The Cambrian Explosion and the evolutionary origin of animals with Professor Paul Smith

welcome to to everybody and what i want to talk about tonight uh is the origin of animals this event that we call the cambrian explosion and and really look at what we mean by that and then focus in on one particular locality that as jack says um i've spent quite a bit of time looking at as part of my my greenland research so you'll be familiar with these references to the the first animals exhibition now sadly behind locked doors in in the museum of natural history in oxford but fortunately it's all available online and if you want to explore the the fossils that i'm going to talk about and the environments that i'm going to talk about in a bit more detail then go and have a look at that website it has most of the exhibition uh content and i'll repeat that url at the end of the lecture so what we're going to talk about tonight is not a new question i wouldn't pretend that in fact it was one that darwin identified when darwin published on the origin of species in 1859 in november 1859 just six months before the museum opened in oxford he recognized a real difficulty that the fossil record posed for him with his new theory of evolution by natural selection and that is as he said there is another an allied difficulty which is much braver i allude to the manner in which numbers of species of the same group suddenly appear in the lowest known phospholiphouse rocks so what he was trying to say with that was that clearly there must be some evolution that's missing in the rock record if multiple numbers of species of the same group appear at the same time rather than spaced out through time and through the rock record so it was a problem that darwin was never able to solve in his lifetime and on and off it's occupied scientists ever since 1859 but with a real acceleration in in the last few decades and a huge amount of research effort has been uh put onto this question of the origin of species um and really just in the past couple of years i think we've begun to make some real inroads into to understanding what the questions are and what the different answers to those different questions are so if we think of a modern marine ecosystem if we think of a a place on the planet with great uh animal diversity we might think of of something like this uh a modern reef with lots of different species of fish very visibly of vertebrate animals the groups in which we belong but also of corals and lurking in those crevices and crypts a lot of other species representing different animal groups but if we go back before the cambrian it's very different for the first three billion years of earth history an equivalent photograph to that would look something like this it's a microbial film growing on a sediment surface and you might be able to see that there are little um pustules um these little bumps that are visible and that's where decay gases from older um mats as they rot away are pushing up against the under surface of the mount that's methane bubbling away but there are also other bubbles one here and that's a bubble of oxygen being released by by photosynthesis so these are photosynthesizing microbial mass created by cyanobacteria or blue-green algae as we used to call them and they're building these films as storms rip across that surface they rip up flakes of the sediment and transport them so it's beginning to turn into rock it's beginning to semi lithify certainly consolidate such that it can be reworked by by current action and and that was it that was marine ecosystems for the better part of three billion years until animals came along and changed things really radically and now these sorts of microbial systems are are not very common they're present in just a few places around the world you might ask how we've got a photograph of a precambrian ecosystem this is actually a puddle in the english lake district a long-term puddle that doesn't dry out and just happens to to mimic those those environments so we can get an understanding of the processes that happen but if we go to the rock record and have a look at what the rocks tell us we've got similar features so these are now pre-cambrian rocks on the east coast of greenland about 74 degrees north on an island called ello and and these are microbial films this time they've actually grown into a reef um so we can get quite complex structures developed from microbes and bacteria and if we zoom in uh that stick is a meter long the yellow stick if we zoom in we can see that in detail what we've got is a series of columns of these bacteria competing with each other and as films and growing up towards the sunlight to capture as much sunlight as possible for their photosynthesis so this is typical of rocks about 800 million years ago and if we zoom back out of that scene um what we've got here is a pretty complete succession from from 800 million years ago up to about 450 million years to what we would call the division so we go through the end part of the precambrian into the cambrian and those distant hills are the the lower part of the ordovician and it's tropical limestones almost as far as the eye can see except for this cliff here and this cliff here and in those two cliffs something very different happens that is really significant to our story because in those cliffs we have rocks like these not looking very limestone at all these are actually two lights they're the deposits from glaciers and the red blocks that you can see are granites the gray blocks um our limestones the slightly yellowy colored blocks are sandstones they've all been scooped up by glaciers and deposited so what we're looking at here is a major glacial event similar to the one that happened in the northern hemisphere until 10 000 years ago but if we map these localities out they're absolutely global and it's an event that we call as it says there snowball earth and reconstructions of snowball earth events um look something like this an entirely snowy planet there's a bit of debate over how much open water there is in fact there's a paper out just this week arguing there was probably rather more open water and and less glaciation than many people have thought but to all intents and purposes the planet locked solid for tens of millions of years it's actually really difficult to unlock when you do that what you need to do or the planet needs to do is wait uh until enough volcanic heat builds up from mid-ocean ridges and that gradually builds enough of a temperature differential because as a frozen planet uh it bounces off all of the sunlight so there's very little heat gain from from solar radiation so if we look at the geological record of these snowball earth events we begin to see an interesting pattern they really terminate this this long period of microbial dominance that's gone on for it for three billion years there are three main events um at the end of the pre-cambrian uh the sturgian from 720 to about 660 million years uh the marine owen from 650 to 635 there's that rock section that i showed you uh was sturdian with mario known on top and that close-up of the rock was the marinon and they're global events they're genuinely global events um there's a third uh glacial event the gas gears which is less global but still a really significant glaciation event uh and that's at 518 that leaves a gap of just 40 million years between the gas gears and the base of the cambrian and it's in that interval as we've heard in earlier lectures that the interesting things begin to happen we begin to see fossils like these and these are the first evidence of really abundant multicellular life existing in quite complex ecosystems and we've we heard jack matthews talk about these in the first lecture of the series and then frankie dunn talked about them a few lectures ago and they've been very enigmatic for a long period of time for many decades since their first discovery but there's now increasing evidence that these are actually primitive animals and one of the lines of evidence comes from from frankie dunn's work that she talks about a few weeks ago looking at how these organisms grow and then relating them to the growth patterns of animals alive today and and using that technique of developmental biology frankly was able to to determine uh the the most by overwhelmingly most likely position on the evolutionary tree for these ediacarans as we call them um organisms multicellular organisms from ideocar and time is that they sit down at the base of the animal tree and there are other lines of supporting evidence including preserved organic molecules are beginning to back that hypothesis up if we roll on a little bit further through the idiocar and through that post-gasculous glaciation time we then begin to see things like this these are animal burrows and from their pattern we think that they must have been