A Big Step for a Fish – The Evolution of Four-Legged Land Animals

okay good morning everyone and welcome to the 2015 edition of the royal tyrrell museum speaker series today the Royal Tyrrell Museum and it's cooperating Society are proud to present our very own dr. Don Henderson Don is the curator of dinosaurs here at the Royal Royal Tyrrell Museum Don obtained his undergraduate degree at the University of Toronto in geology and physics he came to Alberta to take courses in paleontology and zoology at the University of Calgary and to work as a technician at the Royal Tyrrell Museum during the summers of 1994 and 1995 subsequently Don went to the University of Bristol in England to pursue his PhD in vertebrate paleontology where he studied theropod gate through computer modeling Don then moved to the u.s. to hold an answer postdoctoral fellowship at the Johns Hopkins University School of Medicine in Baltimore Maryland after which he returned to the University of Calgary as a research associate and sessional instructor in biological sciences then in 2006 don't join the curatorial staff at the Royal Tyrrell Museum Don's research interests focus on dinosaurs and the various transitions between land dwelling and aquatic animals today Don will present an overview of what the fossil record says about the initial transition from sea to land among vertebrates so without further delay I present you dr. Don Henderson so I for good and bad I'm interested in an awful lot of different things and one of them are these major transitions and I'm especially interested in how you go from being an aquatic animal to becoming a terrestrial one and so I just want to go over the some nice aspect to this story and also this I people have been interested in the since the 19th century and 450 years it was pretty much a mystery but maybe the last 30 years the fossils have really come along and our insights into how this has happened it's as had a huge change and it's a much more complete story now but as usual there's always questions to be answered so so if I've used the phrase their four legs land animals and I should clarify what I mean so technically I'm talking about tetrapod vertebrates so backboned animals with limbs and I'll go into a bit more detail to explain this and the basic message in this talk is going to be how do you get from a fish like this to an animal like this so you're going something live fully in the water with fins to going something with hips and knees and fingers and toes how does that work well what do we know about this transition so this is a rough plan I'll sort of stick to it so I want to review all the different components hopefully partly historically partly geologically biologically how they happened so the first thing you need to explain is what are vertebrates does they're certainly the central players in this story today we recognize five major groups in old times they will be called classes and they all seem radically different from each other as we see them today but they actually share a bunch of features when you really look that they're all very similar and just for some starters so one thing they have is left-right symmetry the way we develop and grow that the left and right halves are mirrors of each other that shared with other animal groups as well not just dumb vertebrates they have two pairs of appendages for movement so even in something like the Piranha on on the left there they've got front fins the pectorals and they've got rear fins the pelvics and that carries across all those even snakes snakes might look peculiar but they're very closely related to lizards and the fossil record shows us that primitive snakes did have limbs the sense organs are all concentrated at the front of the body and the brain that has to process the information from these sense organs are all protected in a bony case at the front and you might think that bony case is for mechanical protection but there's the alternate idea that the bone is actually acting as an insulator and it's person you think nerves are electrochemical devices and if you've got electrical interference between nerds you're going to mess up your signals so it's a it's thought that the bone might acting as an insulator it'll related to that we've got the spinal cord which carries signals from the brain to the rest of the body you want to protect that that's absolutely vital and so it's in vertebrates is protected by a jointed sequence of bony elements and so this collection of organisms it's not just a random jumble there's actually a structure and a degree of relationships between one some are more closely related to others and I've put this in for Dennis's education and entertainment so the modern way of looking at organisms is to use cladistics and instead of trying to distinguish how organisms differ from each other you want to turn it around and say what features do they share and how are they distinguished from others by features they've acquired through evolution so as I mentioned vertebrates they've all got back bones and skulls so something is radically different as a grizzly bear on the right to a piranha they share that as well as other things but if we move over one will come to this group called the tetrapods