The delayed rise of angiosperm families

Flowering plants, with more than ~290,000 species in 435 families, are the dominant and most important group of terrestrial plants. However, the origin and diversification of flowering plants (angiosperms) has been surrounded by controversy ever since Charles Darwin called this an ‘abominable mystery’ back in 1879.  As far as the fossil record goes, flowering plants suddenly appeared in the Early Cretaceous (~140 million years ago) and almost immediately (by geological standards) started to diversify dramatically, eventually resulting in one of the most transformative revolutions in Earth’s history.  However, the initial burst of diversification of flowering plants did not translate into these being dominant in terrestrial ecosystems. No one knows exactly when this happened or which groups of angiosperms (living or extinct) were involved in the transition to angiosperm-dominated ecosystems as we know today. This is exactly what we wanted to answer in our most recent publication in Nature Ecology and Evolution (Ramírez-Barahona et al. 2020).

In our new study, we document the age of origin of all 435 angiosperm families (that is, families’ stem ages) and explore when did they first started to diversify into their living representatives (that is, families’ crown ages). We painstakingly revised family-level phylogenies (information available at the Angiosperm Phylogeny Website) to identify those species that would represent the ‘crown’ node of every family and assembled the corresponding molecular dataset. With these data we constructed a new complete family-level phylogeny of flowering plants (including 435 families), which was then time calibrated using fossil data. Fossil calibration is the single most critical step in any divergence time analysis, so we wanted to do a good job at documenting fossils and do justice to the fossil record of angiosperms. Thus, we assembled and documented the largest high-quality fossil dataset for angiosperms (238 fossils), making every fossil in our list traceable and open to scrutiny by providing a 384-page-long document (available here).

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In sum, with our new time tree we estimated stem and crown ages for angiosperms families. Our results show that the rise of modern flowering plants (that is, families’ crown ages) was delayed for 37–56 million years after the origin of families (that is, families’ stem ages). It is important to note that, most extant families did not start diversifying until after the end of the dinosaurs (66 million years ago), despite the fact that most of diversity had already evolved by the middle Cretaceous (110 million years ago). To put this into context, this time-lag corresponds to around a third of the entire duration of angiosperm evolution. This implies that angiosperm-dominated ecosystems emerged relatively late during angiosperm evolution.

Finally, we gathered good-quality geographic distribution data from the Global Biodiversity Information Facility: 16 million high-quality geographic occurrence records for 248,606 flowering plant species on a global scale. With these data we wanted to test the idea, which is not novel,  that older families are disproportionately represented in tropical regions. Although the results were unsurprising, these analyses led to the discovery that the rise of modern angiosperm families was delayed for longer in tropical ecosystems than in temperate and arid ecosystems. Overall, these results provide a good example of the decoupling of taxonomic diversification and ecological dominance.

This study adds a key piece to the puzzle of the evolution of terrestrial ecosystems over the last 100 million years.

 

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