1 00:00:00,390 --> 00:00:05,970 Okay. Good afternoon, everybody, and welcome to this week's Physics Colloquium. 2 00:00:06,630 --> 00:00:11,310 So it's a great pleasure today to welcome Steve Forbes to give the talk. 3 00:00:11,310 --> 00:00:14,310 Now, Steve is by now quite well known to many of us, 4 00:00:14,310 --> 00:00:23,310 but he started out his physics career as undergraduates at M.I.T. because his doctorate at Berkeley, he did a post-doc at Princeton. 5 00:00:23,910 --> 00:00:29,879 He was a professor at Virginia, and then later at the Annexe in Paris. 6 00:00:29,880 --> 00:00:34,680 And in 2012, he moved here as the as the new civilian professor. 7 00:00:35,580 --> 00:00:41,400 He's he's he's famous for his work in astrophysical fluids in the general area of astrophysical fluid, 8 00:00:41,910 --> 00:00:54,230 and in particular for famous work with John Hawley about instabilities in accretion disks for which they won the 2012 Shaw Prize, 2013 Shaw Prize, 9 00:00:54,240 --> 00:01:05,490 sorry, and which is probably the senior prize in available theoretical astrophysics if you get the chance to watch the award ceremony on the video, 10 00:01:05,490 --> 00:01:13,649 on, on the website, it's really quite something, but that's just by way of a little bit of entertainment. 11 00:01:13,650 --> 00:01:17,370 So today Steve is going to talk to us about where we all came from. 12 00:01:17,610 --> 00:01:22,770 But instead of it being from a biologist point of view, which is what you normally get is from a physicist point of view, Steve. 13 00:01:24,690 --> 00:01:29,900 Thank you, John. Sure I'm on. All right. 14 00:01:33,360 --> 00:01:44,040 So it's a pleasure to be here and to talk about something which I think is a little off the beaten path, at least for most people in the audience. 15 00:01:46,860 --> 00:01:56,060 I think one of the charms of doing astronomy, at least for me, is that there are profound truths to be had, 16 00:01:56,070 --> 00:02:01,800 and they often begin with the most innocent astronomical observations. 17 00:02:03,090 --> 00:02:06,600 Classic example is Olbers Paradox. It gets dark at night. 18 00:02:07,200 --> 00:02:12,600 It shouldn't, at least in the simplest models of the universe, an eternal Euclidean universe would not. 19 00:02:13,110 --> 00:02:15,420 The universe had to have had some kind of the beginning. 20 00:02:16,680 --> 00:02:25,770 For my purposes, my starting point is simply to note that the sun and the moon, you may have noticed, look about the same size in the sky. 21 00:02:25,770 --> 00:02:29,580 They have the same angular size, about half a degree, 30 minutes. 22 00:02:30,720 --> 00:02:37,680 So in most textbooks, if it's mentioned at all, this is just it's written off, just a coincidence. 23 00:02:38,070 --> 00:02:48,930 Let's move on to serious things. Well, we should begin by noting that the existence of the sun in the moon, in the sky, 24 00:02:49,290 --> 00:02:53,910 at their locations that they have, does something besides make nice eclipses. 25 00:02:55,350 --> 00:03:01,560 Their gravitational fields, of course, exert tidal forces on the earth. 26 00:03:03,660 --> 00:03:10,590 You all remember what a tide is. I don't need to go into the details, I think, for this audience. 27 00:03:10,590 --> 00:03:15,329 But there's basically a equilibrium between the centres of mass. 28 00:03:15,330 --> 00:03:19,800 In this case, I'm just looking at the moon and the earth. So if I go to a point on the surface, 29 00:03:20,460 --> 00:03:27,300 it's not actually an absolutely precise force balance because the centre of mass points 30 00:03:27,960 --> 00:03:33,480 perpendicular to this vector are the actual forces slightly off pointing to the moon. 31 00:03:33,870 --> 00:03:39,090 And it's this difference which represents the tidal force. It's not difficult to work this out. 32 00:03:39,390 --> 00:03:47,760 It's a sort of an elementary calculation. The tidal potential turns out to be proportional to the P to spherical harmonic. 33 00:03:48,150 --> 00:04:01,650 And if I erect this u v w coordinate system centred on the earth, then the gravitational tidal force in the form of a vector is minus U and minus V. 34 00:04:01,650 --> 00:04:09,270 It points inward along the U and V acts UN vaccines, and it's repulsive along the w axis. 35 00:04:10,020 --> 00:04:14,370 Key thing is that it is proportional to one over are cubed, 36 00:04:14,490 --> 00:04:19,920 not a one over r squared force because it's the difference between one over r squared terms. 37 00:04:19,920 --> 00:04:23,840 It's a one over ah cubed in its spatial dependence. 