Dinosaur extinction

Species go extinct all the time. Scientists estimate that at least $99.9%$ percent of all species of plants and animals that have ever lived are now extinct. So the demise of dinosaurs like T. rex and Triceratops some $66$ million years ago wouldn't be especially noteworthy—except for the fact that around $80%$ percent of all life alive at the time also died out, in what scientists call a mass extinction.

Mass extinctions—when at least $70%$ of all species die out in a relatively short time—have happened a handful of times over the course of our planet's history. The largest mass extinction event occurred around $251$ million years ago, when perhaps $95%$ percent of all species went extinct.

Five mass extinctions

Ordovician-Silurian Extinction: Small marine organisms died out. ($442$ mya)

Devonian Extinction: Many tropical marine species went extinct. ($365$ mya)

Permian-Triassic Extinction: The largest mass extinction event in Earth's history affected a range of species, including many vertebrates. ($251$ mya)

Triassic-Jurassic Extinction: The extinction of other vertebrate species on land allowed dinosaurs to flourish. ($201$ mya)

Cretaceous-Paleogene Extinction: ($65.5$ mya)

$66$ million years ago: a mass extinction

Scientists refer to the major extinction that wiped out non-avian dinosaurs as the K-Pg event (formerly called the K-T extinction) because it happened at the end of the Cretaceous Period and the beginning of the Paleogene. Why not C-Pg? Geologists use "K" as a shorthand for Cretaceous based on the German word “kreide”, for which the Cretaceous Period with its chalky sediments is named.

The K-Pg event is so striking that it signals a major turning point in Earth's history, marking the end of the geologic period known as the Cretaceous and the beginning of the Paleogene period.
We can see the impact of this event in the fossil record. Fossils that are abundant in earlier rock layers are simply not present in later rock layers, after the event. A wide range of animals and plants suddenly died out, from tiny marine organisms to large dinosaurs.

Changes in the environment

What happened to cause such widespread devastation $66$ million years ago? Scientists agree that species go extinct primarily as a result of changes in their environment. The extinction of many species around the world at one time reflects large-scale changes in the global environment.

To explain what caused this mass extinction, scientists have focused on events that would have altered our planet's climate in dramatic, powerful ways. The leading theory is that a huge asteroid slammed into Earth $66$ million years ago, blocking sunlight, changing the climate and setting off global wildfires. In recent years, researchers have also been investigating whether other forces, including massive volcanic eruptions and changes in sea level, may have contributed to the environmental changes.

Paleontologists continue to investigate what caused all non-avian dinosaurs to die out 66 million years ago. Most agree that the impact of a large asteroid played a major role, while volcanic activity and climate and environmental changes caused might have contributed. Still, paleontologists can say for certain that some proposed explanations for the extinction of non-avian dinosaurs have proven false.

Did dinosaurs...?

Get too big to survive? No. Most of the largest dinosaurs lived millions of years before the mass extinction.

Go extinct because their eggshells got too thin? No. Although some dinosaurs did have unusually thin eggshells, many others did not. Many survivors, including birds (living dinosaurs), crocodiles and turtles, all have thin eggshells.

Die out because mammals drove them to extinction? Did mammals eat all of the dinosaurs' eggs? No. Dinosaurs coexisted with mammals for over 150 million years, so there's no reason to think that mammals would have suddenly caused dinosaurs to go extinct.

Go extinct because their babies hatched as all males or all females? No. This theory—which is based on the fact that the sex of baby alligators and crocodiles is determined by outside temperature—can't be easily tested. But it cannot explain why other animals that rely on temperature-dependent sex determination survived.

Yes. In fact, we currently consider birds to be highly modified dinosaurs, because in modern paleontology we classify animals in groups according to their evolutionary origins. Other than birds, however, there is no scientific evidence that any dinosaurs, such as Tyrannosaurus, Velociraptor, Apatosaurus, Stegosaurus, or Triceratops, are still alive. These, and all other non-avian dinosaurs became extinct at least 66 million years ago at the end of the Cretaceous Period.

Many shared evolutionary characteristics can be found in the skeletons of birds and other dinosaurs. One can follow the development of these evolutionary innovations by following along the evolutionary tree of dinosaurs from larger groups into successively smaller, more recent ones.

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Hip sockets and hollow bones

Birds' hips have a hole in the hip socket, just like all other dinosaurs. This feature was inherited by dinosaurs from their common ancestor and allowed dinosaurs to walk with an upright posture. Next, the bones of birds are hollow, and the hind feet of birds have three toes that point forward and one toe that points backwards, just like the hollow bones and three-toed feet inherited from the common ancestor of all meat-eating or theropod dinosaurs, including Tyrannosaurus.

Finally, the wrist of birds is built around a crescent-shaped bone called the semi-lunate carpal. This wrist bone was inherited from the common ancestor of birds and other dinosaurs that belong to the group called maniraptors. As documented by these characteristics, birds evolved from the first dinosaur, from the first theropod dinosaur, and from the first maniraptor. So scientifically, birds are members of all of these groups of dinosaurs.

Birds are a living group of dinosaurs in an evolutionary sense, and although no non-avian dinosaurs, such as Tyrannosaurus or Triceratops, still exist, dinosaurs are not extinct! You see dinosaurs flying around in the sky and perching in trees every day!

The extinction of non-avian dinosaurs except birds at the end of the Cretaceous has intrigued paleontologists for more than a century.

Volcanic eruptions are one of the most powerful forces of nature. The Deccan Traps are a series of geological formations located in western India that record one of the largest volcanic eruptions in Earth’s history. These eruptions started close to the end of the Cretaceous Period and produced massive lava flows that spanned more 600,000 square miles.

