So far in this course, we've been talking a lot about the day to day, and sometimes year to year lives of dinosaurs. But these dinosaurs died tens or even hundreds of millions of years ago. In this lesson, we're going to talk about when different groups of dinosaurs lived and start thinking about what paleontologists often call, Deep Time. To guide us through this lesson, we're going to be using something called the geologic time scale. This special interactive module shows you how the history of Earth has been broken down into smaller and smaller chunks, usually based on major geological or biological events. Go ahead and have a look at our interactive geologic timescale right now. You can manuever around this module with your mouse or using touch controls if you're watching on a tablet. Feel free to investigate the various eons, eras, and periods that compose the history of Earth. We'll bring you back to this interactive time scale later in this module to illustrate events at specific points in time. But for right now, just go ahead and play. We know when certain events happen, because we use radiometric dating on rocks that formed during that event to determine how old those rocks are. Radiometric dating uses the natural breakdown of radioactive isotopes in the environment to calculate the age of a rock. When an igneous rock solidifies from its molten form, it starts out with an isotope ratio equal to that found in it's environment. Over time, the radioactive atoms in the rock break down. Using a mass spectrometer, we can measure the ratio of radioactive isotopes in the rock compared to the atom that the isotope has decayed into. This ratio allows us to estimate the age of a rock. Do you think that sedimentary rocks can be used in radiometric dating? Sedimentary rocks cannot typically be used for radiometric dating. Sedimentary rocks are formed from the broken up parts of other rocks rather than cooled and solidified molten rock. This makes dating fossils usually found in sedimentary rocks difficult. However, we can use volcanic ash layers, which are relatively common and found within sedimentary rocks as a way to get a good estimate for the ages of fossils nearby. Using radiometric dating techniques, geologists have estimated the oldest rock on earth is about 4.4 billion years old. Rocks on the moon are at least 4.5 billion years old. Our best estimates tell us that the earth formed about 4.6 billion years ago. The earth has been around for about 4.6 billion years. But it's hard to visualize just how many years 4.6 billion really is. So, lets talk about geological time, to see some of the major divisions of the history of earth, and some of the important events that occurred in the development of the planet and the evolution of life. We'll start when the solar system is formed, 4.6 billion years ago. The first minerals on earth are just starting to solidify. A rogue planetoid collides with the earth and the moon is formed. It's a tumultuous time. Earth is being constantly bombarded by meteorites, the surface is partially molten, and there are innumerable active volcanoes. The first oceans formed during this time. We call this the Hadean Eon, named after the Greek god of the underworld, Hades. About half a billion years later is when we think the first self-replicating RNA molecules formed in the oceans. The RNA molecule is the building block of all later lifeforms. Four billion years ago marks the beginning of the Archean Eon and this is where we find the oldest fossils. These were single-celled organisms, probably bacteria and archea. One of the most important events in the history of life occurred about 3.5 billion years ago. The evolution of photosynthesis by sciono bacteria which ultimately produced the oxygen in our atmosphere. Life was pretty much represented by single-celled bacteria and arquea for a long, long time. What span of time do you think is shorter? Is it A, the time from when the Earth formed to when the first multicellular life appeared in the ocean. Or is it B, the time from when the first multicellular life appeared to today. It took about 3 billion years for the first multicellular lifeform to appear on the planet. But, it's been less than two billion years since the first multicellular lifeform appeared. So the correct answer is B. We are now passing into the Proterozoic Eon. The oceans are dominated by single-celled organisms, and the atmosphere becomes more and more oxygenated. Around 1.7 billion years ago marks the origin of the first multi-cellulared organisms. But fossils this old are very rare. Single celled organisms are small and soft and unlikely to fossilize. Additionally, rocks of this age often underwent metamorphosis, turning them into different kinds of rocks. Unfortunately, this process of metamorphosis destroys fossils. As such, our understanding of the fossil record of single-celled and earliest multi-cellular organisms from this era is pretty poor. The end of the proterozoic is a weird and special time in the evolution of life. The last part of the proterozoic eon, starting about 630 million years ago, is called the Ediacaran Period. And it is here we have some of the first large and well preserved multi cellular organism. The problem is that the Ediacran fossils are so weird, and unlike anything alive today, that it can be hard to pinpoint what groups of animals they represent. And here we are at the beginning of the Phanerozoic Eon. Phanerozoic means visible life and it is within this eon that the most familiar forms of life evolved. The Phanerozoic eon is divided into three eras; the Paleozoic, the Mesozoic and Cenozoic. The first period of the Paleozoic Era is the Cambrian Period. 