made by uh organisms one-like organisms uh that have a plane of bilateral symmetry down the middle of the body like ourselves so that's the majority of animals except for jellyfishing and enemies and things like that and sponges so some evidence of slightly more advanced animals beginning to come in as we creep towards the end of the idiocarron and then as we heard in the last lecture from duncan murdoch right at the end of the ediacara we begin see evidence for animals making shells and making what we call biominerals making crystalline inorganic mineral as as part of their metabolism for either protection and or um predation so if we put all that together into a single um overview of geological time this is quite an old diagram now but it it still holds true in in its essence that immediately after the gasket's glaciation we get the incoming uh of the idiocar and biota the idiocar and fauna and at least some of those are animals we can be pretty sure of that then a few million years before the base of the cambrian we get the first trace fossils that we think were made by animals and then we begin to get animals with skeletons coming in in the last few million years right before the camry so you might ask if we've got good evidence for animals prior to the base of the cambrian what's the big fuss what why is it called the cambrian explosion what is the cambrian explosion it's an event that replaces environments like this this is a reconstruction that was part of the exhibition by martin lysak of that newfoundland biota that frankie and jack have described um and here we've got these early animals um sitting on microbial mat still very microbial mat dominated um in quite deep water and they're replaced by really quite dynamic ecosystems at the base of the cambria and so we can define the cambrian explosion in a number of different ways in green here we've got the labeled eb we've got the ediacarans the idiocar and biota and they have a reasonable number of different species attacks particularly towards the end of their their time range but then look at that really abrupt transit translate transition into the cambrian phyla animal fibre really reflect the highest level of body organization they for example our own group is the core data the echinoderms includes the sea urchins and starfish and they're unified by very very broadly similar anatomies at the next level down classes we're looking at still very distinct groups of organisms but for example the sea urchins form a class the starfish form a class the vertebrates our own group form a class so what we can think of when we think of the numbers of filing classes is the number of different body plans that are present and what we can see as we cross into the cambrian is a sudden explosion in the number of different body plans that are present so it's a spurting evolution it's not the origin of animals but it's the establishment of many of the body plants that we recognize at the modern day when we go down to the beach or when we go to an aquarium and you'll notice that that increase very much stabilizes as we go through the cambrian and into the ordination since the division there have been very few new phylum or classes added at all um to to the the animal complement so that's one way of looking at the cambrian explosion another way is to think of it in terms of ecosystem change so we talked in that reconstruction by martin lysak of of the ediacara and biota on the left um sitting three five four two million years ago the base of the camera sitting on these microbial mats a largely passive existence there were some that could move across that sediment surface but not many across the camera in boundary and as it says here they give the fancy title of the agronomic substrate revolution what it basically means is that animals become much more dynamic and the interactions between animals become much more dynamic we get organisms burrowing down into the sediments and beginning to engineer the sediment we get animals swimming up into the water column and these are newly evolved lifestyles that are associated with the cambrian another way of looking at it is to focus in on those trace fossils those burrows um and the way that organisms interact with their with the sediment and that's something that that gabriella mangano who will talk later on in this lecture series uh did in a paper a few weeks ago looking at the number of different trace fossils that we get uh as we go uh from low down in the the uh the precambrian crossing into the cambrian and look at the number of different trace fossils that begin to appear and the way that they're beginning to engineer the substrate they're beginning to bio engineer their own habitats and that enables a number of different processes to interact and it increases the diversity of organisms that can live in those spaces another way this is quite a complex diagram but i'll just steer you through the essentials another way of looking at it is in terms of the way different ways that animals build their skeletons that we've just mentioned and this was what duncan murdoch talks about in the last lecture so here we've got geological time from oldest on the left and the bar at the bottom through to youngest on the the right and we've got an evolutionary tree of different animal groups and sitting on its side above that and the different coloured bars represent different types of mineral that animals use and it's a painstaking assembly of a big data set but what we can tell from this is a that biomineralization the creation of skeletons happened multiple times different group b different groups chose different minerals in an evolutionary sense some chose calcite like starfish some like ourselves chose calcium phosphate and appetite a few like the sponges um chose uh silica very few animal groups once they've made that evolutionary choice remake that choice it became becomes entrained in their developmental biology and so if organisms begin to um make their skeletons out of appetite they carry on making out of appetite and we can actually break it down still further into different types of calcite as well different types of calcium carbonate and one of the other points the is that the evidence of use of hard tissues of skeletons for predation appears at more or less exactly the same time as that for protection so it's a co-evolution it's an evolutionary arms race between um hard tissues evolving for predation and heart tissues evolving for protection so that's another way of looking at the cambrian explosion and these are all equally valid ways of defining the cambrian explosion most recently people have begun to think of it not in terms of individual aspects but of really looking at a unified interdisciplinary approach and again this is by rachel wood who will talk later on in the lecture series and what she's done with phil donahue and other co-authors is try and unite different data sets to look at patterns and processes across the base of the cambrian and the base of fanner the phanerozoic as it says here the top of the neoproterozoic on the left so what she's trying to do here is unite the evidence we've got from jeans on the left informing us about evolutionary trees and um about the timing of the origins of groups with geochemical evidence in the middle with the sedimentary evidence the the burrowing evidence um indicated by the biotition index with the fossil evidence the the largest and the exceptionally preserved fossils and if we begin to look at the interaction of all those processes we're likely to get a much better understanding of the dynamics of the cambrian explosion this is something that dave harper and i tried to do a few years ago i mean in the green boxes we we've got biological processes in the blue boxes we've got geochemical processes and the red boxes we've we've got geological processes and although it's a very complex diagram what it indicates is that all of these processes are interacting and they're interacting by what we call feedback loops um where the process goes round and round and begins to amplify the signal so this is another way of these these interacting processes of looking at the cambrian explosion how can we get evidence for all of this what what's what are the lines of evidence well one of the key tools that we have is that we're very fortunate in the cambrian to have a lot of localities with exceptionally preserved fossils and that's really the theme of the first animals exhibition we call them lager statin the same uh word that informs the the beer lager storage and preservation um there are many localities and i've just chosen some of the key ones here um we'll hear about the burger shell from derrick briggs in a few weeks time and about the changing which is also part of the exhibition uh also in a few weeks time um but what i want to do is focus today on the the least well