38 00:04:24,360 --> 00:04:29,069 There are two tidal bulges, one on either side the first time one sees it. 39 00:04:29,070 --> 00:04:35,100 It's a little counterintuitive. You can't understand why the far side is in kind of sucked along toward the moon. 40 00:04:35,550 --> 00:04:43,350 But in fact, there's a precise force balance at the centre of mass as I go a little bit closer, of course, the gravitational force is bigger. 41 00:04:43,620 --> 00:04:48,480 This is in orbit. So there's a centrifugal force. The centrifugal force is correspondingly weaker. 42 00:04:48,750 --> 00:04:54,000 As I move away, the centrifugal force gets a little bit bigger and the gravitational force diminishes, 43 00:04:54,150 --> 00:04:59,970 producing at least leading order a symmetric bulge. 44 00:05:02,250 --> 00:05:14,190 So here's another coincidence. Even though the masses of the sun and the moon differ by many orders of magnitude, as do their distances, 45 00:05:15,570 --> 00:05:20,610 when you calculate the tidal forces from the sun in the moon, they are they're almost the same. 46 00:05:20,610 --> 00:05:28,049 They're within a factor of two of one another. And that's fairly remarkable because I've done the M over R Cube calculation there, 47 00:05:28,050 --> 00:05:33,090 and you can see in the numerator you have terms which differ by eight orders of magnitude. 48 00:05:33,780 --> 00:05:40,560 We have a term which differs by three orders of magnitude and phenomenon and you qubit at the end of the day, you're within a factor of two. 49 00:05:43,020 --> 00:05:50,490 Well, the matching angular size and the matching tides are not two different coincidences. 50 00:05:51,630 --> 00:06:09,600 It's one. If I set the moon's gravitational amplitude over our cubed equal to some factor f of order unity to the sun's gravity tidal amplitude. 51 00:06:09,960 --> 00:06:15,930 And I write the masses as some average density times a diameter cubed or proportional to that. 52 00:06:16,590 --> 00:06:25,620 Then you can quickly see with one line of calculation that if I write that relationship in terms of angular size, 53 00:06:27,120 --> 00:06:35,190 the angular size of the moon and the angular size of the sun are equal to one another, except for this factor here. 54 00:06:35,610 --> 00:06:41,909 And if F is slightly bigger than one, the density of the sun being a gas and the moon is slightly less than one. 55 00:06:41,910 --> 00:06:49,860 And it comes into this one third power and it is never a number which differs very much from unity. 56 00:06:50,430 --> 00:06:55,950 So the point is, is that even a rough agreement in the tidal strength produces a very close agreement. 57 00:06:56,400 --> 00:07:03,000 In theta, it's like having a very tight focal plane. If you move something a little bit out of the focal, it goes completely out of focus. 58 00:07:03,000 --> 00:07:10,920 If you're anywhere near being in focus, then your position is very tightly constrained and you can see this graphically as well. 59 00:07:10,920 --> 00:07:23,220 If you just plot the title strength versus position, then there's a big title spread within a factor of two with a very, very tight position. 60 00:07:25,530 --> 00:07:33,020 So the real importance, I think, of the angular size argument is that it tells us an Isaac Newton, this isn't anything new. 61 00:07:33,030 --> 00:07:36,120 This is something even argued. It's not a difficult calculation. 62 00:07:36,600 --> 00:07:39,749 This is something that Isaac Newton was able to use. 63 00:07:39,750 --> 00:07:42,810 In fact, he has a he has a well-known, 64 00:07:42,840 --> 00:07:50,850 ingenious argument for why the density of the moon has to be bigger than the density of the sun based on tidal forces, 65 00:07:50,850 --> 00:07:54,839 the same angular size and the ratio of spring tides to need tides. 66 00:07:54,840 --> 00:08:02,210 So this is hundreds of years old. But somehow it kind of gets lost. 67 00:08:02,230 --> 00:08:04,600 It's not something that is widely appreciated. 68 00:08:05,740 --> 00:08:17,120 The point is, if we're trying to understand a coincidence, it makes much more sense to put it on the dynamical plane rather than the perception plane. 69 00:08:17,140 --> 00:08:24,850 Right. Dynamics have content and we can begin to ask serious questions about what the consequences of the equal tidal forces are. 70 00:08:26,260 --> 00:08:34,460 So is there something important to us being here now that is related to the sun and moon having comparable tidal forces? 71 00:08:34,480 --> 00:08:38,590 In other words, is there some sort of an anthropic basis for this? 72 00:08:39,370 --> 00:08:42,850 And I will try to make the case that there may well be. 73 00:08:45,280 --> 00:08:52,240 So if we want to calculate tidal forces the way we do this is we put the. 74 00:08:52,510 --> 00:08:56,230 We go back to Ptolemy. We put the Earth at the centre of the universe. 