in the last segment we discussed how we know that continents have changed their position through Earth's history and we saw the agglomeration of supercontinence as well as their breakup let's start this segment with Pangea the supercontinent that was present at the Permian Triassic boundary from the paleogeographic reconstruction we can see that populations of terrestrial organisms could have biogeographic distributions that touch the entire land mass there were no barriers to Simply walking to the next place as we move through the Mesozoic Pangea gradually breaks apart and by the Cretaceous pillaging boundary the continents are nearly in their modern positions and the world begins to look recognizable in each of these palegeographic Maps we can see the different elevations of land depicted in shades of green and brown and the different depths of the oceans moving from light blue shallow Seas to dark blue deep oceans we can see that the breakable continence is driven by rifting that emplaces oceans between them we can also see that high sea levels can sometimes cause great Continental flooding and there are many shallow Seas of top continental crust during portions of the Mesozoic graphing these changes in the percent of the Earth's surface that is land and the percent of the Earth's surface that is shallow Seas through the whole phanerozoic we can see that the Mesozoic was a time of increased land masses compared to the Paleozoic although there were fewer shallow seas in the mesozoa than the Paleozoic there were many more Continental Seas for much of the Mesozoic than there are today the change in proportion of land masses may have had an impact on the diversity of land animals dinosaurs included today looking at the number of species we find for a land mass of a given area we see a positive relationship in which larger areas tend to contain more species this is called the species area effect and is one of the many factors that can affect biodiversity on our planet climate and environment can also have an impact on biodiversity in the last segment we talked about how we expect certain types of sedimentary deposits in different climate bands for example we'd expect to find coals preserved in warm wet environments then today we associate with equatorial jungles if those warm wet conditions occurred at higher latitudes though due to an overall warmer paleo climate then we might find those coals distributed at higher latitudes similarly there are certain plants that commonly only live in warm wet environments like palms and mangroves and we might use their fossils to track the distribution of tropical environments certain minerals there are also Hallmarks of chemical weathering that occurs in warm wet tropical environments and so when we find these in sedimentary records we can also infer the presence of tropical environments when we use fossils and different types of sediments to infer climates we call them proxies because they allow us to indirectly measure environmental factors on the other hand there are certain sedimentary rock types we would expect for arid dry environments like evaporites which are cells that precipitate out of water as the water evaporates and types like paleosols or ancient soils that are characteristic of dry environments looking at the distribution of these rock types in the sedimentary record lets us infer where we have arid environments as we talked about last time a bit glaciers can also leave behind their Mark as glacial striations on bedrock as glaciers move across the land surface they pick up rocks that freeze to their undersides and get carried with them as they advance when those glaciers eventually melt they leave behind jumbled poorly sorted masses of sediment or glacial till that lets us know that a glacier was once in that location sometimes we even find large rocks dropped into otherwise fine-grained sediment that we know must have been moved by a glacier because so much energy is needed to move a boulder then is needed to move mud so liquid water alone could not have done it with these proxies in mind we can look across sedimentary rocks of different known time intervals in the Mesozoic and map out their Distribution on paleocontinental maps we can then infer the climates of different regions based upon the sedimentary rocks we find if we look at the middle to late Permian when Pangea was at its greatest extent we see that the middle of the supercontinent there were a lot of evaporites and dry climate paleo Souls that tell us that much of Pangea was an arid environment it is only at the higher latitudes and on smaller land masses in the tropics that we see coals palms and mangroves typical of warm wet environments these warm wet environments were even at the South Pole only at the very northernmost portion of the globe do we see drop stones that imply the presence of periodic glaciers in the middle Triassic we have less data but still see a broad arid region around the center of the globe with warm temperate and wet environments at the poles in the Jurassic as Pangea begins to break apart we see warm tropical environments in what today is Asia and persistent arid conditions across what will be North America South America and Africa in the early Cretaceous Pangea continues to break up and we see the expansion of wet conditions with the preservation of minerals that occur due to chemical weathering in warm wet environments and more coal is deposited throughout the highest latitudes by the late Cretaceous the air conditions are isolated to North Africa parts of South America and Asia and we see warm wet conditions throughout North America and dotted across the map notably missing from these Maps accepting the Permian map at the start we don't see glacial tills or striations or drop stones if we Zoom back out to the phanerozoic and look at a timeline of when we see glacial drop stones in the sedimentary record we see very few instances in the Mesozoic the y-axis on this graph shows the latitudinal extent of glacial deposits and when we do infer glaciers in the Mesozoic they are restricted to the highest paleolatitudes the time we live in today is quite unique in that the latitudinal extent of glaciers is much more equator word than during most of Earth history and we have not