known of these largest and the the serious passage at biota of greenland this is a picture from the the locality an exquisitely beautiful spot right up on the north coast of of greenland um discovered at about the same time as the chiang jiang biota but but far less known because people have been able to visit it much less often and get much less material out now i want to spend a little bit of time talking about where it is the discovery um and and how we built up knowledge about serious lagos static many people will be familiar with the story of the discovery of uh or at least the apocryphal uh story of the discovery of the burgess shale uh exceptional preserved deposit when a geologist's horse charles doolittle walcott's horse stumbled um and he found the fossils and he traced them up the hill and found the currency that they were coming from and the discovery of the serious lag status is not quite so romantic but but it's a story worth worth looking at this is where the locality is right up on the northern tip of greenland there's virtually no land further north than sirius passage there's a tiny little bit of greenland and a the smallest sliver of canada on the northern coast of ellsmere island um but the rest is is just frozen arctic ocean right the way out to the north pole we zoom in on that bit of land at the northern tip of greenland um it's difficult to comprehend the scale actually the the locality is is over here this is serious passage indicated by the uh the red dot um the nearest human habitation is station nord right over on the east coast that's a a weather station uh and that's actually 400 kilometers away so those are the nearest human beings uh this alert in helsmere island is about the same distance and then going northwards um the north pole is about twice that distance uh going off the top of the diagram uh from the red dot on the left the red dot on the right about 800 kilometers to the north pole the serious passive locality is situated in in the world's largest national park and they this next slide gives you a bit of a feel for for the scale of the terrain we're looking at so the northeast green the national park is a shade over 970 000 square kilometers it occupies the red area of greenland difficult to envisage of course what 970 000 square kilometres looks like but if i was to tell you that that red area is equivalent to the area of france plus germany plus the netherlands plus belgium that gives you a feel for the scale of the northeast greenland national park um and it's relatively recently explored the early exploration um was summarized by laughter um in a beautiful series of maps in um in 1940 and he looked at the survey of north greenland and this was his summary of um the the total knowledge of the north coast of green greenland you'll see that the the coastlines were beginning to emerge at this point probably only 10 people had ever been to this locality uh surveying it and mapping it of course there'd been lots of indigenous peoples who passed across this area at earlier stages but the map was only just beginning to come together in 1914 you'll see that our red dot is plonked right in the middle of a blank piece of land and it was only really with the the advent of aerial photography and then satellite imaging that we've really begin to begin to get detailed maps of of this area the sirius passa lacalla the fossils were found by these two gentlemen they they were found during the uh the systematic geological surveying of north greenland um that i was fortunate to be part of just after my my phd um tony higgins a british geologist but based for most of his career at the geological survey of greenland in copenhagen a structural geologist but one with uh a a really encyclopedic knowledge of geology just as happy doing basin analysis and sedimentology as he was making structural geology maps and he was working uh with jack super who's as well known to the the rock climbing community he basically was one of the founders of modern rock climbing but but also um in the 1960s was part of one of the last great heroic sledge survey um expeditions sledging around the north coast of greenland making a geological map as they went and they were working together and over the radio one evening in in july 1984 we heard them very excitedly saying that they found fossils and that they'd found a load of trilobites and a bunch of grass we tried to decipher what the bunch of grass might be in in fossil terms it definitely wasn't a bunch of grass it turned out actually to be a sponge um and so jack went back the next season in 1985 and collected some bulk samples and on the back of those bulk samples sorry tony went back in 1995 and on the back of those samples john peeling in copenhagen one of the paleontologists at the greenland survey at that time recognized soft bodied fossils fossils with preserved soft tissue in a similar style to the burger shale and that led them john peel to get the money together for an expedition to go and collect the serious passage uh locality for the first time um and that expedition took place in in 1989 and i was part of that expedition there was a bit of a logistical challenge the the geological survey was mapping a thousand kilometers further south on the east coast so we were really stretched in terms of the logistics um we knew that there was a little landing strip here um but the locality as it as you can see from the red dot is over here that's 25 kilometers away so we if we could only land at that uh brainerd brainerdson's airstrip we would have to take all the equipment 25 kilometers across what became known as sirius passes it didn't have a name at that time um and then all of the fossils all the way back again so we were quite keen to find uh an alternative solution um and that's where the the icelandic pilots of the twin otter aircraft short takeoff and landing aircraft really came into the fore because we took all the equipment off at brain arson and then went looking for a locality um where we might be able to get the twin otter down with the equipment um and we circled around we circled around we circled around uh the pilot would go in on a likely looking strip and thump the wheels in to see whether it would support the weight of the aircraft when it was landing put the engines back on spiral rounds far around thumping one of the more nauseating experiences of my life and in the end they told us to hang on um and they landed the plane and it clattered to a halt in literally 50 meters now normally um if we're working in these remote areas the ideal solution as we had last summer was to work with helicopters but this time we only had the twin otter aircraft to support us so that meant that we had to get both in and out using the twin otter aircraft and the place the icelandic pilots managed to land it um was right on the limits of what these things can land up now you might think this is an artfully posed photograph um of with holding the camera at a tricky angle but if you look the pilot is standing perfectly upright and it's the plane that is a slightly wonky tilted to the right landing uphill angle um they managed to get us down but it was made clear to us that there was no way um that they'd be able to land there again and take us off with all of the rock so one of the next challenges is was that not only did we have to find the locality but we had to build an airstrip before we could get back out again as well the locality we knew was somewhere in those hills in the far distance boom um in pre-gps days and we didn't have an extremely precise fix on where it was so the plane dumps us with all of our gear about a ton of equipment and here's john peele who raised the money and led the expedition waving the fund farewell and then we had to get all of our equipment across to a camp where we might be able to build an airstrip this was the the photograph of the team and the team for this first exploration um was john peel on the left then of the geological survey of greenland but um later university of uppsala in the white coat simon conway morris university of cambridge um in the middle next to simon paul schuller um who at that time was with the geological survey of greenland as well but but later went to work for the geological survey of new zealand and then me on the right and that's about a ton of equipment surrounding us that we had to get across uh to to the locality uh before we could be picked up so this is an indication of of the terrain the locality is really easy to spot on aerial photographs um it's this white black and white stripe over on the right hand side um that you can see there and the plane put down it doesn't look far it's only five or six kilometers uh but you'll notice that there's a somewhat um substantial river that comes down between the two and builds this big delta here um and quite a lot of water spilling