which are distinguished from fishes at least in one way they've got limbs with digits or in the case of snakes their ancestors had it but they've subsequently lost it and we can move on further within tetrapods what distinguishes amphibians from everything else the other reptiles the mammals are belong to a group known as amniote a-- they lay a shelled egg or in the case of mammals their ancestors later shelled egg and there's a big split within amnio - between Mammalia who are distinguished by having fur milk too young and a placenta and Reptilia which don't although they're now finding that there's reptiles actually have some form of placenta it's about was that's quite a surprise so for this story the main part we're interested in is how do you get from the fishy side to the tetrapods side and you might think what we've got on fib Ian's today we'll just compare them to fishes but there's a problem with that that modern amphibians are just too specialized sure the split happened you know hundreds millions years ago between these two groups but damn Vivian's didn't stop evolving the modern amphibians today are completely unlike the ancient ones the salamanders frogs and Sicilians it's hard to believe this thing at the bottom is a it's a amphibian aren't even a vertebrae I'll show you soon but their bodies of the reproductive strategies are so different that we could not even begin to guess that they've what happened a long time ago based on the living forms so we really do need the fossil record for this and just to show you that this thing in the bottom right is an amphibian if you just rip the skin off the head and look on the underside you will see jaws and teeth and muscles and and the basic features you would see in other vertebrates this is really like a vertebrate trying to be an earthworm it's really hard to believe and there's even aquatic forms of these things as well so come back to this what I want to show in this talk is put these animals in context so we're really focused on vertebrates because we're vertebrates ourselves and we're biased what we think most people think it's most interesting and also we're talking about tetrapods we're tetrapods we might think that we're a substantial and significant group but I just want to show you some numbers so there's about 4700 mammals and off more about half of them are rodents and the next big group is bats there's about 10,000 different kinds of birds 8,000 types of lizards and snakes vote 4000 types of amphibian but there's not less than 30,000 fish and there's several new fish being described every week I think so if you add them up on the right it's still less than the numbers of fish so you would think if going on to land is such a big deal and such a good thing why are there still more fish I think it's related the fact that 70% of the earth's surface is covered with water and I think in water in the oceans particularly you've got a three-dimensional habitat so you don't just have horizontal area to occupy you can occupy vertical space in the water column so I think the fishes will always trump land animals at least on this planet and the next bit of contact setting is to try and get away from this central vertebrate bias is if you look at all organisms plants and animals vertebrates are a very thin slice of the pie you can see insects vastly outnumber everything in fact more than half the insects or at least half are beetles in the nineteenth century Charles Darwin said that God must have an inordinate fondness of beetles because he was when he was in the tropics he was astounded at the beetle diversity so you have to keep in mind that although we're focused on vertebrates they really are a small picture of the of the bigger picture interestingly for this talk there are very few aquatic insects it seems their ancestors came out of the water along with things other things but the insects have had trouble going back into the water I can think of like diving beet there's aquatic beetles and water Striders dragonflies and mosquitoes have an aquatic larval phase but they adults are here on land and although we're talking about vertebrates coming on to land all the organisms here have had to make this same transition and so the you know show later vertebrates weren't the first plants and small things came on and the real thing I think it's really amazing if you think consider crustaceans shraddh crabs shrimps and lobsters there's a thing called a coconut eating crab it climbs trees to go and get coconuts that is just so bizarre I also think how could you even exist so get enough subsistence eating coconuts anyway I digress so we another contact setting is we want to put this in a geologic time scale so the current accepted age of the earth is about 4.6 billion years that's stabilized for the last few decades I think and but we're really only interested in the last little percentage the last five six hundred million years is when complex multicellular life really gets going and so we'll just expand on that and you'll start to recognize names and another context setting Alberta dinosaurs occupy maybe the last 20 million years of the Cretaceous but for today we're going much further back in time to the Devonian period so at 419 to 