540 million years ago, a special event we call the Cambrian Explosion occurred. It wasn't a literal explosion, but an explosion in the diversity of multicellular life forms. Many familiar kinds of animals appeared at this time, like sponges, mollusks and worms. The extinct arthropods called trilobites appeared at this time. There were also a variety of unusual arthropod predators, without close relatives like Anomalocaris, or Opabinia. And finally, the very first relatives of the vertebrates occurred like Pikaia. The Burgess shale in British Columbia, Canada, is one of the best records of life during the Cambrian explosion. From here on out, the oceans become a pretty happening place. The seas are full of trilobites, echinoderms, nautiloids, bivalves, brachiopods and corals. By the middle of the Ordovician period, about 460 million years ago, the first fish the ostracoderms had evolved. But jaws would have to come later, about 30 million years later, with the appearance of the placoderms and acanthodians, the first organisms ventured on to land about 450 million years ago. In the Silurian, period 420 million years ago, land plants had begin to diversify and millipede like arthropods scuttled beneath. Back in the seas low fin fish, or sarcopterygians, appeared at about the same time and would eventually give rise to the first amphibians. About 370 million years ago in the Devonian period, amphibians first ventured on to land during the Carboniferous period, a time of vast swampy forests of tree-like plants, unrelated to living trees. There forests form the basis for many of our coal deposits today. The first reptiles appeared towards the end of this period, about 310 million years ago. During the Permian Period, the continents collided to form one giant continent called Pangea. This is also when reptiles evolved into the three main lineages of hard shelled, egg laying vertebrates. The synapsids which included the mammals dieapsis which included crocodiles, birds, snakes and lizards and the anapsis which included the turtles. The end of primeon saw the greatest mass extinction that ever occurred and this event marks the end of the Paleozoic Era. The mass extinction at the end of the primean period, which closed out the Paleozoic Era led to an incredible time in earth's history, the Mesozoic Era. The Mesozoic Era is the main focus of all the material in this course because it is best known as the age of dinosaurs. Now, we're going to describe the three periods of the Mesozoic in more detail shortly. But for the time being, I've got a question for you about vertebrates and the Mesozoic. There were several groups of vertebrates that existed during the Mesozoic Era. Dinosaurs were the dominant group throughout most of the Mesozoic. Mammals were also vertebrats.Did the first mammals appear before or after the non-avian dinosaurs went extinct at the end of the Mesozoic era? A is the correct answer. The earliest mammals appeared about two hundred and twenty-five million years ago, very shortly after the first dinosaurs, and the first birds evolved during the Mesozoic era. Another catastrophic extinction at the end of the cretaceous period mark the close of the Mesozoic and the beginning of the Cenozoic era. After the Mesozoic era came the Cenozoic era. This era's often called the age of mammals. Mammals became quite large and dominated the wildlife on the planet during the Cenozoic. The earliest primates evolved shortly after the end of the age of dinosaurs, but apes did not evolve until about twelve million years ago. From there, it's just a short amount of time to the earliest humans which evolved from apes about 7 million years ago. Our genius and species homosapiens have existed for about 200,000 years. All of recorded human history has occurred in the last 10,000 years. When you think about the long history of earth, we've only existed for the blink of an eye. If all of earth history was compressed into a single day, such that 4.6 billion years was equal to 24 hours. For how much of that 24 hours have humans existed? Is it one second? 30 seconds? One minute? Or five minutes? If all of earth history were compressed into a single day, modern humans would've have only existed til the last second of it. The earth is incredibly old, moreso, than most of us can truly comprehend. Take a few minutes and have a look at our interactive geologic time scale once again. You can review all of the eras and periods that we've talked about so far in this module. What is the era that we live in today? Is it the proterozoic, the mesozoic or the cenozoic? We live in the cenozoic era, so C is the correct answer. The non-avian dinosaurs we are studying in this course lived in the mesozoic era. Speaking of which, it's time to start talking about rocks from the Mesozoic era. More specifically, those from the Cretaceous Period. Here's Dr. Phil Curry at Dinosaur Provincial Park in Alberta, to talk about stratigraphy and Cretaceous rock formations.