out and melting the ice um in the field so we then had to navigate that the river's not insubstantial particularly in the middle of the day when the melt is is at its highest and that's a very perplexed hands in pockets me uh wondering how on earth we were going to get across john p had come up with a fantastic plan of building what he called tundra trolleys we took them up uh broken down and were able to quickly assemble them um and it turned out that these were quite a good way of getting across the terrain they're not ideal for getting across big rivers and so the only way to do it was basically heft we spent an entire day my notebook says that we spent eight hours of continuous work to move the camp 200 meters uh and it really was absolutely exhausting work because once we got to the other side of the river we then had to get it up the hill on the left hand side on those trolleys once we were on the top of there it was relatively easy going and within a few days we were able to find a nice horizontal camping spot that's the the first of the trolleys that we dragged across and we were then able to go and actually excitedly look for the the serious passive locality up on the hill above so this is us on the very first morning heading up the hill in the vague direction of the locality that tony higgins had described to us not knowing exactly where to look and just combing the screen as we went for any fossils and as it turns out the very very first fossil we found is one of the most iconic serious plastic fossils as we were walking across some screen we noticed this particular specimen and we now know it's a very primitive mollusk but a very unusual mole it's got a shell at either end and these um calcium carbonate spicules covering the body so it's a slug-like animal um maybe five or six centimeters long and was really the sign that we we'd honed in on the the locality uh and this was the locality in 1989 a really snowy place uh working underneath this overhanging cornice that kept breaking away and a pretty chilly place to work as well and it could not look more different these days um this is us trudging up to the locality a little bit later on in in the expedition and the locality is is over here um over on on this hill um but you can see that this isn't just a temporary there is a recent snowstorm in this photograph but most of it is unmelted seasonal snow and and now the locality looks like this it's grassy um the snow is more or less all gone there's virtually no permanent snow you can see for scale jacob vince from university of bristol they're just beginning to carry down some fossils there and so 20 years of rapid polar melting and climate change really has changed serious concept beyond all recognition but it was a pretty snowy cold expedition um and that meant uh lots of time sat in in the tent um so again the expedition team paul schiller john peele me and simon conway morrison he noticed the old greenland hands get the end of the tents that a has the candle b has the bottle of whiskey and then on the as the weather improved there was still the small matter of building a runway so that we could get out of there before the winter arrived and so one of the unlikelier entries on my cv uh is runway construction airstrip construction but that really opened up the locality for future visits because now um we had a camp with good water supply we had an airstrip right next to the camp um and it was a couple of kilometers walk and 400 meters of ascent and up to that next locality up to the locality and this is the runway a few years ago um ready to receive a door-to-door service of twin otter um more or less landing on top of the tent and picking our gear up the next time i was able to go up was 2009 in in the interim john peel and simon conway morris went back um in 1991 and 1994 and then um another short trip in 2006. and we went up in 2009 and 2011 and a south korean team with jakob winter and university of copenhagen have been up in uh 2016-17 so still only eight short trips up to syria's passing which is why we know it uh not as well as as the other cambrian lava setting so some geology back to some geology um we took a systematic approach um in in 2006 and 2009 that's been carried on in the the later expeditions and mythology had arisen that you could only find the fossils in the screen which is not true they're they're just as abundant um but slightly harder work to get out of a solid rock so whilst jacob winter on the right um scours for for exceptionally preserved fossils uh emma hammerlund on the left our geochemist at the university of london in 2011 um is logging a section with me um and as we construct the sedimentary log um she's collecting the geochemical samples that would be uh critical to our understanding of the environments in which uh serious past animals were living and which we will come on to and then once the section was logged it was basically uh rock breaking chain gang stuff um and so so this is the main expedition in 2011 emma hammerlund on the left martin stein anna torto nielsen head down in the blue jacket uh yanaud and rasmus and jacob vala and dave harper recording the notes and around the corner sitting on the naughty step uh jacob so that was a big team that meant we were able to make a lot of progress and and really begin to to to tear apart um the serious passive locality both in in terms of its fossils but also uh in terms of its paleo and environments this just gives you a feel for the field work um but still we're able to take out relatively small volumes of rock because we're constrained by twin otter palette so what fossils occurred at serious passage this is um in a way one of my favorites because it's emblematic of a serious passage uh hulkeria the fossil i showed you first is the poster child of the serious pastor locality um but of the soft-bodied organisms that have real detail in their anatomy uh arthro aspis here with a 20 centimeter notebook for scale um is is perhaps uh really emblematic of that locality and this is a just a composite plate of the the range of of organisms um this this is is the trilobite uh bouinellis higgins eye named after tony higgins who discovered the locality these are the sponges um that were probably the bunch of grass um and what i want to do is just take you through some of the components of the fauna and to give you a flavor from as it says there there are 45 species and counting but uh one of the tricks that paleontologists use is is to to create what's called a collector curve and to see whether you're you're beginning to carry you you're still collecting but you're not finding new species um and we're we're still at the point where we're finding new species so there there is more to come out of serious passa that's for sure it's not just that it it's it's a formerly the corporate locality it may not be as diverse as as uh burger shell and chen chang but it's certainly more diverse than 45 species uh this is a reconstruction first of all of helkiria just to show you you know how slug-like it is with this armored upper surface um and and this foot on the under surface mouth on on the right um and we'll look at where that was living and how that was making uh uh how again it's nutrition in just a moment one of the other common fossils of the the soft-bodied fossils those when we talk about soft body fossils we mean those that don't have skeletons uh mineralized skeletons um what we call logopods they're most closely related to peripherals the velvet worms of the modern day um and on the left and on the right we've got two of the different lobopod genera and species and we've been able to add more detail to um kerrygila from material that we collected on the recent expedition that's allowed a revised uh anatomical reconstruction that you can see here that we we published in nature communications a couple of years ago um of course one of the most um configured organisms that you see in both the virgin shale and chang um here illustrated with with lagania and these these were the apex predators of cambrian seas uh growing up to a meter two meters long um with this radial mount they belong to a group called the radiodance um and these appendages uh stuffing food into the mouth but but not all anomalocarines were fierce um and the one that we has been described by by jakob and colleagues uh from sirius pasa is actually very different in terms of his feeding note to that and you'll see on the left that it's got um these feathery appendages that stick off uh the the main arm of the appendage um and that this appendage could be flexed and curled up uh and the reconstruction uh that jacob and colleagues thought most appropriate was that actually it's using these as a mobile sieve and that to messiah carys was actually the cambrian equivalent of of a baleen whale that it was wafting these giant appendages through