360 million years and that chunk that 60 million years represent just but one point just over one percent of Earth history but an awful lot happened and what did I want to show here yeah and it's in particular with the demony it's the late devonian the last 24 million years so you recall this 4.6 billion this 24 million is about half a percent of Earth history an awful lot happened in a very short time it's for our purposes a lot happened I'm sure there's other things for other geological and biological events but for this of things it's quite tiniest what and as I mentioned earlier tetrapods were not the first on land and there's lots of evidence for that for earliest evidence for terrestrial life organisms at least moving on land it's appears in the Silurian middle sailor in and with these diagrams the schematic set up so that marine conditions are on the long left the transitional brackish and freshwater is on the right so you can see middle Siberian you've got your rib turrets and some other arthropods I think as as if assurance or not stir up turds anyway and those are marine fishes on the on the left and you can see we've got evidence for a little tiny Club he plant sort of things what an alga algae are fungus coming on near shore if we come a bit further forward to the early devonian we still got the marine fauna we still got these large arthropods but now we're getting freshwater fishes and so these are fleshy sarkar aging fishes these are jawless fishes this is a group known as a cathode Ian's and this is another jawless fish and you can see there's more plant coming along now and when you come to the late devonian there's a huge increase things really started to move along you've got a huge variety of lobe-finned fishes you've got sharks marine or brackish a cathode Ian's fresh water a cathode Ian's you've got still got jawless fishes and you've got other freshwater Sark up to regions and marine ones and so that's this time block here and the other thing to know is that tetrapods are still minor component at this time it's the the vertebrate diversity still governed by fishes the other thing that sought to be significant is once you start getting what we would recognize as forest now with a canopy and an understory and leaf litter what you have a look at where have these fossils come from so the here on the diagram I've marked where if not exact tetrapods things very closely related to them fishes on their way to being that and you can tell you can see it tends to be concentrated in the northern hemisphere and it's also in very populous regions and I think there's two factors going on here is that the greatest population density in North America is on the east coast where people have been living for me people with education appreciation for science have been living here for longer than they have over here and again the same in northwestern Europe the East Greenland occurrence of early tetrapods has an interesting connection Amundsen the Norwegian explorer who was first to the South pool he was bored after that name as further adventure him and this crazy Italian we're gonna take this balloon trip over the North Pole but they vanished as soon after no one ever heard from them so they sent out all these expeditions to try and find them and one of the spots says East Greenland they came across these Devonian age rocks sedimentary rocks and found these amazing collections of fish fossils and people started goat the Swedes started going there in the 1920s and they started discovering tetrapod fossils so there's that was an interesting connection so it's not just body fossils we also have to track ways of these animals as well things moving around in shallow water leaving imprints or maybe even on land so we have two forms of fossil record for tetrapods also you've got this weird occurrence down here for Australia both tracks and body fossils you think how do why is mostly stuff concentrated up in North Western Europe and in Australia I think it's partly where all the big universities and an educated population that would recognize that and it's the same down in south eastern Australia it's where most of the people live the big cities are down here as well I suspect eventually we will find tetrapod fossils in Devonian rocks all over the world but right now it's a population and education bias and some sites closer to home the gaspé Peninsula big washa has an amazing collection of Devonian fossil fish and I'm sure eventually we will start to recognize tetrapods in there if we haven't done so already because the fauna Merson put the fauna you get at McGuire is very similar to funds fauna and found in Scotland as well in rocks of the same age so if we wind the clock back and reconfigure the earth as to what it was like in the Devonian so here we are 370 million years ago and orient you here's the equator and the continents that would the region that would become North America and Western Europe are this other continent most of it lying south of the of the Equator and if you remember that diagram where I showed where all the fossils were so there's there's fossil Devonian age fossils all around here and the Greenland stuff is in here deep far inland but we've also got evidence for tetrapod