diversification, changed across the three gymnosperm groups during the Jurassic-Cretaceous transition. The shared patterns across these groups, as well as the more idiosyncratic ones, will be compared with the timing and scale of the climatic oscillations of the period, as well as with the increase of angiosperm diversity starting in the Early Cretaceous. This will help us understand the causes of this major worldwide floristic transformation, and help us to point towards possible challenges to the global flora caused by current and future climatic changes.

The transition between the Jurassic (201-145 million years ago) and the Cretaceous (145-66 million years ago) is a crucial moment in the history of life. The emergence of flowering plants (angiosperms) about 130 million years ago drastically changed terrestrial ecosystems and led to an increase in the diversity of insects, mammals and other animal groups. This was the birth of our modern world. However, the causes of the emergence of flowering plants are still largely unknown. On the one hand, the rapid climatic changes during this period may have created new conditions for the spread of flowering plants; on the other hand, the decline of previously existing vegetation may have left ecological spaces that flowering plants could exploit. According to another theory, flowering plants displaced gymnosperms thanks to their new, better properties, such as fruits or more efficient leaves. Furthermore, these abiotic and biotic factors may have interacted in complex ways. In our project, we tried to understand the context of the rise of angiosperms by looking at the evolutionary dynamics of the gymnosperm groups that were supposedly dominant during the rise of the flowering plant, and were displaced by these superior competitors. We first looked at the Gnetales, a group that supposedly radiated during the same timeframe as the angiosperms. A review of their fossil record supported this view, though many unknowns about the dynamics of their evolution remain. We then focused on the cycads, a group thought to be dominant during the Mesozoic. The diversity of this group was supposedly diminished by competition with angiosperms, leading to a depauperate living flora. Our results completely overturned this classical view: by integrating the fossil record with DNA evidence, we were able to show that extant cycads expanded both in terms of species diversity and geographical span during the Jurassic-Cretaceous transition, and that the morphological diversity of cycad leaves was not impacted by the rise and origin of the angiosperms, but rather expanded up to the present. This suggests that the origin of the angiosperms was part of a wider evolutionary radiation across all the seed plants. Moreover, we found evidence that some of the superior traits of the angiosperms were potentially unlinked to their success in the Cretaceous. We realized that a fossil leaf from the Triassic of Greenland, thought to be a relative of flowering plants, was rather part of a group of 'seed ferns' that went extinct at the end of the Triassic. This further suggests that a simple view on the trigger of the angiosperm radiation is unsubstantiated, and that a new perspective on this event in Earth's history is needed.