the water sifting out plankton and putting them back in into the mouth and then swimming alongside um we have some of the other characteristic organisms of a serious matter so symmetric occurrence was large and multi-meter long and gigantism is is something that we see in in serious past and other cambrian localities um this is a thing called syrilorica uh described by john peel um it it's most closely his closest living relatives that we will be familiar with things like nematode worms uh begins to it belongs to a group called the loriciferons um but the thing about laura suffers at the modern day is that they're tiny if you look at this scale bar down here oops try again scale bar down here that's 25 microns um 25 thousandths of a millimeter whereas siri laurica is tens of centimeters long so the cambrian representative although anatomically really very very similar his orders of magnitude bigger the group that was swimming alongside semester cars is the group that i'm perhaps most interested in in syria's past the the veterans but a lot of debates about what they're most closely related to here we have on the left we've got a species that jacob dave harper and i described in 2011 uh dejira and it's a very characteristic veteran we've got these these gill slits opening down the side we've got a large um oral opening a large mouth so we think that the water was flowing in through the mouth and then out through these gill slits but at the back of of that chamber um is a tail that we can see is segmented and capable of flexing um so we think this was capable of free swimming um as the model um by uh has been horn in 10 times indicates on the on the right uh and here for comparison is um an example from chen zhang so this is where veterans were first described from you can see very clearly here the oral opening um as this slits have been has been flattened on the left and and here are the gill slits where the water would come out so this thing was taking in large volumes of water again it was filter feeding um and it was almost certainly taking the food out of the water on cilia and fine hairs that lined those gill openings down down the side but propelling itself through the water column to get the maximum volume of water and food ins into that chamber that open chamber um if we reconstruct it and again this is one of martin lycek's reconstructions of a cheng zhang fossil this is what it would look like some protection at the opening so the mouth and the right so it could close it off and protect it but to feed it would open those up um and then the gill slits down the side and the mobile tail um the exhibition website also has some some lovely animations um and swimming and feeding and these are really quite abundant they're present in quite large numbers um they haven't been found uh in the first few expeditions but uh once you get a search pattern in your eye they're quite badly decomposed often um and so we now have many hundreds and we're in the process of writing those up and describing them so where were all these animals living what's the environmental setting that we were trying to understand through that sedimentary logging and geochemical sampling well this is a simplified geological map of north greenland the sirius pass locality is in jp fjord which is um is just here you can ignore the red box that's for a different purpose but the key point take a message from this summary geological diagram is that we can divide the geology into two parts we've got the pale blue rocks to the south now those are cambrian or division and silurian shallow marine deposits continental shelf deposits mainly limestones and deposited in for the most part a few tens of meters of water depth but then we go over the edge of the continental margin onto the continental slope into much much deeper water quite abruptly into deeper water and there um we get mainly the deposition of fine-grained sediments and some sediment that is is pouring off the edge of the plant so sirius passes is pretty much on the boundary between the continental shelf and the continental slope or as it says on this diagram the shelf and the trough if we draw a cross section pretty much um parallel to the eastern side of that red box it looks something like this so now we've got a vertical cross section through through the earth's crust and through the the sedimentary deposits um and sirius passa is located here so it's right on that flexure um as we go from from the horizontal continental margin on the left down into the deep water on on the the right um and you'll see that on the left it's indicated that there are sandstones there's continental shelf sandstone similar to what's deposited in the northern sea and west of the british charles of the modern day we then go into finer and finer deposits as we get out of the range of the sediment supply um and down into mudstones and the the serious plastic locality have actually drawn this arrow a bit too high sits just above that boundary that transition boundary and that we can see there so it's at a point of transition um and a point of change in sediment supply is the first thing that we can conclude if we zoom in and look at the geological map that we we made um you can see that point and emphasize so in pale blue we've got the the older rocks that the sirius passa is sitting on the serious passer locality is actually within a geological unit called the boon formation and the boon formation itself changes quite radically as it goes over the edge of the continental shelf so in the south in yellow we've got the typical shelf sandstones that we saw in the last diagram in the north in the pale green um we've got the the trough mudstones um and in in the two darker shades of green um we've got the uh the transitional boon as it's sometimes been called um which is where the serious passive locality is located and everything's been a bit compressed because it's been scrunched up by later geological activity but we can still disentangle um really quite easily that original depositional replay relationship um which is much more difficult to do with some of the cambrian localities so we've got a lot more um sedimentary information and paleo environmental information to play with slightly over printed by metamorphism these rocks have been heated up to a few hundred degrees so the minerals have begun to change but we can still do the sedimentology this is the locality looking eastwards again we've got these um shelf sandstones of the boon in b on the right we've got the older portfield limestones which is what everything sits on the locality is this black and white stripe here um then we've got the transitional rocks um of the shelf slope break and then we go out into what called the frig view of mudstones which are the the trough mudstones if we just zoom in on the locality itself this is what it looks like so this is the serious passive locality you can see um here the you see here the uh the small quarry that has been opened up to to extract the fossils and to measure the sedimentary section the these arrows here are actually younging arrows and they v in the younging direction so in the white rocks these are younging upwards that's as you would expect rocks to be the serious passive locality is actually upside down it's been inverted by this um later geological activity but we can still read the rocks pretty easily and this is what the locality looked like on that first trip before we began digging away at it so we can see the rib of the most fossiliferous material heading down the hill to simon conway morris in the blue down jacket um and if we zoom in on on that sediment it looks something like this it's pretty undistinguished as setting up goes it they're very thin platey dark gray to black mudstones but one of the really distinctive things is that they're coated in these yellow powders and these are iron oxides and hydroxides that are the result of the the oxidation of pyrites iron sulfide and that's a a really key part of our story so we put all of that together into a sedimentary log it looks like this it's a very thin section and the fossaliferous section is even thinner so um you can see that in the darkest browns those are mudstones uh going through to the palest brown stones through intermediate silty mudstones and muddy silk stones it as for those of us who make sedimentary logs this is about as dull as it gets frankly but really informative in terms of the environment you'll see that in the label uh i've mentioned that there are abundant microbial mats and that's a key feature of the locality in red we've got the horizons from which soft tissues are are recovered and in yellow we've got where these rusty sediments are these yellow powders and on the extreme right um we've got what the sediment looks like as we've just seen but then at the bottom right a thin section through one of