We now know that the Deccan volcanic eruption occurred throughout 700,000 years in multiple events or pulses, and that one of these pulses was far larger than the others (responsible for 80 percent of the erupted volcanic rocks) and its age places it awfully close to the K-Pg boundary.

A massive volcanic eruption like this one would have released enormous amounts of volcanic gases into the atmosphere, which could have altered the global climate, making the Deccan Traps a serious candidate for contributing to the end-Cretaceous environmental changes and mass extinctions.

In the late 1970s, at about the same time that the Deccan Traps were proposed as a potential cause of the end-Cretaceous mass extinctions, Walter Alvarez, a geologist from the University of California, Berkeley, was studying sediments of the K-Pg time period in the Italian Apennine Mountains (Fig. 4). Alvarez noticed a distinct 1-inch-thick black clay layer that marked the boundary and separated very distinct marine organisms below and above it. He wondered how much time that inch of black clay represented and asked his father, the Nobel Prize–winner Luis Alvarez, for ideas about how to measure it.

The Alvarezes had in front of their eyes an unequivocal indication that a massive asteroid must have collided with Earth right at the Cretaceous-Paleogene boundary. This finding was published in 1980, introducing what is known as the Alvarez hypothesis. The authors suggested the asteroid must have been about 6 miles wide, with the resulting impact crater approximately 100 miles wide. The impact of this giant bolide, traveling at 20 times the speed of a bullet, would have created a blast equivalent to 100 million nuclear bombs. The impact would have ejected tons of dust and debris into the atmosphere, with some of it even reaching outer space and eventually falling back to Earth. Up to 99 percent of sunlight would have been blocked for days or months, stopping photosynthesis and causing the collapse of ecosystems on a global scale.

Years before, in the late 1970s, geologists working for a Mexican oil company had discovered a ringlike structure near the Yucatan Peninsula while prospecting for possible drilling locations. This ringlike structure was partly submerged in the Caribbean and turned out to be the remnant of a giant crater formed about 66 million years ago. Moreover, it was about 110 miles wide, matching Alvarez’s prediction for the asteroid impact! However, the discovery was not published at the time, partly because of the secrecy of the oil industry and partly because it predated the publication of the Alvarez hypothesis, which prompted the search for a large crater.

In 1991, American geologist Alan Hildebrand obtained permission to publish the crater and other important data. This announced to the world that the crater of the dinosaur-killer asteroid had been found. The Chicxulub Crater, named after a nearby Mexican town, became instantly famous as the missing piece of evidence proving the Alvarez hypothesis.

The timing and the magnitude of both the Deccan Traps and asteroid events makes it impossible to ignore either of them as potential causes of the end-Cretaceous crisis. In 2014, Mark Richards, a geologist from the University of Washington, suggested that the seismic waves of impact from the asteroid could have triggered the onset of the major Deccan Trap pulse, reactivating the already existent volcanic system. The devastating effects of both the asteroid impact and the mega volcanic eruption could have been acting at the same time!

This idea is known as the one-two punch hypothesis, and its merit is still being debated. It is now clearer than ever that improvements in technology and the accuracy of radiometric dating will be key for telling apart the different events that resulted in the end-Cretaceous mass extinction event.

Grab your notebook or computer and write out your answer to each of the following questions. Then compare your answer to the suggested answer found in the “Answers to exploration questions” article. Have fun! (Hint: the information needed to answer these questions can be found in the video and the articles.)

1\. If species go extinct all the time, how do scientists know when a mass extinction occurred? How do they know which organisms went extinct?

2. Scientists think a giant asteroid impact and huge volcanic eruptions may have contributed to the mass extinction 65 million years ago. List at least three ways that these events could have changed the environment.

3\. Extinction theories suggest that blocked sunlight would have decreased photosynthesis in plants. Which kinds of dinosaurs—plant-eaters or meat-eaters—would have been affected by a change in photosynthesis? Explain your answer.

4. Scientists say that dinosaurs are not extinct. In fact, you see them every day. What do they mean?

If you are wondering where the suggested answers came from, you can review the video and the articles in this tutorial.

1. If species go extinct all the time, how do scientists know when a mass extinction occurred? How do they know which organisms went extinct?

Answer: Scientists know about mass extinctions by studying fossils. Fossils that are abundant in earlier rock layers are not present in later rock layers. When a fossil disappears like this, scientists determine it went extinct. When this happens to a wide range of many organisms at one time, scientists can infer that a mass extinction occurred.

2. Scientists think a giant asteroid impact and huge volcanic eruptions may have contributed to the mass extinction 66 million years ago. List at least three ways that these events could have changed the environment.

Answer: Scientists think the asteroid impact and volcanic eruptions could have led to other changes in the environment, such as:

• Blocked sunlight from debris in the atmosphere
• Decrease in photosynthesis in plants
• Freezing temperatures
• Long-term global warming
• Acid rain

3. Extinction theories suggest that blocked sunlight would have decreased photosynthesis in plants. Which kinds of dinosaurs—plant-eaters or meat-eaters—would have been affected by a change in photosynthesis? Explain your answer.

Answer: A decrease in photosynthesis would have affected all dinosaurs. A decline in plants would mean less food for plant-eating dinosaurs. But meat-eating dinosaurs prey on plant-eaters for food, so they would have less food to eat too. Without enough food, all dinosaur populations would eventually disappear.

4. Scientists say that dinosaurs are not extinct. In fact, you see them every day. What do they mean?

Answer: Dinosaurs are not extinct because birds are a living group of dinosaurs. They descended from a group of carnivorous dinosaurs. So every time you see a bird, you see a dinosaur!