or near tetrapod in marine settings and there's that Australian setting is still way down there you might think how do you get Fibby ins all the way down here across an ocean modern amphibians today are extremely salt intolerant it's absolutely lethal to them there's one example there's a a crab eating frog that lives in mangrove forests it can tolerate brackish conditions but it's thought these old earlier amphibians where the better tolerance for salt and they could have dispersed through these shallow seas to get to other locations I just need a splash of water so the Devonian has been called since the 19th century the age of fishes they used to like to say the age of fishes age of reptiles age of mammals but we realize that that's sort of artificial once I fought that idea the fossil record improved Dave Unwin has a nice comment it's never the age of any reptile or mammal like he says it's always been the age of bacteria but the vertebrates are better at public relations so I just want to highlight some of the bizarre fish that we had in the Devonian there was this amazing diversity quite different we what we have today is actually it's quite limited so we had things like Ostia Stratton's the jawless fishes and when I read about this is that these things as a kid I always imagined how could you have a mouth without a jaw so I just assumed these things didn't have a mouth and then I was always mystified well how did they feed did they suck the food in through their gills no you can have a mouth it's just not supported by bone another group came along was the akan thorium's these were quite diverse they were try almost trying to be like ray-finned fishes but instead of having multiple arrays I just had single spines at the leading edges of the fins there were sharks there was many different kinds of sharks and Devonian there was what we would consider a standard looking shark but then there was all a lot of weird ones as well there were the plaka derms and these were armored fishes they did really well on the Devonian but went extinct at the end and I just wanted a note about the name here it's often pronounced Dunkleosteus but it was actually named after a man called James Dunkel so it should be promised Dunkel ostia s' Jim McCabe had a funny comment he talked about the uncle Dunkel debacle another group words that the early rafe in fishes you can see there if you look at the structure their fins they're a step up from these things they've got raised supporting them though they probably had muscles at the base maybe to control the fin rays and then the other group that appears are thus our copter agents the fleshy lobed fins and that's the ones that are central for our story so I just wanted to highlight this diagram showing that there was this amazing diversity of fishes the plaka Dern's didn't go very far the a cathode Ian's didn't go very far the jawless viscious did really well but today we've only got two kinds left that's the lamprey and the hagfish there's about 300 types of shark left there's about the 30,000 ray-finned fishes there's seven types of lobe-finned fish left if you don't count this group and then from the lobe-finned fishes came the tetrapods and so there's about fifty 55,000 tetrapods today and so it's this Devonian period we want to concentrate on so in the late devonian we start to see more sophisticated freshwater environments as well as this developing forest and undergrowth and it's whistie the thinking is being pretty was getting kind of crowded here you've got all these different kinds of fishes and you've got this habit unexploited this habitat above water that's not being exploited by things with jaws so you've got starkov regions you've got the akan 13 got solar placa terms and you've got these tetrapods and there's been several ideas of why this would have happened so you would think why would you give up a cushy life in water for a possibly tougher life on land some possible ideas are there's new food sources there's been this forest here for tens of millions of years there's all sorts of little arthropods in there possible new food source if it's so crowded with all these things with big teeth maybe you want to escape predators you hide in the shallows to avoid being eaten and then maybe you discover that you can live out of the water for a bit it may be a safe place to lay eggs there's actually a South American tetra it's called the splash tetra they live in there in the Amazon basin the female actually lays her eggs on leaves outside the water and the male has to jump out of the water and fertilize them and that's thought to keep those eggs away from from egg predators or it's probably a mixture of all three so there was the old idea that early tetrapods and their limbed fishes were crawling out of drying ponds to go find another pond and that seemed to hold for many years but then people began to think well why would you change your body to get out of the water to go back into the water to be a fish again it just didn't make sense and also as our improving geological knowledge not all tetrapods were found in arid landscapes there was there's moist humid humid environments so these things were living in as well well forested so it wasn't all bleak