Gunnera
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Gunnera is a genus of herbaceous flowering plants, some of them gigantic. The genus is the only member of the family Gunneraceae.
Evolution
Gunnera is thought to be a rather ancient group, with a well-documented fossil history due to the presence of fossilized pollen spores, known by the palynotaxon Tricolpites reticulatus. It is a Gondwanan lineage, having originated in South America during the Cretaceous. The earliest fossilized pollen is known from the Late Cretaceous (Turonian) of Peru, about 90 million years ago, and within the following 10 million years, Gunnera had achieved a worldwide distribution, with fossil pollen grains being found in areas where it is not found today, such as western North America, mainland Australia, and Antarctica. Based on fossil pollen recovered from drilling cores, Gunnera is also known to have inhabited the now-submerged islands of the Ninetyeast Ridge during the Paleocene, likely having dispersed there from either Australasia or the then-emergent Kerguelen Plateau islands.
Due to the widespread distribution of Gunnera during the Cretaceous, it was previously thought that the modern disjunct distribution of the genus was a relic of this period. However, phylogenetic analysis indicates that the majority of Gunnera species, even those found on entirely different continents, diverged from each other during the Cenozoic, indicating that the modern distribution of Gunnera is a consequence of long-distance dispersal from South America to other parts of the world, rather than relics of a former cosmopolitan distribution. The only species that diverged prior to the Cenozoic is Gunnera herteri of southeastern Brazil, which is thought to be the most ancient species of the genus, its lineage having diverged during the Late Cretaceous, roughly concurrent with the oldest Gunnera fossil pollen from Peru. The persistence of the Gunnera crown group since the Cretaceous makes it unique among flowering plants, and may have been facilitated by strong niche conservatism, dispersal ability, and being able to aggressively colonize disturbed land.
