This is a blog about paleontology (the study of the history of life on Earth through the fossil record) with an emphasis on vertebrate paleontology, the study of extinct vertebrates (animals with backbones). The methodology and findings of paleontology will be discussed, as well as related issues such as evolutionary theory. The blogger is a vertebrate paleontologist specializing in the Triassic Period, the Beginning of the Age of Dinosaurs.
My posts are presented as opinion and commentary and do not represent the views of LabSpaces Productions, LLC, my employer, or my educational institution.
Sorry about the delay since my last post. I've got a series of blogs about the evolution of dinosaurs and closely related reptiles that I want to do, but as I started writing it, I realized that I ought to do a series of blogs giving a little background into how paleontologists know what they know about the history of life first, and also about what exactly dinosaurs are and how they fit into the big picture of life. So, first things first. Also, I AM paying attention to questions that people ask in the comments section, and will try to cover them (eventually).
This first blog is about how we put events in the history of life in order. Imagine a novel being written in collaboration by a bunch of different authors. Some of them are writing about characters, some about settings, some about specific events which occur in the plot...but they are writing in isolation, and no one has any idea how these people, settings, and events are ordered. What happened first? Did the characters go to Paris after their trip to Mars, or before? Did the schizophrenic dog kill his owner before or after his affair with the monkey? This stuff is important. Everyone is just writing their own pages, but until someone takes all the pages and put them in order, there is no story.
Fossils, the remains of extinct organisms, are preserved in sedimentary rocks. These are rocks made of sediments, which include mud, sand, and gravel (all of which are basically tiny ground up little pieces of other rocks). In many different environments, sediments tend to pile up in layers over time. For example, in rivers, sand, gravel, and mud are moved along by the current, and get piled up at the bottom of the river and along its banks. When rivers flood, mud and sand get carried by floodwaters over the surrounding area, and are deposited there. Any animal or plant which had died in or near the river has the potential to get buried by these sediments, and over hundreds, thousands, and millions of years, these sediments can get piled into stacks of layered sediments hundreds and thousands of meters thick, with the remains of dead animals and plants mixed in.
Now, here is an extremely important point, which is also extremely simple: it is hard to bury something that isn't already there. In other words, if one stratum (a single layer of sediments) is covered by another stratum, the stratum on the bottom must have been there first. Make sense? This means that if you have layers of strata stacked on top of each other, the ones at the bottom are oldest, and they get gradually younger going up. This principle is called the Law of Superposition. Imagine if you get a daily newspaper, and every day you throw the paper on top of the one from yesterday, until you have a big stack of newspapers. If you went through the stack from top to bottom, you would find that they get older and older as you go down through the pile. The study of strata and the order they occur in is called lithostratigraphy. A nice added bonus is that geologists (people who study rocks) can figure out what sorts of environments strata formed in by comparing the sediment layers and various characteristics that they have to sediments forming in modern environments. This means that we can not only figure out the relative ages of strata, but of environments. If a series of strata resembling those formed by a modern river has strata resembling those forming in a modern desert stacked on top, then you know there was a river there first and a desert there later. Cool, huh?
Now, here is another point stemming from the first. If the strata get younger going from lowest to highest, so do the fossils they contain. A fossil buried in one layer must have died and been buried before a fossil in the layer above it. The study of the order that fossils occur in a series of strata is called biostratigraphy. So, just as we can look at a vertical sequence of strata and figure out the order in which ancient environments existed, we can look at the vertical sequence of fossils and figure out the order in which different animals and plants lived.
The following figure is intended to help illustrate superposition (start at the bottom).
We now have the basic tools we need to put the history of life in order, but we have a potential problem. Those buried layers of sediments are, well, buried, and therefore potentially hard to study. Fortunately, in certain places, the Earth very considerately exposes layers of strata. In the western United States, a series of upheavals over the past 65 million years (I'll talk about where we get those numbers later) pushed up thousands of feet of strata. The Colorado Plateau, which basically covers the four corners states (Arizona, New Mexico, Colorado, and Utah), is a huge area of uplifted strata, most of of which formed over the last 550 million years or so (again, I'll talk about how we get those numbers in a later post). Imagine a little kid punching his fist into the bottom of a birthday cake, pushing a chunk of stacked cake and frosting up through the top of the cake. I don't know why a little kid would do that, but that is beside the point. Little kids are weird.
Rivers do not just deposit sediments; they can also erode them away. When land is uplifted, rivers running across the land surface will cut down through them and generally wash away sediments instead of piling them up. The Colorado River, which flows from the Rocky Mountains in Colorado to the Gulf of California (Sea of Cortez), cuts right through the middle of the Colorado Plateau. The Colorado and its tributaries have cut down through thousands of feet of strata, gradually washing the ancient layers of sand, mud, and gravel down the river. The Colorado Plateau is gradually getting washed into the Gulf of California. This is kind of sad, but it is also nice, because the rivers are carving out nice cross sections through the strata so that we can examine them (such as at Grand Canyon, shown below). They are also exposing the fossils buried in those strata so that we can collect and study them.
NOW: the Colorado Plateau is a big stack of uplifted strata, oldest on the bottom, getting younger going up. The Colorado River starts way up in the mountains and flows southwest and down in elevation to the Gulf. SO, if you follow the river down through the plateau, you find yourself generally going back in time, as the river cuts down through lower and lower layers of strata. The rocks exposed in Grand Canyon (in northern Arizona) are older than those exposed around Lake Powell and further northeast (in southeastern Utah and western Colorado), and the different layers of strata exposed as you go up each contain a record the environment they formed in and the animals and plants that lived in them, allowing us to reconstruct 550 million years of history in western North America. If you want to learn more about the history of the Colorado Plateau, I highly recommend a book called Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney.
That works great for one particular area like the Colorado Plateau, where you can see the strata stacked on top of each other. But how do we know the relative ages of strata in different parts of the world? For example, I work on rocks in Arizona which I know are about 220 million years old, and I know that there are rocks in South America and Germany which are about the same age. How? I'll talk about that in the next couple posts.
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This means that we can not only figure out the relative ages of strata, but of environments.
I work with several people who take soil, and by running the extracted carbon through a mass spectrometer, can tell you which sort of plants grew in that soil in the time period that that soil layer was built/deposited. Different plant types will vary in the carbon isotope (C12 or C13) they use, so by looking at that ratio in a soil, you can determine what type of plants grew there. It's fascinating stuff!
"Imagine a little kid punching his fist into the bottom of a birthday cake, pushing a chunk of stacked cake and frosting up through the top of the cake. I don't know why a little kid would do that, but that is beside the point. Little kids are weird."
Big fan of this analogy.
I wish I could go fossil hunting with you! I took a course that took the class fossil collecting in the Santa Monica mountains (Los Angeles). It was fantastic.
This post was fantastic. Clear, to the point, and you were able to leave us wanting more.
MOAR says the interweb!