Evolution Basics: The Placental Revolution, Part 1
Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we discuss the rapid diversification of placental mammals following the Cretaceous – Paleogene (K-Pg) mass extinction approximately 66 million years ago.
In the last post in this series, we discussed the origin of the eutherian “body plan” as a gradual shift away from an egg-laying reproductive strategy to a placental one. While crown-group placentals (i.e. modern-day eutherian species, their last common ancestral population, and all species descended from that common ancestral population) are remarkably diverse (think whales and bats, for example), these species are thought to have arisen from a common ancestral population that lived approximately 65 million years ago – either just prior to, or just after, the extinction of (non-avian) dinosaurs.
From early eutherian to the crown-group placental ancestor
The earliest-known eutherian stem-group species, Juramaia sinensis, is dated at ~160 million years ago (in late Jurassic period). The eutherian fossil record from the Jurassic (and the more recent Cretaceous) is sparse: the discovery of Juramaia was significant in that it extended the known range of eutherians 35 million years back from the oldest eutherian then known at the time (Eomaia scansoria, at ~125 million years ago). So, while stem-group eutherians were present from the late Jurassic on, it seems they were not common (and they serve as a reminder that the fossil record is biased towards common, and widespread species). Despite their relative scarcity, this diminutive lineage had found a niche – likely as small insectivores/scavengers – in a landscape dominated by dinosaurs.
All this was about to change, however. About 66 million years ago, an event would alter the course of vertebrate evolution on a global scale: the impact of an asteroid about 10 kilometers (6 miles) in diameter at Chicxulub on what is the modern-day Yucatan peninsula. The impact released the energy equivalent of millions of atomic bombs, caused tsunamis of staggering size, and littered North America with debris. The impact precipitated a mass extinction event that eliminated all non-avian dinosaur lineages: the remarkable tetrapod diversity of flying reptiles (i.e. pterosaurs), fully aquatic reptiles (such as ichthyosaurs) and large terrestrial, non-avian dinosaurs was lost in the (geological) blink of an eye. This event would bring the Cretaceous period to an end, and usher in the Paleogene (these periods are abbreviated as K and Pg, respectively, with the impact defining the K-Pg boundary). In the aftermath, ecological niches lay open – and mammals would step in to fill them.
Crossing the boundary
What is non-controversial among paleontologists is that the post K-Pg fossil record shows a remarkable diversification of placental mammals. The fossil record, however, is not well-suited to resolving events on a fine scale, as we have discussed previously. As such, it is still unresolved if the last common ancestral population of crown-group placental mammals lived before, or shortly after, the K-Pg boundary. What is not controversial, however, is that in either case, crown-group placentals undergo a significant burst of speciation events in the Paleogene:
The placental radiation: a case study in convergent evolution
The placental diversification in the Paleogene is highly interesting from an evolutionary standpoint since niches that were vacated by reptiles were in many cases filled by placental species – species that were shaped over time to fill those niches. Though the starting point for this diversification was a small insectivore, convergent evolution (a topic we have examined in detail previously in this series) would reshape species over time in ways remarkably similar to how it had shaped reptiles previously. Consider modern-day cetaceans (whales, dolphins, and porpoises) and ichthyosaurs – both groups show exquisite adaptation for a fully aquatic lifestyle, with flippers, a streamlined body shape, and a tail for propulsion, among other features. While cetaceans and ichthyosaurs are both tetrapods, and thus had tetrapod features in common as ancestral characteristics, these lineages were shaped independently from non-aquatic ancestors to a similar overall form (with some differences, reflecting their distinct trajectories – such as the up-and-down motion of a horizontal tail fluke for propulsion in cetaceans compared to the side-to-side motion of a tail in ichthyosaurs, or the fact that ichthyosaurs had four flippers, whereas cetaceans have almost completely lost their hind limbs). And as we expect for any major transition over time, the terrestrial mammal-to-cetacean transition is attested to by a robust fossil record of stem-group “whales,” showing the gradual loss of hind limbs, the movement of the nostrils to the top of the head to form a blowhole, and other features characteristic of modern cetaceans. (For those interested in this fascinating chapter in the evolution of placental mammals, I have written a short series on whale evolution that might be of interest.)
Other examples of convergence could be cited – eventually large, terrestrial placental mammals would arise and take on herbivore and predator niches previously held by dinosaurs, and bats would converge on powered flight using membraneous wings, as pterosaurs had done millions of years before – though flying mammals would share the skies with the one dinosaur lineage to survive the K-Pg impact – avian dinosaurs (i.e. birds). While these convergences may have (eventually) appeared in placental mammals, the K-Pg impact and the subsequent extinction of non-avian dinosaurs certainly played a large role in the timing and extent of mammalian evolution at this time.
Enter the primates
The placental diversification in the Paleogene is also when the earliest-known primates enter the fossil record. Since primates are the group to which humans belong, it is not surprising that much effort has been made to shed light on the origins of this group. In the next post in this series, we’ll begin to trace characteristics of our own species as they are revealed by stem groups on our own lineage.