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The 5th extinction and the rise of the mammals

Watch: Cretaceous extinction event and the rise of the mammals
Hi everyone. Last time, we spoke about the end of the Triassic extinction event, and you may remember that one of the main consequences of this event was the fact that it cleared the way for non-avian dinosaurs to become the dominant land animals. In this talk we’re going to focus on the last of the big five extinction events, the so-called end of the Cretaceous extinction event. During the Jurassic, we see that non-avian dinosaurs rapidly diversify. There are many different kinds of dinosaurs, some of them weighing as little as a few kilograms to others weighing over a hundred tonnes. It is no exaggeration to say that during the Cretaceous, all around the world, dinosaurs dominated terrestrial landscapes.
Given this remarkable radiation of dinosaurs for over 160 million years, it is astounding that their reign comes to an abrupt end at the end of the Cretaceous. But it’s not only dinosaurs that go extinct. We see that about 50% percent of animal families are wiped out at this time. This includes about 75% of all species, which includes dinosaurs as well as pterosaurs and even early birds. In marine environments, we see that the giant marine reptiles, such as the plesiosaurs and mosasaurs, also go extinct. Invertebrates such as the ammonites, belemnites and brachiopods also die out at this time. There’s much debate about how long this extinction event lasted.
Some scientists have suggested that it may have been just a few thousands of years, others have suggested that it might have been long, drawn-out over 2.5 million years. Indeed, what caused the extinction event is also hotly debated. There’s been many hypotheses that have been proposed. The three main ones are the asteroid impact, volcanism and sea-level changes. So what is the evidence for an asteroid impact? In fact, a son and father team of scientists in the 1980s proposed that there may have been a meteorite impact and they base their findings on the fact that there was an iridium anomaly which showed more than 30 times higher than normal levels of iridium present at 85 Cretaceous-Paleogene boundary sites.
At this time, there was no impact crater known, but later on there was an impact crater identified in the Yucatan province of Mexico called the Chicxulub crater which measured about 180 kilometres in diameter and was 20 kilometres deep. In fact, there are also other similar age crater sites known. At these impact sites there is also an iridium anomaly, there’s also glass spherules and we see that there is shocked quartz which, we know, forms under extremely high pressure. What is the evidence for volcanism? Interestingly enough, we know that there was huge volcanic eruptions in west central India round about this time. There are huge lava beds spreading over 500,000 square kilometres.
In some places these deposits are two kilometres thick, making mountains in places such as Mumbai and Mahabaleshwar. These volcanic features are some of the largest volcanic features known on earth and we know that they date to about 63 to 65 million years ago. At the end of the Cretaceous we know that the continents were moving to more or less their current configurations, and this would have resulted in massive sea level changes and environmental changes that would have impacted on both marine and terrestrial ecosystems. It is very likely that it wasn’t just one of these events that caused the extinction event, but rather more a combination of these events that led to the end of the Cretaceous extinction event.
After the end of the Cretaceous we see that, although many animals went extinct, there were also many survivors. In the water, we see many fishes and other invertebrates survive. On land, we see that crocodiles, snakes, tortoises and even some birds and some mammals also survive. There’s no evidence for the selectivity. What we do know, then, is that in the Paleogene, there are no non-avian dinosaurs and the stage is now set for the radiation of mammals. Mammals, as you may recall, evolved about 200 million years ago and, until the end of the Cretaceous, they were contemporaneous with the dinosaurs, but throughout this time they were small, inconspicuous, probably nocturnal, burrowing animals.
We know then also that, at this time, there were different kinds of mammals that were present. There were the multituberculates, there were monotremes, marsupials and placentals, and all of them survived, even some of them with substantial losses. But, after the extinction event of dinosaurs we see that mammals experience remarkable adaptive radiation. There is a prolific divergence of new forms and these begin to fill all those vacant ecological niches left by dinosaurs. Within 10 to 15 million years, we see 130 new genera of mammals. There are about 4,000 different species that evolve. At the Messel locality, which dates to about 50 million years in Germany, we see that there are many specialized adaptations among mammals at this time.
There are examples of horses and bats and even primates. So, placentals become the dominant vertebrates in the Paleogene. Later this week, we will take a trip to the West Coast Fossil Park where we will have an opportunity to see a fossil site dating to about five to seven million years ago. At this site we will see the wonderful array of animals preserved here from that particular time period and it gives us a glimpse into an ecosystem from a long-gone time that no longer exists. Relatives of humans only make the entrance onto earth about 7 million years ago. We will talk to Becky Ackermann about how humans came to dominate the planet.
Later this week, we will talk to Professor William Bond about the spread of grasslands and what might have been the trigger for the spread of grasslands.

With the fifth and last mass extinction at the end of the Cretaceous, 75% of all species were wiped out including non-avian dinosaurs, pterosaurs, and many early birds. In the marine environment, there were also large scale extinction of species such as the ammonites and large marine reptiles, as well as smaller invertebrates.

There is still much research being done into the details of this extinction event – including whether it was short-lived or lasted over a few thousand years. There are several likely causes the details of which are hotly debated among scientists. Nevertheless, there is some consensus that the main culprits were: asteroid impacts, volcanism and sea level changes. In my opinion, also shared by many other, it is more than likely that the End Cretaceous extinction event was probably the result of a combination of these causes.

Whatever the details of the cause, it was another great rolling of the dice that ended the domination of some species and opened up ecological niches for new species. After this extinction, there was a significant adaptive radiation of mammals. Earlier, I talked about the small early mammals that co-existed with dinosaurs – it turns out that although mammals also experienced some extinctions some of them survived the End Cretaceous extinction event – and from these surviving taxa, over 130 new genera comprising 4 000 different species radiated afterwards during the Palaeogene.

Next, I talk with three researchers to discuss their perspectives on different aspects of the development of life over the more recent past. They are William Bond, who is a terrestrial ecologist, Pippa Haarhoff, who is a palaentologist and Becky Ackermann, who is an archaeologist.

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Extinctions: Past and Present

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