75 00:08:56,740 --> 00:09:02,350 Then the sun goes around in the ecliptic. Here's the celestial equator, the north celestial pole, south celestial pole. 76 00:09:02,560 --> 00:09:11,200 Just a line running through the axis of the earth. So Ecliptic is inclined, of course, at 23 and a half degrees relative to the equator. 77 00:09:11,710 --> 00:09:17,890 And the moon is also on the celestial sphere, roughly speaking, not exactly, but roughly speaking. 78 00:09:17,890 --> 00:09:26,950 Also going around in a circular orbit, elliptical orbit, but circular orbit on the celestial sphere inclination angle of 28.6 degrees. 79 00:09:26,950 --> 00:09:36,100 And then the net tidal force that we experience is, of course, on the frame of the rotating earth, watching the sun and moon go around us. 80 00:09:37,300 --> 00:09:40,480 So the moon orbits with a frequency. Well, let me do it. 81 00:09:41,230 --> 00:09:46,120 I have it down on the slide in terms of frequency, a period of 27.3 days. 82 00:09:46,690 --> 00:09:51,610 The sun would go around, obviously, 365 and a quarter days. 83 00:09:52,120 --> 00:09:55,560 So these are slow. They proceed around slowly. 84 00:09:55,570 --> 00:10:01,840 They don't whiz around on the sky. But of course, we perceive being on the earth much larger frequencies. 85 00:10:01,840 --> 00:10:07,030 They're Doppler boosted by the one day period of the Earth's rotation. 86 00:10:07,030 --> 00:10:17,200 So in fact, the effective frequency that we perceive and the tidal forces of the moon in the sun are actually very nearly equal because of this, 87 00:10:17,200 --> 00:10:22,810 but not exactly because of the difference in the frequencies of their own particular orbits. 88 00:10:23,380 --> 00:10:35,410 Well, we know what happens when two periodic functions superposed and have comparable amplitudes and nearly but not quite the same frequency. 89 00:10:36,550 --> 00:10:45,010 I don't need to remind this audience about beats. We get a beat frequency and their result is that there is strong amplitude modulation. 90 00:10:48,680 --> 00:10:59,780 What this means physically is that the net tidal force and the resulting tidal displacement, the height of the sea, will be strongly modulated. 91 00:10:59,810 --> 00:11:07,460 And that in addition to having this one day or twice a day frequency component, 92 00:11:07,850 --> 00:11:14,480 in other words, from one high tide to another, the incoming sea level won't be the same. 93 00:11:15,530 --> 00:11:25,670 That means that an inland tidal pool formed at one high tide may not be replenished the next day or the next day or the day after that. 94 00:11:26,510 --> 00:11:33,020 And over the course of time, depending upon circumstances and the precise layout of the land. 95 00:11:33,320 --> 00:11:37,160 You can imagine a complex network of such tidal pools being created. 96 00:11:39,110 --> 00:11:48,440 Schematically, there's a high tide, and then the next high tide could could quite easily leave an isolated tidal pool. 97 00:11:49,340 --> 00:11:52,910 And that poor little fish may be stuck there for a month. 98 00:11:54,710 --> 00:12:00,260 So how do we actually calculate the height of a tide? Let's call it Zeta. 99 00:12:01,430 --> 00:12:07,159 A standard textbook way of doing it is basically to calculate how far and actually 100 00:12:07,160 --> 00:12:12,770 potential surface is displaced by the presence of the moon or in more physical term. 101 00:12:13,850 --> 00:12:19,480 Energy equals force times distance for your GCSE. 102 00:12:21,410 --> 00:12:26,060 So memorise this equation g times zeta is potential energy. 103 00:12:26,150 --> 00:12:29,240 So G is the earth's gravitational field. Z The tidal height. 104 00:12:29,690 --> 00:12:39,430 P The potential energy. Well, that's fine, except hey, you know, we're talking about water and water doesn't do that. 105 00:12:39,440 --> 00:12:44,000 Water sloshes around. So these are the so called the plus tidal equations. 106 00:12:44,360 --> 00:12:50,149 And it's actually reasonably you can sort of get a handle on what's going on. 107 00:12:50,150 --> 00:12:56,809 We have a Coriolis force term, Theta and Phi are just the theta and Phi coordinates on the surface of a sphere. 108 00:12:56,810 --> 00:13:02,690 Theta is called Latitude Phi. As a smooth Zeta I can advance this. 109 00:13:04,920 --> 00:13:07,950 Zeta is the height above the average sea level. 110 00:13:08,430 --> 00:13:13,080 H is the actual depth of the sea positive number. 111 00:13:15,510 --> 00:13:21,510 And so my equations two equations for f equals m e I include. 