Dawn of the Planet of the Angiosperms
The origin of modern rainforests
∘ Prelude from outer space
∘ Rise: Enter the Angiosperms
∘ And the aftermath was…
Prelude from outer space
One of the most common research topics by academics and enthusiasts alike within the field of paleontology/paleobiology is the Cretaceous-Paleogene boundary. The K/Pg boundary (for short) is a name given to a geological signature, a “landmark” that stamps the end of the Cretaceous Period and with it the end of the Mesozoic Era.
In this fragment of Earth’s history many ecological changes shaped the world as we know it today, partly because of the effects of an asteroid that impacted the Yucatan Peninsula. This cataclysm brought havoc and disrupted the terrestrial communities that used to thrive, including non avian dinosaurs, which are one of the most popular groups among media.
But not only did dinosaurs meet their destiny that fateful day. Terrestrial communities suffered a dramatic change and in some cases an abrupt end; it’s in fact one of the five mass extinction events that have taken place throughout Earth’s history. Even so, focusing only on extinction, chaos, and destruction in these situations is a short view. Like people say: “Something ends, something begins” and in this particular case the “fall” of Gymnosperms (groups of seed-producing plants that includes conifers and cycads) brought to fruition the rise of modern Neotropical rainforest alongside Angiosperms (flowering plants).
Rise: Enter the Angiosperms
Speaking about plants, Angiosperms are newcomers in comparison to other groups, with the oldest fossils being from the early Cretaceous Period (about 140 mya to 100 mya)(1). In comparison, the oldest fossils of plants date back at least 500 million years ago, 100 million years earlier than we originally thought(2). For nearly 434 million years, non-flowering plants proliferated through the continents, from the very first colonial Bryophytes (liverworts, hornworts and mosses), to the vast forest dominated by Pteridophytes (ferns) and later by the Gymnosperms.
So one of the big questions would be, how were the conditions before the great explosion and the aftermath of it? Thanks to a study conducted by Carvalho et al. (3), a description made of several outcrops gave us an insight of the composition of the forests of yore and its biotic interactions in northern South America, in what’s now modern Colombia. Through palynoflora analysis (study of plant pollen and its diversity) and the examination of leaf fossils, they determined the environmental conditions that were before the massive cataclysm. The landscape prior to the end-Cretaceous contained a mixture of Angiosperms, ferns and conifers (members of the family Araucariaceae being the most representative), and the forest presented relatively open canopies, with an intense and host-specific insect herbivory (3).
And the aftermath was…
On the other hand, Paleocene evidence, which is the first Epoch within the Paleogene Period, suggests that rainforests that thrived after the event were more similar in fashion to their modern counterparts, with multistratal closed canopies, dominated mostly by Angiosperms but with low diversity (3). Probably the ashfall from the Chicxulub impact added phosphorus, which acted as a fertilizer leading to a quick evolutionary radiation of this group of plants.
Members of the Fabaceae family (bean family) experienced a major diversification event in part due to the low nitrogen concentration present on the soil. Root nodules filled with fixing nitrogen bacteria, common in these groups, allowed further fertilization of the soil, leading to its rise during the Paleocene and enhancing the growth of other Angiosperms over other groups, in short stimulating forest productivity (3).
Although differences exist between Cretaceous and Paleocene floras, similarities in leaf physiognomy suggest evolution under the same selective pressures, such as a wet tropical climate (3).
In any case, even if it’s the past or the present, what we now call The Amazon is an amazing place, don’t you think? Many different questions would surely arise around this topic, making it interesting to delve into. Do you think that the asteroid did in fact have a major effect on the flora? Or what other factors might have been taken into consideration?. I would love to know your opinions on the matter. Please let me know more about it in the comments below.
