fossils are the remains of animals and plants or traces of their presence that have been preserved in the crust so very often the actual body parts of the organism aren't preserved in the crust and we just have things like imprints like you see here these are trilobites this is an ammonite fossil here we have a plant fossil and we can also have fossils of of basically just traces of the organism like burrows okay so those are actually considered to be fossils as well so it doesn't have to be the actual body part of the organism it's a trace of their presence something that shows us that there was an organism that lived there at one time so fossilization is the process that turns a once living thing into a fossil and so we'll look at the different ways that that can happen but the fossil record is very much incomplete okay so for something to be fossilized it has to be rapidly buried after it dies right if something dies for instance on land and it is not rapidly buried it is going to be exposed to the elements so it's going to undergo decay it could be scavenged by other organisms and so it wouldn't actually be preserved so rapid burial is required and usually only if if any of the body parts are actually preserved it's only going to be the hard parts things like shells or bones or teeth okay so these types of organisms that we see in these three images here are very much over represented in the fossil record because these are marine and vertebrate organisms so when they die they settle to the bottom of the ocean where they um have a much better better chance of being buried rapidly than something living on land and their outer parts of their bodies are hard and and they're easier to preserve okay so the fossil record is very much biased towards organisms like we see here as opposed to organisms that have a lot more soft parts to their bodies okay let's look at the different types of fossilization that can occur actual preservation is where the actual material is being preserved the actual body parts like in this case the the insect is trapped in amber so its entire body is preserved that's very very rare or the preservation of you know a whole dinosaur in terms of its bones where the actual material or the hard parts like bones or teeth are preserved and there's not just an imprint of that material left so this is the the most rare type of fossilization to occur because the entire organism would either need to be buried very rapidly like this dinosaur must have been or encased in something like amber okay next are molds and molds are just impressions of the body part there are exterior molds or external molds okay that would make an impression of the outside of the body part and internal molds make an impression of the inside of the body part and so these impressions are just left behind after the body part has dissolved away so over long periods of geologic time groundwater will be moving through these rocks and that water can dissolve the materials that the original organism was made out of and then that material is gone but there's just an impression left behind okay so we see both here this is the external mold of a of an ammonite all right so we see the the impression of the outer part of the of the organism and this is an internal mold of an ammonite so showing what the shape of the inside would look like okay next are casts so if that mold if there is basically a void left after the material dissolves away after the original material dissolves away and that mold gets filled in with minerals or sediment it forms a cast so a cast is basically an exact reproduction of the organism but made out of some other type of material all right so like sediment um or other minerals that have crystallized in the shape all right so this is again an ammonite cast where the entire shape of the body has been preserved but this is not the actual fossil itself it's it's sediment or mineral material per mineralization is a replacement of the body part with minerals okay so the that what's that's what happens when on a molecular level that each of the the molecules that are forming the organism are actually replaced with minerals so here we've got another ammonite and it's being replaced with calcite and iron oxides in its in the sections here okay this is an ammonite that has been completely per mineralized with the mineral pyrite or fool's gold and this is a wood fossil so this is a piece of wood that some of the fibers of the wood have actually been replaced with opal all right so this is when the the the kind of shape of the body part is preserved but parts of it or the whole thing are being replaced with minerals that crystallize in that space carbonization is common in plants so this is when you have organic material that is getting compressed between layers and what happens is the organic material kind of dissolves away but it leaves an impression of the carbon behind so it leaves kind of a silhouette of the of what the original organism looked like so like i said very very common in plants so you have like a fern here there's none of the actual fern material is left behind except for an impression of carbon same with this organism over here and then we have trace fossils so trace fossils are indirect evidence of life that have been left behind basically just some kind of sign that there was an organism there but not actually a body part and not even actually necessarily an impression of a body part so like the burrows that we saw at the beginning so these could be clams that burrow down into the sand and then those burrows were preserved that's called a trace fossil footprints tracks those are all trace fossils these are called coprolites this is fossilized poop basically so that is also a trace fossil we don't actually see the organism there but there is evidence that is left behind that that organism