and nasty and drunk and drought stricken and so there's now new reconstructions of what Devonian forests would it be like if you didn't look too close so you think they just look like today but there'd be no pine trees no flowering plants no grasses but there would have been loads of organisms living in and on the trees and the vegetation and then the leaf litter and in the soil so there's this food source waiting to be exploited i might i tend towards that the feeding angle for why things would go on the water so this is a basic evolutionary tree that I'm going to keep referring to it's about 17 years old but the basic story what's it hasn't remain unchanged is this one major edition which I will get to so what I want to highlight in this diagram Dennis take note it's another cladogram there's a big split in what you would can call fishes between the rafe and fishes and every and the others the circuit the flesh finned fishes for our story we're not interested in the Ravens I just briefly review this these distinction so but most of the thirty thousand fish are ray-finned fishes like this salmon we still have I think it's now seven types of living Sarcoptes Jennifer Tabriz there's three types of lung fish and there's two types of coelacanth seal I have to say something about the coelacanth it's a great story the first one was caught and eyed by the trawler off the Southwest southeast coast of South Africa in 1938 and this guy here professor Smith at Cape Town was an ichthyologist and he was absolutely obsessed with this thing and by the time he got to this one it had been gutted because it was smelling really bad it was starting to decay in the hot tropical climate was their summer so he put out this reward if anybody sees this fish he Plast up and down that South West Coast all the way up to plate Lake Rhodesia and other places on the coast of Madagascar if you see this fish get your reward and contact contact Smith finally in 1952 one was caught much further north than where the previous want to be found and the reason this guy is smiling Eric hunt he caught the fish and he's the vote to get his big cash reward but Smith was absolutely obsessed and he managed with his connections he got the South African Air Force and Navy they flew he flew up to make sure it was the right fish and they got the Navy to send a ship to come and get this fish and here they are on the deck of the fit of the thing and you can see he's actually having to hold up the fish and Smith was was convinced that Sheila cats were on the direct line towards tetrapods and he always referred to this thing as old forelegs but now with better knowledge and better fossils we realize seal accounts are often off the main line we come back to this line there's a coelacanth we're not going to talk about them anymore but lung fishes are also in the 19th century they were thought to be possible candidates in fact there was a huge debate our lung fish as fishes or our the amphibians and it was Richard Owen made a definitive study saying no lung fishes are fishes one thing I want to highlight here is this very rapid time slice and if you look at these - these dots that's the known physical occurrence in the rock record of all these intermediates fishes and almost tetrapods and tetrapods so all this happened in a very narrow time and if you think back to that picture of the globe were conditions sufficiently different a Devonian that this sort of apparently dramatic evolutionary change could happen in that in that time period another thing is that these early tetrapods and near tetrapods they're big animals you tend to think of evolution happening with small animals with small body size and short generation times none of these things is less than a meter so either this is the true picture or what is it what if it's a bias of the fossil record that only the big robust things are getting preserved and the small things are getting eaten or their bodies rapidly decay and we lose them so I want to go and look in detail at some each of these characters in this sequence we'll start with you stand Opteron it's been known since 1881 from the gaspé Peninsula and that for a very long time it was considered to be a possible tetrapod ancestor and you would often see it in paintings and museum exhibits of this thing crawling out onto the beach and you can see why they would think that because when you look at its skeleton it's got a hips it's got a single leg bone here or like the equivalent of a femur it's got the next two bones tib-fib and if you look in the shoulder and arm region it's called what could be a shoulder blade and upper arm bones and the pattern of bones in the skull is very similar to later amphibians but with new fossil discoveries use the knob turns being pushed off it's it's not on the line and that's probably related to something like this fish called pendrick theis so it was known since 1930s but it wasn't described until the 1960s and this is a much better candidate it's got this flat body it's got the side mounted fins its eyes are in the top of its head this clearly says says this animals a shallow water dweller and you can see it's also lost its dorsals wears money it's lost its dorsal fin so fish is living in a deep or