seen this much ice since the late Paleozoic some 300 million years ago we have other climate proxies as well and instead of relying solely on sedimentary rocks and their fossils we can also examine the chemistry of fossils to determine changes in temperature one commonly used proxy examines the ratio of two stable isotopes of oxygen these Isotopes are stable meaning they do not Decay over time like the Isotopes we use for radiometric dating the auction isotope palear thermometer relies on the ratio between o16 and o18 which differ in their Mass we call this ratio Delta o18 because o16 is less massive it tends to be preferentially evaporated from bodies of water and thus is over-represented in precipitation if this precipitation snows out where Glaciers are forming that o16 becomes trapped in the ice and the ocean becomes relatively enriched in o16 yielding higher more positive Delta o18 values the shell-making organisms that live in the ocean and make their shells of calcium carbonate thus have relatively more o18 available to them to make their shells during cold times with high ice volume thus we may measure their fossils we see that they have higher delto18 values when climates are warm and the o16 of the precipitation is returned to the ocean because there is less ice to trap those Isotopes on land in an ice-free World there is still a relationship between temperature and Delta 18 such that warmer conditions will have lower Delta 18 values there have been calcium carbonate shell making organisms throughout much of the phanerozoic and so we can measure the oxygen isotope composition of those fossils to reconstruct temperature from the Cambrian to today most different organisms are common at different parts of the thanozoic we have to switch the organisms we use which is why the points in this graph are changing colors we can see that the Mesozoic is relatively warm compared to today and is also relatively warmer than the lake Paleozoic this corroborates evidence we have from the lack of glacial deposits and give us confidence that the Mesozoic was a warm interval in Earth's history but like all intervals of Earth's history the climate was not constant and if we zoom in on the Mesozoic we see an oscillation between relatively warmer and relatively cooler times today if we look at the diversity of organisms we see the diversity is highest in the tropics where there are warm wet rainforest environments given many of the land masses of the Cretaceous had a similar environment we might expect the diversity of terrestrial animals was high at this time because this environment increased over the course of Mesozoic we might expect to see the diversity of dinosaurs increase along with it another feature of Mesozoic paleogeography is the continual fragmentation of the land masses from that large supercontinent Pangea to the more smaller isolated continents of today we can quantify this fragmentation through the phanerozoic I can clearly see that during the Triassic and Jurassic Continental fragmentation was at a minimum and fragmentation increased rapidly from the late Jurassic to the end of the Cretaceous this separation of continents and emplacement of oceanic Pathways that serve as barriers to the movement of land animals isolated their populations and restricted the ability of animals to breed with populations and other land masses these Geographic barriers can promote speciation because as populations become reproductively isolated their Gene pools become separated and each population can involve unique characteristics if this breakup is driving speciation among vertebrates on land we would predict the number of species on Earth or its species richness would increase to the Mesozoic and when we look at the fossil record and ask how many tetrapods we find in assemblages we do see a general increase in diversity from the early Mesozoic to the late mesozoa but keep in mind that the preservation of fossils also plays a big role in the diversity that we see the changing connections and disconnections among continents not only isolate populations of dinosaurs driving driving speciation but also isolate whole dinosaur faunas and let each disconnected land mass evolve its own unique species this creates a world in which dinosaur faunas have shared evolutionary histories with groups that exist on the same land masses but evolve independently from those on other land masses and we see this paleogeographic history reflected in the geographic distribution of the dinosaur fossils we find today for example in the late Jurassic land masses containing North America Africa South America all the way to Australia we're connected and walkable north to south but they were disconnected from Land masses that now make up Asia this isolation leads to a dinosaur fauna that is shared between North America and those southern land masses that make up the paleocontinent gondwana but as distinct from the dinosaur faunas on the Asian continent if we move forward in time to lay Cretaceous we see a different configuration of continents with connected continents in the northern hemisphere that are separated from continents in the southern hemisphere so now when we look at the similarities among dinosaur faunas we see a northern hemisphere fauna that is distinct from the dinosaurs of the southern hemisphere taken together we've seen that the changing Continental positions and changing paleo environments through the Mesozoic kept the world in which dinosaurs were living and evolving in a constant state of change these changes impacted where we find dinosaurs the diversity of dinosaurs we find and the biogeography of different communities of dinosaurs in the next Lab module you will use the paleobiology database to map where key fossils important for recognizing the past positions of continents are found these fossils also provide evidence to support the claim that there was a supercontinent in the late Paleozoic that broke up during the Mesozoic you will then consider how the changing positions of land masses and oceans affected the biogeography of dinosaurs using data from the fossil record to help you get familiar with using the paleobiology database there's a mini assignment and tutorial video to view before you move on to the laboratory module