these individual mudstone layers and you'll see that it's white and black stripe but that from where my pen is now up to here it goes from black a sharp base black gradation is in white gradationally into black and what that's telling us geologically is that these are quartz grains and those quartz grains are progressively replaced by mud so in other words we would say that this is fining upwards and that's indicative to geologists of decelerating currents we interpret this as being the result of sediment slurries that are going across the continental shelf perhaps kicked up by storms and and depositing these very thin millimeter thick um horizons but they're transporting animals with them as well um so that leads us into um a distinction in the fossils that we find at sirius passage we can identify as i mentioned microbial mats and these are uh readily recognized by the fact that we can see burrows running parallel to the mat so horizontal burrows that pop up and go down up through the the mats um and if if they're going underneath the mats they pop the mat up just like um jerry would in a tom and jerry cartoon um but then if they pop up onto the mat surface they make grooves so you can see that this is a a positive relief burrow this is a negative relief borough um and that's and we can see the transition from one side to the other as we follow burial traces um these often occur around uh fossils they seem to invade any decaying organic matter you might just be able to see in this picture um segmentation this is one of those large arthrosis arthropods that i showed you a few slides again now living on those microbial mats is a very distinctive fauna that we call the microbial mats guild so um top right we've got one of these uh distinctive arthroaspis being mined for its nutrients by these these burrows the worms making these burrows um we've got polykey worms that are living on the burrows not burrowing into it no vertical burrowing um here we've got one of the arthrospas in the middle a halkeria almost certainly grazing um on on the microbial mats top left and then a sponge sitting and filter feeding on the mat bottom left so that's what we call the microbial map guild and they're preserved at the base of these little storm layers underneath the course of sediment they're smothered they're killed by the input of these plumes of sediment um from from the storms within the thin sediment layers carried by the storms are the rest of what we might call the typical cambrian fauna um we've got bits of uh phenomenal normal aquarius and the the tumisia cars that we mentioned we've got um free swimming pollocky annelids we've got vegetalicolions free swim vegetalicoliums but it's also ripping up burrowers um as well and some things living on the surface so these are preserved in a much more typical burger shale type way than the things that are living on the mats which tend to be preserved more in three dimensions we can then tie that preservation in into the geochemistry and this is quite a complex diagram but i just want to focus in with this column on the left next to the sedimentary log so by looking at a number of deposits a technique has been developed to calibrate um what's called the fe hr the highly reactive ion to fet total ion ratio so iron in sediments occurs in lots of different forms but the highly reactive form of iron is the the iron that occurs um in the oxides the hydroxides and the sulfides now that's used as a proxy for the oxygenation of the water column if the fe hr fet ratio is more than 0.38 we can say that that water column where the sediment is is effectively without oxygen it's what we would call anoxic between um 0.2 and 0.38 it's very low in oxygen uh we would say that that's hypoxic or or dyson you'll see both terms and slides coming up um and you'll see that the the fauna that's living there and being transported a short distance we don't think any of the phone is traveling very far is quite frequently within that low oxygen interval and the low oxygen intervals are indicated by these horizontal rulings so how does that compare with the distribution of fossils well actually if we open that diagram out and we look at the abundance of fossils and the diversity in red so this column here the abundance and in red the diversity and also um the representation of the most common fossils in terms of the the number of um of specimens we were found in finding per unit rock area and you can see that the peak abundances of fossils um are coincident with the anoxic periods now one could say well they're just being preserved because of that well that's partly true but we also have good evidence that they're living there so these organisms are living in low oxygen environments that's not so much of a surprise because emma hamerland has also been looking at the chenjung deposit in china and the chang looks like a beautiful beige limestone but actually boreholes have shown that it's almost identical sedimentologically to the serious passage it's these thin storm layers um and the fe hr fe fet um plots look almost identical to sirius passing these organisms are living and being preserved in low oxygen very low oxygen environments if we look at modern oceans um that's quite similar to the sort of situation we find in what are called oxygen minimum zones they also occur on the edge of continental shelves oxygen is then trained into the sea water and in the photic zone it's being produced by photosynthesis it's also being bashed in by wave action but as we go out of the light the depth to which the zone to which the light can penetrate through the photic zone um photosynthesis stops and that means that animals are just using up the oxygen in respiration there's no new oxygen being generated and that results in a collapsing in oxygen levels but we still see a very characteristic fauna as it says on the left the occupy the margins of oxygen minimum zones they're they're frequently quite dense aggregations quite abundant organisms but they're low diversity and so the opportunists who can survive there when not many other organisms can um but they encompass quite a range of different groups and we think that this is not a bad model actually for for the the a comparison uh with what we're finding that serious person and that's quite contrary to a lot of earlier ideas where people speculated that actually an increase in oxygenation oceans was what drove the cambrian explosion and now we're beginning uh to find evidence that that actually the inverse is true that the first animals are preferentially located in terms of their diversity in low oxygen environments and emma has explained in this recent paper that shouldn't be so much of a surprise to us actually we look at oxygenation from the perspective of successful terrestrial organisms who rely on oxygen but actually uh a large part of the animal kingdom lives in much lower oxygen levels fringe oxygenated levels um and as emma says on on the left you know she she calls this the normal world and actually it's coping with oxygen in many ways is what's challenging um and so she then refers to uh that low oxygen world has been an ancestral niche and what we are used to is actually the concord niche so we shouldn't be too surprised um the early animals occur in low oxygen levels um we know for example that the tumor cells um in cancers uh can withstand very very low oxygen levels and still thrive one of the tools that people have looked at to try and cure cancers um is to starve cancer cells of their oxygen but actually it turns out that they can withstand very very low oxygen levels and so maybe we should reverse our thinking of animals and think more about the norm um being that they evolved in low oxygen environments so last few slides the we can put all of that together into an environment in which the serious passive locality is indicated um by the red star it's sitting right at the limit of where sediment transport in these storms would reach so we've got storms on the shelf greatly increased greatly exaggerated gradient storms on the sandy shelf kicking up sediments it flows out across the very gentle slope below storm wave base into that oxygenated zone it buries our microbial mats it preserves our serious organisms right at that shelf slope rate so if we look at what was driving the cambrian uh evolution and the cambrian explosion we can be more or less certain that it wasn't oxygen because if it was oxygen that was keep the key control they will be up on the inner shelf where there was a higher level of oxygen and yet what we've got is the sediment the organisms living here and being transported a short distance in to that locality so they're there in as we've seen quite a low oxygen um environment if they were primarily driven by um nutrient supply also they would be further into show they will be within the photic zone where the peak