did live there okay index fossils are fossils that we can use to kind of correlate different rocks over large geographic regions so they are specific fossils that are going to give us a lot of information about where on the geologic time scale these particular organisms fit in so there's a few things that are required for a fossil to be a good index fossil it has to be easily recognized we have to easily be able to tell what type of organism this is and what its specific species is it has to have a wide geographic distribution meaning the more places that we can find it the better okay if we can find a fossil that lived in many many places on the earth at around the same time that is going to make a good index fossil because we can then correlate rocks from wide from places that are very far apart from each other and say something about the time that those rocks formed and their their best if they existed over a very limited period of geologic time now of course geologic time is long 4.56 billion years but the best index fossils are going to have a limited period of geologic time where they're found in the in the fossil record that's going to make them very very useful so some organisms that are really useful index fossils are trilobites ammonites and of course dinosaurs okay the geologic time scale so those types of fossils index fossils were used to produce these divisions of the geologic time scale all right the geologic time scale is essentially the calendar of earth history and it is split into these various divisions right so we'll talk about um what happens in each of those but the divisions are not based on like length of time so not all of these divisions are not the same length of time they are based on the changes in plants and animals over time what we see in the fossil record and that is called the principle of fossil succession okay so evolution over earth's history has produced a succession of unique fossils that correlate to the units of the geologic time scale assemblages or groups of fossils that lived in the same time period contained in strata are unique to the time that they lived and can be used to correlate rocks of the same age across a wide geographic distribution okay so if we see an assemblage of fossils in one part of the world in a particular rock and we see that same set of index fossils in another part of the world we can correlate those we can say those rocks must have formed at the same time even though we're looking at two places in entirely different places on the earth okay so let's talk a little bit about correlation we can correlate rocks so here we're not even really thinking about fossils rock correlation can happen over a region just based on the stratigraphy the the order of rocks of these layered rocks sedimentary rocks so this example is from the colorado plateau and this is called the grand staircase series and it goes from the grand canyon north to zion national park and then to bryce canyon national park so if we start down at the grand canyon so that's down here in arizona we see that the top two strata of the grand canyon formation are the kaibab limestone and the moon copy formation if we go to zion the rocks the oldest rocks that are exposed at the bottom of the series of layered rocks are the kybab limestone and the moen kopi formation okay so they're similar rock type and we can see just based on their elevation that these are correlated okay and so that's what the lines here indicate that these are the same layers and we just can't see this part of the layer if we go to the top of zion we have the navajo sandstone and the caramel formation in bryce canyon if we go up the staircase the oldest rocks that are exposed at the at the bottom of the series of rocks are the navajo sandstone and the carmel formation okay so we know that these were all being formed in this region subsequently to each other and we can correlate those rock layers with each other so we can do the same thing with fossils as well alright so in this example we've got two different assemblages of fossils in two different rock units so in rock unit a we've got sea stars we've got maple leaves we've got a fern a scallop and a dinosaur all right so what we're looking at here we have to find the time where all of these fossils in this assemblage overlap with each other okay so that's the pink bar here where we had the scallop the fern the dinosaur the maple leaf and the sea star all existing at once all right so rock unit a we can kind of constrain its age based on that same thing with b right so we've got a trilobite a fern this is called a brachiopod a scallop and this and the c star right so here we have the trilobite the the scallop the fern the sea star and the brachiopod all overlapping with each other just in this range so we know that the the age of rock unit b has to be somewhere in this range and that it's older than rock unit a which we can also tell based on the principle of superposition okay so as we're going to see some of the major divisions of the geologic time scale are reflecting mass extinctions so mass extinctions are these events where many many species go extinct at the same time and there have been five major mass extinctions in earth history shown here we'll just look at the the three most recent so 200 about 250 million years ago we had the largest mass extinction so at this time most life was still in the ocean but there were land organisms and 95 of all of the species in the ocean went extinct and about 70 percent of all the land organisms that existed at that time went extinct so this was a huge mass extinction that wiped out most of the species on earth the main hypothesis for why this happened was a huge amount of volcanic eruptions in siberia so massive massive amounts of volcanic eruptions would have changed the composition of the air could