November 8. Flowering plants

Today I’m going to tackle flowering plants – obviously a huge topic, so we’ll only touch the surface of this subject.

Plants grew on land from sometime during the Silurian, at least 425 million years ago, or perhaps even earlier. Forests thrived by Devonian time, 380 million years ago. Plants were so abundant that their remains in the form of coal give their name to the Carboniferous Period. But for close to 300 million years, plants were without flowers.

Historically, fossil flowers seemed to appear so suddenly in the fossil record that they were a problem for Charles Darwin’s evolutionary theory, and he called it an “abominable mystery.” Since then, enough transitional fossils have been found to demonstrate the episodic evolution of angiosperms, flowering plants, from gymnosperms like seed ferns. But the details of that evolution are still pretty sparse, and the question of why did it take so long to get there remains unanswered.

Magnolia photo by WibblyWibbly (public domain)

There are some possible flowering plants from the Jurassic, and there’s some fossil pollen from the Triassic that has similarities to angiosperms, but the oldest pretty much accepted flowering plant is called Archaefructus, and it dates to about 125 million years ago, the middle part of the early Cretaceous in northeastern China, found in part of the famous Jehol biota that includes well-preserved insects, dinosaurs, and mammals, as well as the typical floral assemblage of cycads, gingkoes, and conifers. Flowers were definitely rare, but they were present.

Whether or not Archaefructus is a basal ancestor to modern flowering plants is debated – it might be a sister group rather than a direct ancestor. There are some other early Cretaceous examples as well.

It took an additional 25 million years, until about 100 million years ago, at the middle of the Cretaceous, for flowering plants to really take off. It’s called the great angiosperm radiation, when a dramatic increase in flowering plant diversity took place. But even that is no longer seen as quite the sharp event it was formerly viewed as. Many modern groups, including magnolias, had developed millions of years before then. Nonetheless, geologically speaking, there was still a real ramping up of diversity and numbers of flowering plants in mid-Cretaceous time.

The radiation of flowering plants has traditionally been tied to a co-evolution with insects, with both evolving a symbiotic relationship that continues to this day. The relative sudden explosion of flowers may be an example of a highly successful trait that initially evolved in some isolated ecosystem, such as today’s Galapagos Islands, where Darwin’s finches famously evolved various traits to fill a diversity of ecological niches. It’s also been suggested that angiosperms, present but dominated by the conifers and cycads, were opportunistic plants that invaded niches where the competition from gymnosperms was less, establishing a foothold that simply grew and grew.

The attractiveness and food sources provided by flowers and fruits wasn’t lost on mammals, either. A typical view holds that the evolution of flowering plants was a boon to mammals that scrounged on forest floors or climbed into shrubs, bushes, and trees. The relationship between plants and mammals was symbiotic, like their connection to insects. Mammals ate plant fruits and helped spread their seeds. However, a 2013 study found that the mid-Cretaceous, when the great flowering plant radiation took place, was actually a time of decreasing diversity among mammals. This may seem counter-intuitive, but for whatever reason, it appears that insectivores, and not plant-eaters, were the mammals best suited for survival in the Cretaceous. Those insectivores basically gave rise to all modern mammals except the egg-laying platypuses and spiny anteaters.

There’s even been a suggestion that grazing by those huge, hungry herbivorous dinosaurs might have been a factor in stimulating flowering plants to evolve.

Yet another look at the possible causes of the angiosperm radiation suggests that fires paved the way for their diversification. The Cretaceous atmosphere was probably something like 25% oxygen, compared to today’s 21%, so wildfires would be more likely. Fires would have cleared the gymnosperm forests, allowing for invasion by angiosperms. To my mind, one problem with this is why didn’t it happen earlier, or why did it happen so suddenly, relatively? Of course there might have been some threshold that was crossed, but if angiosperms were present at all, it seems that they might have been able to take advantage of wildfire situations before the mid-Cretaceous. But maybe not. The modern savannas, grasslands on many continents, appear to have evolved over a short period, say 10 million years or so, and wildfires may be a factor in their evolution, too. Stay tuned.

By the end of the Cretaceous something like 80% of modern groups, including oaks and maples, had been established. The forests were much more modern in appearance than they were at the start of the period.

I have links below to reports on three of the research topics I’ve mentioned today.

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Today’s birthday is Aleksandr Fersman, a Russian mineralogist born November 8, 1883, in St. Petersburg. His work in economic geology helped provide for the needs of the Soviet Union during its post-World War I industrialization. It’s perhaps a minor footnote to his career, but he was responsible for evacuating 80,000 mineral specimens from the Moscow mineral museum during World War II. As a mineral collector myself, I can relate to that.

—Richard I. Gibson

LINKS:

Origin of angiosperms
Cretaceous angiosperm radiation
Mammals declined with plant expansion
Fires and plants

Magnolia photo by WibblyWibbly (public domain)