nutrients are there are a few specialists uh some vertical borrowers called scolitas um that that live in um the shallow water zoning in at this time until that you see in northwest scotland for example but one of the things that might be controlling where these early organisms dwell is substrate mobility it might be that they really weren't very very tolerant of mobile substrates that would be moved around by currents and that they're sitting on microbial mats um as just about the most um stable substrates so maybe we should think of the sirius pass and chang and other cambrian lagosta of other exceptionally preserved cambrian deposits as being organisms that lived in deep dark and rather smelly water and not the crystal clear tropical um waters that you see in in some reconstructions and this uh rather nice diagram artwork by martin lysett for the exhibition i think characterizes this for the changing but this is pretty much the environment that i would envisage for serious passa as well and i'll leave it there thanks very much for listening thank you so much to paul for a wonderful wonderful talk um do keep your questions coming in on the chat and we'll try and get as many answered as we can before we get on to that just a reminder that you can find all our previous talks and indeed polls will be up uh in the next week or so on the museum youtube channel i've just put a link in the chat um right now and just a reminder that if you've enjoyed tonight's talk and then do feel free to share that online do share the link with friends and family who you think might be interested as well and then at that point we will just quickly switch across to that because we are very pleased to say that we have our schedule sorted for the months to come so our next talk in two weeks time is going to be by professor sarah gabat from the university of leicester who's going to be talking about some wonderful research that she's done looking at experimental decay and what this tells us about the preservation of fossils and the link to sign up to that is up already and i've just popped that up for you then two weeks later than that we've got professor xu hai zhao from virginia tech two weeks after that professor derek briggs from yale we've got a parade of professors for you and then as uh as paul said september the 9th um professor gabriela mangano from the university of saskatchewan in canada will be joining us and to wrap it all up on september the 23rd we've got professor rachel wood from the university of edinburgh again you've got a bit of a flavor for what she might talk about from one of the slides tonight but do put those in your diary and as we get more details from our speakers we will put some registration details up on the website and i will of course share them at other talks and just before we get on to answering some of your many questions just a reminder that we love to get a bit of feedback and we'd be especially uh pleased to hear your thoughts on this question has this lecture changed your opinion on the evolutionary origin of animals or added to your understanding of the topic if so how and if you've got any thoughts on that do pop that in the chat but at this point let's go to some questions and i thought we would start with some lovely questions on your your field work because it was a lovely insight into how field work happens in these uh very challenging areas so we had some questions from let's group them all together um or let's do them in two sorry we got firstly a question from morrison how has the transition from snow to grass because of global warming has that made it easier or harder to find the the fossils that you're after and margaret asks a really good question she's wondering if any of the indigenous people have stories or understanding of the locals um in terms of the ease of um a finding it i suppose it's what we will say swings and roundabouts you win some you lose some so my overwhelming memory of the early expedition was and the early expeditions at that time was sitting in slightly overcast conditions um really quite cold you couldn't feel your fingers but when searching for fossils at serious passage good light is really critical um and in recent expeditions it might be look but we have much better light the downside is that that increasing grassiness and increased moisture has resulted in huge numbers of mosquitoes that we never used to get in that part of north greenland um and there are days when as is said in an old arctic exploration uh account i was reading a few days ago the only way you could fit more mosquitoes into a cubic meter would be if the mosquitoes were smaller um and and so that can be a bit unpleasant at times but overall uh stable sunny weather like this photograph would be my option every time i mean it's still only four degrees perhaps but um it's great weather for for sitting and collecting fossils all day um in terms of the indigenous peoples there haven't been any indigenous peoples this far north for quite some time there are a few archaeological remains of uh of paleo-inuit settlements they were probably transient settlements um dotted around the coast they probably never came very far inland because that's not where the food was um but uh so i'm not aware of any indigenous accounts of geology super and sticking with field work uh three more uh short sharp questions laura wants to know if you have to worry about polar bears um morrison wants to know how do you jacket or otherwise protect the specimens um do you have to worry about the cold um and for transportation and then william asks us of these fossils from this expedition in the museum in oxford i'll take the last one first um yes they are some are being studied so there's some in my office the the vegetalicollions i mentioned and some uh were on display as part of first animals and you can see on the exhibition website um the the main collections of serious fossils are sitting in copenhagen um in in the the national museum of natural history there but they're really just being kept um in stewardship because the ultimate aim is is to get them back to a museum in greenland in the fullness of time so all the material that we collect belongs to the the geological the to the to greenland and the green land people and government um and that's where the material will go back that's where it it should go back remind me of the other questions jack uh the other questions were um how do you protect the specimens for transport and do you have to worry about the cold and uh laura wants to know if do you have to worry about polar bears yeah so in terms of protecting them from the cold that's not so much of a problem more of a problem is the fact they're reasonably fragile so we do pack them very carefully um either by wrapping them in in fabric and cardboard um john peeler the really good system of taking up flat pack cardboard boxes and rolls of foam rubber um and we would then just put a layer of foam layer of fossil foam and then fill the box tightly pack it down and you can more or less kick those down the runway and the fossils will be undisturbed um in terms of polar bears we've not been troubled by polar bears but they do live there um i've seen them on the ice in jp cockfield that that field uh in the background uh from a helicopter which is one of the best ways to see polar bears but we we do carry big guns big right by big high color for caliber rifles uh with us and we keep them in the tents at all times just in case we're disturbed most recently it's been wolves actually wolves have begun to become more abundant in north greenland and and there's a a pretty active pack in sirius passat but they they keep out of our way they're fairly shy for the most part and on to some questions about some particular specimens and the fossils themselves we'll start with a question by zhang who asks does halkirius two shell-like segments suggest a close affinity to brachiopods or bivalves or is the resemblance superficial only so it's largely superficial when when simon was first looking at it that very first specimen he came up with the the lovely hypothesis that it was it was the missing link between brachiopods anilid worms and mollusks we now know that they sit in very different parts of the the animal evolutionary tree um but work by jacob vince as part when he was a student actually a master's student showed that alkyria is a primitive mollusk so the way that it's growing it's its shell is analogous to the way that the clams or bivalves make their shell um so the two shells at either end are more similar to bivalve shells but then they've got all these spicules in between that are part of this this armor these sclerites as we call them parts of the arm and joe botting wants to know how similar are the assemblages from bed to bed well joe botting the expert on serious passive sponges the man who was published