change the temperature it could block light from coming in from the sun and so all of those things may have been factors in this mass extinction event about 200 million years ago there was another one at the triassic jurassic boundary this one is also thought to be from volcanic eruptions but in this case volcanic eruptions of massive amounts of basalt at the bottom of the atlantic ocean all right so that would have changed the composition of the water it would have changed the temperature and it could have made it so that it was not habitable anymore for the for the organisms that did live there and then our most recent mass extinction was 65 million years ago so this is the one that took out the dinosaurs and um it is believed that this was due to a major asteroid impact in what is now the gulf of mexico all right and that's what's thought took out the dinosaurs so when mass extinctions happen they influence the evolution of species because for instance if we look at the permian triassic if you had the majority of the organisms going extinct the organisms that that made it out that are left behind are going to have a lot of new niches that they can fill right maybe their maybe predators don't exist anymore maybe they don't have to fight for food anymore and so that can cause an evolution of species where you get different types of organisms becoming more dominant on earth and so we can see that that's reflected in the geologic time scale okay so here's another look at it so what you are responsible for knowing are the main division the four main division so that's the precambrian the paleozoic the mesozoic and the cenozoic you do not need to worry about the finer divisions here okay so let's start with the precambrian the precambrian goes from the origin of the earth about 4.56 billion years ago to 542 million years ago all right so if you do a quick calculation there that is over four billion years of earth history all right so you can see that based on the scale this is not to scale right it's showing just this small amount is the pre-cambrian but that's actually the bulk of earth's history right there four billion years so the reason there aren't a lot of divisions here is that again these divisions are based on life and the earliest fossil record of life is about 3.8 billion years old and that is single-celled organisms things like bacteria so um that's the earliest thing we see in the fossil record and then we don't see anything more complex than that in terms of life until 542 million years that is when the earliest shelled animals evolve so the earliest organisms that had hard parts right shelled animals marine invertebrates and that marks the end of the precambrian the the um the appearance of these earliest shelled animals in the fossil record that's where we start the paleozoic okay so paleo means ancient and zoic means life so ancient life um that goes from 542 million years to 251 million years okay so during this time we have a lot of things evolving we have the first fish the first land plants the first amphibians reptiles and during this entire period marine and vertebrates are the dominant species on the planet so most things are living in the ocean and most of them are these shelled organisms these invertebrate organisms right now at 251 million years that's where we get the permian triassic mass extinction the one that wipes out 95 of all those marine organisms and 70 of the organisms living on land okay that starts the mesozoic era meso means middle and zoic means life so middle life and this is where the first dinosaurs and mammals start to evolve as well as birds so from 251 million years to 65 million years is the mesozoic and again this is marked by a mass extinction the most recent mass extinction which made the dinosaurs go extinct and that brings us to our most recent era the cenozoic that means recent life so we are still in the cenozoic so the cenozoic goes from 65 million years ago till the present we are still in the cenozoic and this is when um because of the extinction of the dinosaurs mammals are really allowed to thrive so the cenozoic is the time of mammals mammals become the dominant species the earliest humans uh evolve and bringing us all the way up to the present okay so those are the four divisions that you want to know about and you want to know what the dominant life forms were in each of those okay so again this geologic time scale was originally put together just based on the fossil record but now that we have radiometric or absolute dating the radioisotopic dating methods that we discussed we've actually been able to put ages on these that's where all those numbers come from okay so we've used those methods to put ages on the various divisions of the geologic time scale which were originally subdivided based on the fossil record so again we see here that the precambrian so in this image it's shown as just this little tiny slice at the bottom remember that represents four billion years right and then this is just kind of showing the different types of life that come in and how abundant they were right so we in the pre-cambrian we only had single-celled organisms and then at the very end of the pre-cambrian invertebrates come in right in the mesozoic reptiles really become the dominant organisms and then in the cenozoic mammals evolve to become the dominant organisms and last but not least this is what the geologic time scale actually looks like so this is what a geologist would actually be using i'm just showing you this to let you know that it's a lot more complicated than what i've just shown you but as we find more and more fossils we are able to refine these into even smaller sections into these ages and so these are these geologic time scales are revised you know every i don't know five to ten years as more and more information becomes available