Taxonomy & Evolution

“Long fuses” delayed angiosperms’ global take-over

The time it took to accumulate key adaptations meant delays of tens of millions of years.

Angiosperms – the flowering plants – are phenomenally diverse as well as being ecologically and structurally the basis for the majority of terrestrial ecosystems. The earliest angiosperm fossils appear in the Early Cretaceous (~133-125 mya), the last major group of land plants to appear. By the Middle Cretaceous (~115-100 mya), a burst of diversification had produced most of today’s extant lineages. However, the dominance of angiosperms in terrestrial biomes didn’t come about until the Palaeocene (66-56 mya). Why the large delay? And did the transition happen the same way in different environments?

Photo by James Wheeler from Pexels

In a recent article published by Nature Ecology & Evolution, lead author Santiago Ramírez-Barahona and colleagues explored these questions using a complete, dated, family-level phylogeny that integrated 16 million globe-spanning geographic occurrence records. The phylogeny sampled 1209 taxa in all 435 currently recognized angiosperm families, with 85% of living angiosperms (248,606 species) captured by the occurrence records.

The trees showed that the main angiosperm lineages diverged rapidly through the late Jurassic and early Cretaceous. However, there was a significant time lag – referred to as a “long phylogenetic fuse” – between the first divergence of angiosperms from their sister group (the stem node), and when the group began to diversify in earnest (the crown node). Though the angiosperms were both diverse and widespread at the family level by the Middle Cretaceous, they didn’t diversify into the species richness of today until the Palaeocene.

Though they vary in length from one family to the next, long phylogenetic fuses are a consistent feature across the angiosperms. These fuse lengths are not random, but form geographic patterns according to biome, with tropical biomes producing longer average fuse lengths than temperate and arid biomes, in which diversity arises later, but more quickly. What’s more, families with longer fuses tended to have lower extant species richness. The five largest angiosperm families (Orchidaceae, Asteraceae, Fabaceae, Poaceae and Rubiaceae) mostly have fuses of less than 25 million years. “[W]e estimated that 20–39% of non-monotypic families (74–147 families) have phylogenetic fuses spanning less than 25 Myr, but collectively these families represent 53–75% of the total standing diversity of flowering plants,” write the authors.

“We hypothesize that the temporal decoupling between angiosperm family origins and their crown diversification, and its geographical heterogeneity, has been partially driven by the acquisition of morphological, reproductive and ecological innovations across angiosperm families, leading to the eventual increase in diversification during the Cenozoic. The prevailing long family phylogenetic fuses may correspond to the time necessary for the accumulation of [key phenotypic characteristics], and which may be associated with the subsequent increases in diversification of extant species in many of them.”

Overall, both the geographic and phylogenetic evidence support an “out-of-the-tropics” model of angiosperm diversification in which the ancestral climate was a warm humid one, followed by eventual adaptation to suit temperate and arid biomes.

You can read this paper via ReadCube at https://rdcu.be/b5JjY

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