on syria's passive sponges so not not an entirely naive question joe thanks um they do vary quite a lot so um the overall species composition from we sampled at 10 centimeter intervals all the way through the section just so we could do that census um and it's the same assemblage of species um but but when it's um when it's a particularly high diversity interval you just get the rarer elements coming in so there's not much there's no particular gradation in the fauna from bottom to top and a a nice open question from mark jacobs who wants to know what was your favorite find i suppose it would have to be hulk erie i didn't say it during the lecture but um i more or less tripped over it and so to pick that up and realize that you know we we found the locality and we found a rather interesting fossil at the same time um but i think that's one of the best moments of fossil discovery i've had in my career actually we always like to hear about favorite fossils or favorite areas of of geology moving on to some questions about about trace fossils um i'll put two questions together here um frederick would like to know how can you tell that something is bilateral from its burrow shape and denver asks are the evidence for animals through trace fossils in the ediacaran still valid um as mariotti ettel have shown that microbial mats can form such tracks assumed to be made by animals yeah that was a neat interesting paper um i think it's one of those instances where you know exceptional claims require exceptional evidence and you know it is tempting to leap in um in older deposits in india if your car and deposits and claim that things are made made by animals i i think there are undoubted um horizontal burrows in the late ediacaran that we would all be very happy with um and if frankie was speaking now she would say i think that there is some evidence that um at least some of that idiocar and biota was able to move and disturb the substrate as well um so so yeah there are definite i think from from my perspective definitely good um evidence for burrows there is good evidence for burrows in in the top education um in terms of the bilateral symmetry it it's the pattern of sinuosity and the fact that the bureau can deflect it um that type of burrowing really depends on on peristalsis the the swallowing action as we push a bolus of food down our throat that's peristalsis um and it's peristalsis that is only possessed by bilaterally symmetrical organisms that enables burrowing of that type it's also for what it's worth probably what enables that primitive swimming that we see in some groups as well um so it's the the evolution of peristaltic muscle movements is actually a key innovation in animals and siddharth asks the question which is how can we calculate the age of the fossils i'm going to be a bit more specific how do we know the age of the fossils at uh at sirius passage oh that's a really really good question really good and it's taken us a long time it's much easier to to describe a nice soft bodied fossil than it is to try and work out the age the biostratigraphy is difficult and that's partly because we can't use acrylic these small spores um that we frequently use at this horizon because the metamorphism the rocks have been heated to a point where they've been burnt out they're just carbon the only thing that we can use is the trilobite bouinella's higgins army um but it's only known from sirius passat so we've then got to take um a series of steps to correlate um bouin ellis and its relatives across into more complete sections in um central north north america um where we've got more complete stratigraphy so that's one way that we do it but we can we can error check that because what we can do is use the sequence stratigraphy which is the record of sea level rise that's in the sediment and the serious passage foreign i didn't particularly talk about it is sitting right at the base of a major sea level rise and that's a major sea level rise that we can take right away across the north american continent and right the way across other continents as well it's a very distinctive time conforming uh horizon um and and they both give the same answer that it's essentially sirius pata is the same age as chang and sitting at the base of cambrian series three okay a few f a few questions left i think and then we'll wrap it up but a really good one from sarah why do you think the cambrian explosion happened when it did ah so i i think it was gonna happen regardless um you know everything was in place my instinct is that what acts as the trigger for that real acceleration that we saw in the histograms um is is that sea level rise that i've just been talking about for uh many tens to hundreds of millions of years the the inner parts of continents haven't been flooded um and sea level sat at or below the edge of the continental margin um but for a variety of geological reasons one of which is an increasing plate tectonic activity that increases sea level globally and it floods the interior interior of the continents for the first time that increases the area in which organisms can live within a given um depth of water on a slope you're gonna fit a small small area if you spill all of that water out across low gradients continental shelves you've got a much bigger habitable area and what ecologists know is that species diversity is pretty tightly linked to habitable area so that sea level rise increases habitable area that then provides the evolutionary driver to explosion in my opinion and last but one question from william do you think we will ever have a snowball earth again yes but not for a while and therefore to wrap it up we go for the final question from elena straw who asks if you had unlimited funding what would be the ideal place or thing for you to investigate to help you understand the cambrian explosion i think what everyone would answer to that question is that we need more localities we need them in other places um we need them in so we've got them in australia we've got them in china we've got them in north america um we've got some in russia but what we need is just more localities now given my predilection for things arctic and with my unlimited funding i would take a helicopter and hop my way all along um the the bowen formation in north greenland and because the serious passive locality must occur in other places um and although we've looked in in that small area around the locality and found a few bits um that outcrop belt as we call it that band of rock extends for about 300 kilometers so that's my fantasy helicopter trip thank you so much for that final question eleanor and we look forward to your donations towards uh paul's field fund um sorry to all the questions um we couldn't get to there were just so many but we hope we'll see you back here in two weeks time for our lecture with professor sarah gabat from the university of leicester who's going to be talking about experimental decay and the wonderful world of rotting fish and what that tells us about fossils and preservation should be really really exciting so do join us in two weeks time and the link for that is up in the chat but um and do put your answer to the questions on the screen now in the chat but um from me at the mu the museum and from paul thank you once again to paul for a wonderful lecture and we very much hope to see you again in two weeks time thank you very much you

Channel: Oxford University Museum of Natural History

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Keywords: palaeontology, science, museum, lecture, talk, evolution, natural history, oxford university, fossil, fossils, first animals, oxford university museum of natural history, oumnh

Id: blN-F8n2IAc

Length: 86min 27sec (5187 seconds)

Published: Tue Jul 21 2020

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Our understanding of the evolutionary origin of animals has changed dramatically in recent years. Continuing our talk series, museum director Prof. Paul Smith will look at the beginning of animal life, with particular attention to the Sirius Passet fossil site in the north of Greenland. The exceptionally preserved fossils from this site have been key in the development of our understanding of the ‘Cambrian Explosion’. Paul will discuss evidence for the timing of the origin of animals, as well as the evolution of modern marine ecosystems and food webs. Prepare to journey back in time more than half a billion years… Paul Smith is the Director of Oxford University Museum of Natural History and Professor of Natural History, and has over thirty years of experience on Arctic scientific expeditions. His research is focused on the interactions of Earth systems and organisms, particularly in relation to the origin and early evolution of animals, and combines the study of sediments and palaeoenvironments with palaeobiology and geochemistry.