In the last segment we thought about death and decay and how becoming a fossil requires at least some of that fossil to escape biological and physical breakdown burial is a key way for a carcass to avoid scavenging weathering and other disturbance that it would otherwise have to contend with at the surface in some environments an organism is more likely to become buried or be buried faster. In some cases scavengers themselves will bury carcasses, badgers are known to bury whole cows in most cases though we have to rely on geological processes Looking at our landscape there are places dominated by erosion the removal and breakdown of rocks and sediments and places dominated by deposition the emplacement of sediments that will eventually lithify or harden into sedimentary rocks mountain tops are sites of erosion as are deserts and open fields rushing rivers have enough hydraulic power to be erosive after all it is water trying to flow to the ocean that cuts down river bottoms the crashing waves of a shoreline are also erosive forces other environments though are low energy think a placid lake or the abyssal sea floor where very little erosion happens these are also sites where sediment is carried making them depositional basins where sediments are being delivered and can help bury a carcass floodplains and seasonally wet postal planes are also key sites for fossil preservation this bias in where fossils get buried can lead to some interesting gaps in our knowledge there are probably lots of places that dinosaurs lived that just never get preserved and so we have no evidence that they were there are also some great depositional environments like the ocean but given dinosaurs did not live in the ocean it's pretty rare but not impossible to find one there burial doesn't make our fossil completely safe there's still diagenetic and tectonic processes we have to worry them diagenesis refers to physical and chemical changes that occur through the conversion of sediment to sedimentary rock a fossil is really just a very interesting particle that became part of a sedimentary rock and so is subject to all the same processes that sedimentary rocks undergo and the type of sediment the fossil is surrounded by can matter a lot let's look back out on the landscape and think about the energy level of these environments again where we have rushing rivers and crashing waves small particles are easily swept away and we are left with coarse sands and gravels those grains don't fit together all that well and so there are often a lot of spaces between the grains that form conduits for fluids to still reach the fossil on the other hand if we're on a flood plain or a lake bed with a flow of water is much gentler a lot of fine-grained particles accumulate and those spaces between grains the pore spaces are smaller and better in case the fossil and protect it we mentioned last time that oxygen can speed up decomposition if the sediment grains are small enough even air has a hard time exchanging through the sediments and the pore spaces around the fossil can become anoxic if that sediment has lots of organic matter the oxygen gets used up even quicker which would be great if to gain organic matter didn't produce acid that could dissolve bone so best of the sediments are mostly rock and mineral grains without a whole lot of organic matter so the sedimentary grain size of fossil is surrounded by can affect its preservation and what types of grains bury the fossil are largely controlled by the geologic environment on the path from sediment to sedimentary rock sediments and the fossils within them undergo compaction as more and more sediment is piled on top this compaction can break down into form fossils such that they aren't preserved in the same three-dimensional shape as they were in life even large-scale movements of the Earth's crust can deform and fragment fossils making their anatomy somewhat difficult to interpret minerals and pore waters flowing between the grains of sediments can also penetrate the bone filling in the pore spaces within the bone and an enhancing preservation down to cellular details this is permineralization which can enhance the preservation of bone even once our fossil is buried and safe from all those processes of disarticulation decay decomposition diagenesis and deformation that's a lot of D's we still have to discover it most fossils are discovered because a portion of them is visible at the surface so ironically the fossil has to at least be partially unburied by tectonic uplift and erosion for us to even though it's there other times we have an idea based on the environmental deposition and use geophysical techniques to visualize what is beneath the surface before choosing to dig where we even have a chance of finding a dinosaur though depends a large part on where rocks of the right age intersect the Earth's surface here's a rather simplified geologic map of Texas in geologic maps areas are color-coded based on the ages of rocks exposed on the surface non-avian dinosaurs live in the Mesozoic and on this map those rocks are colored teal light green and dark green there are older rocks here in pink and purple that date back to before we had land animals at all and rocks here in yellow and red that are more recent just the last 20 million years or so just looking at this map and you can get a sense that there are some places worth going to find dinosaurs and some places that one just wouldn't bother trying it also of course matters what environment if we're interested in dinosaurs we'll need to look at terrestrial rocks and all the Marine rocks deposited in the vast inland seas of the Mesozoic would be less helpful although full of mosasaurs which are not dinosaurs and occasionally containing really well-preserved dinosaur surprises once discovered it can take months to years of work in a lab to carefully remove the surrounding sedimentary rock or matrix to uncover enough of the bone to really identify and describe the fossil these fossil preparators are critical part of the fossil discovery process that makes ancient organisms like dinosaurs known to science as technology advances we can also use computer tomography or CT scanning to examine the anatomy of bones encased in rock this can be especially important if the specimen is preserved in a concretion which are exceptionally well mineralized sediments often found surrounding fossils lots of things interact to determine where we find dinosaurs and how many we find it's not only taphonomy and geology but also where there is funding for scientists to search excavate and prepare and study the fossils these funding resources are not evenly distributed across the globe and so there are some places we know more about not just because the biases of preservation but because of biases in the work itself recently the magazine Scientific American published a compilation of where new dinosaur species were found and the year in which they were described there are clear concentrations in the American West and in Europe as well as in East Asia we can see from the time chart that most early discoveries were in North America and Europe but in recent decades a boom of dinosaur fossil discoveries have occurred in Latin America and Asia as well when you see a fossil or read about a new dinosaur discovery think about all the things that had to go right for that data to be here it is astonishing that we know as much as we do given all the things that have to go right for a fossil to preserve a few main things to remember about fossil preservation fossils are rare fossils are incomplete and the fossil record is biased not all groups of animals preserve equally not all environments are equally good at preserving fossils and not all places in the world have been equally searched