deeper water the purpose of the dorsal fin the anal fin caudal fin is to stop the body rolling to help to keep it upright but when you're in the shallows and touching bottom you don't you're not going to roll so why would you need dorsal fin so it seems to be lost and there found this illustration of a shallow water predator but look what its champing on little placa derms both really pissed like things I'm a bit suspicious these road there are some tracks I think that have been attributed to these things crunching along with their peculiar armored fins but anyway it's panda Rexy seems to be a shallow water predator so we can move forward most cases the fossil record is really good but in the last 20 30 years they've gone back and looked at some stuff which was written off as just scruffy fish bits now one of them was el Gander Pathan so when you look at it it's it's no screaming al but Perry albergue and Jenny clack and other people they've really looked at this material and compared it to more recent finds and it does now seem that el ganar Pathan is a valid tetrapod or near tetrapod and there's a restoration so this has been made long after the original finds we had it was probably something like this there although we've only got they're more robust upper arm limb elements they've proposed that this thing probably did have digits as well another nice animal is Ventus take a--from Latvia this is a nice step up from the previous material and it's 3d you can actually get a good sense for the shapes of the skulls one of the thing we even though we haven't got the body they're starting to put fins and fin rays on these a lot of the skull bones and these tetrapods they've got grooves in them that show they housed lateral line canals and that would be absolutely useless in air but they work great in water so these things are still in the water and that's a more modern restoration of what they think it looks like and again this isn't based on Ventus take a material you can see what we've got in the in the lower right picture but based on more recent finds they think it probably looked something like this and that one-off occurrence way down in Australia it's just a jawbone but the albergue and his PhD supervisor Clark they really looked at this stuff and they decided this is definitely a tetrapod so way down on Australia so even than that also in that twenty million year time period in the late devonian you don't just have rapid evolution you've got global distribution as well these things quickly radiated all around the world I'm sure we'll find them in South America and Africa and Antarctica eventually again there's another guesstimate what meta sign office would have looked like so here's the star from the last ten years it's Tiktaalik and I've we have fortuitously we bought a nice 3d mini diorama of Tiktaalik and that's it down there you can come and have a look I asked Brandon if there was a cutesy name for this thing and he said no so I thought were a nice disgusting one would be tikki parcel but so this animal it's you couldn't ask for a better intermediate that's and there was this huge media hype about it and the people at Harvard really got a lot of mileage out of it it even started to get the nickname of fishapod because it was this seemed it was such a good intermediate so it was found on in Ellesmere on Ellesmere Island and the Harvard University people Shubin and I can see him I can't remember his name - they're from Philadelphia and the old guy from it also worked on Jenkins yes they plant they looked around the world said where would be good places to go and find early tetrapods we need rocks of the right age and the right type and they're recording the right environment and they recognized that Ellesmere rocks on Ellesmere Island would be a good candidate they had several field seasons where they didn't find anything and then they hit the jackpot and they got several specimens not just one i'm various stages of growth and it really is an amazing fossil and so just to remind you where we are on this sequence we're trying to get from Euston Opteron to something with legs that would move on land and already you can see that Tiktaalik is a pretty as a really nice intermediate and there was so much media noise about this and it was a whole raft of cartoons appeared so there's a modification of the Darwin fish this one I like I have this one on my office door the reason for this we should build a wall at the same time in the United States they're having that big debate where they should build this giant fortified wall with Mexico to keep the immigrants out and then there was this one as well but it's not off fossil fine gets cartoons done about it so the famous tetrapod animal in between fish and true tetrapods with Axios it was xt estiga for many decades no one since so it was excavated by Swedish Peabody shion's in the 1920 and it got described in the 1930s by Eric yar vaq and he basically built his career on the estiga and he went to his grave thinking that this was what it looked like and it was the intermediate form oh and one thing one thing you should note he assumed it had five digits that was big surprise was to come shortly so in the 1980s and 1990s at Cambridge University in England they started they wanted to go back and look at these old sites so they mounted a series of expeditions and they came back with amazing fossils not just of xeo Stiga but a related animal called a canvas tiga and they also had this woman Sarah Finney who was does amazing preparation work she's so patient she'll go through and pick out all the tiny little grains she's probably almost as good as Donna and mark so it's an idea of as people studied these new accessory it's the estiga remains this was the new reconstruction that came out in the 1990s it looks more like a seal and now look at how many fingers there are Tortosa and another student of clocks recently looked at the existing material again and has come up with this much more radical interpretation of what it looked like this thing looks diseased it's very peculiar so that also from the greased Greenland sites was a canvas Stiga it wasn't purely known previously but after 1980s they've got these amazing fossils some of them got nicknames when you look at this in person I've been to see it the quality of the fossil and the quality of the preparation is amazing all the matrix has been picked out of all these tiny little pits this twisting of the face is a taphonomic artifact it was warped after burial this was another specimen called Boris and they you can see they make him more manageable in the lab they carefully had to cut it in certain parts but they it's on display in fact I'm pretty sure this backing this is what's in the display case that they use even zoology at Cambridge and so this is what new reconstruction of a canvas Stiga this is done by Mike coats and at kansas tiga although it's further along towards tetrapods than these other things it still has some very fishy things one of the big things they noted would was boney gill arches this thing was still primarily a gill breather but it still had openings on the top of its skull for for the rear of the skull fur for breathing and they also had fin raised and when you look at them under the microscope the construction of these fin rays is identical to what you see in fishes today with the way they the things developed so this was clearly an animal fully adapted for water yet it's got fingers and eight fingers and eight toes acanthus egos got seven for the you know for centuries everybody thought five fingers with some magical logical quantity maybe you have a central single digit to on the site to balance but with these new discoveries there's nothing special about v it's just historical accident we've set settled on five if we could have had eight so I just want to show in detail how these new fossils show us how limbs would have developed so come back to good old Houston Opteron he's still useful but don't think of them as a direct ancestor these shading here the dark gray is the humerus the medium gray is the that allowed me the radius and the light gray will be the ulna so Wade 385 million years ago your typical fish fin was mostly fin ray with a bit of bone come forward ten million years you've got much reduced fin ray component and a lot more bone and by the time you get to something like a canvas Stiga you have no fin rays in the limbs it's all bone the function what was taken by fin rays has now been replaced by digits fingers and this definition for what's a tetrapod some people say if the limb doesn't have any fin rays you have a tetrapod because with all these new fossils it's become very hard to say where do you draw the line between fish and non I just want to highlight exist Egon and it's a knit slim so it's got seven digits but you can still recognize the basic bones that we have so we've got the femur here the top of the leg the two tib-fib right in the middle we can recognize ankle bones and then the phalanges that make up the digits and so I've been focused on the limbs but we should say something it's not just the limbs changing there was other changes in the body as well I think that's why this 20 million year event is just so amazing it was so much happening we'll just focus on the eyes in the basic skull shape so if you go back to you stand up turn on the eyes are towards the front and if you think about how fishes feed you've got this nice and fluid around you which is a bit viscous you can come up to your prey quickly open your mouth and expand your gills and you suck your prey in so you've got this huge chamber back here to create a low-pressure zone to suck in the food that works great if the waters deep enough but as you come forward into things like Pandorica these are Tiktaalik the eyes start to move back and you're seeing a bigger component to the front of the face and if you keep coming up further these other tetrapods the eyes are moving back the front of the face is getting longer so the interpretation is that when you're in shallow water you can't use that suction feeding you now have to go up and place your teeth on your prey and bite it and pulled it and once you come on to land you can't use the gale force winds wouldn't even suck it into your mouth but you have to use your teeth so that's just one another one of these changes so all these correlated changes are happening in these animals so I mentioned the track ways the track ways are an interesting part of this story because most of the trackways predate the body fossils which i think is very peculiar you've got these limb fishes or almost fishes are almost tetrapods but then there's least and if they're in the water how are you how can you explain these tracks the first one I want to show is from south west coast of Ireland these were just discovered in 1998 I think by a Swiss geology student who was doing his mapping there and you can recognize here's the two parallel sets and you can map them you see alternating sequence of tracks and to leave tracks like that your body weight has to be pressing down on the substrate and making an impression if you're fully floating in water you can't really put a force down so these the thing that made these tracks was either in very shallow water with not all the body supported or it was fully out in the air and look at the date it's 380 million years it's like 15 million years sooner than things like Ichthyostega and a canvas diga a couple of years ago there's now 390 million year old tracks from Poland and in this paper they've proposed that there was some sort of tetrapod making these alternating sequences and even more interesting they've got digit impressions in these same tracks so to leave these fine marks I don't think you could make them underwater I think this must have been in the air and the mud would stake stuck together and if you recall what the estiga foot looks like something like this D guy may have been pushing around severely so these tracks I've I've always been suspicious about some aspects of the anatomy of xt estiga and a canvas Stiga they've got these massive hips and shoulders and if your limbs aren't feeling your full body weight because you're in the water why do you need such big hips and shoulders and another thing why would you need jointed limbs with digits and I'll show you some counter examples to these so if you're it seems very strange to have these in water if we look at some other animals from the fossil record so things that had land-living ancestors with good limbs and digits and knees and elbows and then they go back into the water they completely rearrange their limbs and hips so the mosasaur is an example the limb bone proportions are completely changed you don't have long elongate limb elements right here if your hind limb isn't taking the weight you don't need a stir connection to your spine you don't have free fingers they've become like a giant flat mitten almost trying to be like a fish fin again that's just mosasaurs was and here you can see the same thing again in dolphins and whales dolphins have a vestigial pelvis it's not even connected to the it's the spine they've abandoned the hind limb the forelimb proportions are completely different and again the forelimb has become this unitary structure almost but you can recognize and all the bones and the five fingers in the hand just two more examples please eeeh sores make these sores um with pleasee sores you can see still see they had five fingers or five toes but the radius and ulna have become almost indistinguishable from the rest of the rest of the hand and and and ichthyosaurs did the same thing it seems to me that this sort of thin structure is much better than a jointed flexed limb underwater and I when I asked clack and company about this they said oh well these things needed these arms to crawl through the weeds and stuff well I'm not buying that I think for if you're a large on with a backbone and a skull the best way to propel yourself is through fins or flipper type structures so I would say what if X the estiga and a canvas Teague are descended from truly terrestrial ancestors and that if we've got these much older trackways that could have been made by these terrestrial ancestors and you could think of igneous Teague and acanthus sigue as animals that went back into the water and retained almost like a larval or tadpole stage the gills the fins would be at that interpretation and not long after tetrapods appeared in true land animals were walking around some of them started to go back into the water really stolen cases this scary animal called Crescent Uranus in fact it's more closely related to groups of tetrapods that were on their way to becoming reptiles that Reptil morphs but it's radically modified its body reduced its limbs there's an awful lot of cartilage the pelvis is reduced so very quickly they can adapt back to this aquatic lifestyle and there's no big pelvis or shoulder to let you saw with XTS tiga and not just cry CJ entus there's a whole range of Paleozoic tetrapods that went back into the water so I think this is a really nice idea so I thought the only way to end this is to see when is the fish not a fish when it has legs so that's why I really like this illustration it shows nicely this transitional form something that's still in the in the water with fins but it's got limbs with digits to leave a mark and I'm done [Applause] you

Channel: Royal Tyrrell Museum of Palaeontology

Views: 3,677

Rating: 4.7446809 out of 5

Keywords: Speaker Series, Royal Tyrrell Museum, Palaeontology, Paleontology, Tetrapod, Evolution

Id: 5OFSRBfl0og

Length: 51min 10sec (3070 seconds)

Published: Thu Jan 11 2018