112 00:13:21,780 --> 00:13:26,519 This is the pressure term with the little bulge of the wave on top, the Coriolis term, 113 00:13:26,520 --> 00:13:33,329 the inertial derivative and phi is the forcing tidal potential so to equations 114 00:13:33,330 --> 00:13:39,600 for the theta and PHI components of the velocity and mass conservation equation. 115 00:13:40,520 --> 00:13:44,300 And G is a kind of convenient variable to work with. 116 00:13:44,310 --> 00:13:50,370 G Zeta plus phi is how I define big g and the equilibrium tide, 117 00:13:50,370 --> 00:13:57,810 which I just calculated before corresponds to g equals zero, v equals zero and no time derivative. 118 00:14:02,060 --> 00:14:04,370 Just to remind you what spherical coordinates are. 119 00:14:05,600 --> 00:14:15,140 So when we want to go about solving this system, it turns out you can take that system of equations and reduce it to one kind of big monster equation. 120 00:14:15,470 --> 00:14:19,790 Although it's not too bad. So g remember has a piece of phi in it. 121 00:14:19,910 --> 00:14:24,800 So it's a little more complicated than it looks because I have Phi on both sides of the equation. 122 00:14:25,340 --> 00:14:32,960 And I have this kind of messy cross product. And this is okay, this is just kind of a Laplace in Helmholtz kind of grouping of terms. 123 00:14:33,980 --> 00:14:37,760 And D is a resonant denominator, omega squared, minus four omega. 124 00:14:38,000 --> 00:14:45,709 I realise I'm going a little fast here. I'm assuming a harmonic dependence of time in the tidal potential since I was 125 00:14:45,710 --> 00:14:50,510 linear ising I should say that linear ais in the equations it makes sense. 126 00:14:50,510 --> 00:14:54,530 248 Decompose which is actually a pretty good approximation anyway. 127 00:14:54,950 --> 00:15:00,679 48 decompose the force in tidal potential and I wind up with this equation to solve 128 00:15:00,680 --> 00:15:06,139 and there's a resonant denominator at a frequency of two omega cosine theta with two. 129 00:15:06,140 --> 00:15:15,980 I made a two omega times the sign of the latitude. Now it's actually I won't do it here because that'll take me a little too far. 130 00:15:15,980 --> 00:15:22,070 FIELD But if you did a purely hydrodynamic calculation for the tides where you got rid of the land, 131 00:15:23,060 --> 00:15:28,600 you know, the spherical cow calculation and just took H to be a constant. 132 00:15:28,610 --> 00:15:34,160 You can show that just the tidal forcing itself actually produces a result which is pretty unimpressive. 133 00:15:35,150 --> 00:15:37,010 You get tides about 30 centimetres. 134 00:15:38,270 --> 00:15:46,910 So clearly to get something more interesting, this function h the depth which will in general depend upon Theta and Phi obviously, 135 00:15:47,090 --> 00:15:51,410 and the outline of the continents themselves are key players in this kind of a game. 136 00:15:52,340 --> 00:16:01,400 Now, to really understand the Devonian tides, we're going to go back to the Devonian and you'll see why I chose that period. 137 00:16:01,820 --> 00:16:10,100 Shortly, we would need a theory of Devonian bathymetry, as it's called, and detailed numerical simulations. 138 00:16:10,430 --> 00:16:13,440 So what's the difference between depth and bathymetry? 139 00:16:13,970 --> 00:16:19,640 Well, bathymetry refers not only to the depths of the seas, but the height of the continents as well. 140 00:16:20,150 --> 00:16:25,610 And when people do these kinds of calculations, they like to know whether there will be inland flooding. 141 00:16:26,030 --> 00:16:30,410 And so they are careful to calculate the slop that comes over the mountains 142 00:16:30,770 --> 00:16:35,660 and whether you can get so-called epi continental seas on the land as well. 143 00:16:36,290 --> 00:16:40,880 And in fact, we are trying to do that as I speak. I have a group of people I'm working with, 144 00:16:41,300 --> 00:16:50,960 and we're using reconstructions to try and get some idea of what the tidal effects really were like 370 million years ago. 145 00:16:51,920 --> 00:17:00,050 But look, we're physicists and there was a time when physicists didn't go whining to their computers. 146 00:17:02,370 --> 00:17:09,860 Was it time you took out of pencil and paper and you had a complicated and you thought about how to solve the darn thing? 147 00:17:11,030 --> 00:17:15,190 Sorry, that's uncalled for, but I'm getting old in ten. 148 00:17:17,570 --> 00:17:20,910 But look, there's a small parameter in this problem. 149 00:17:21,690 --> 00:17:26,129 Yes. G h over four omega squared. 150 00:17:26,130 --> 00:17:28,680 R squared, which appears in the equation. What is that? 151 00:17:28,710 --> 00:17:35,430 That's basically the speed of the wave it will turn out compared to the rotation rate of the earth. 152 00:17:36,570 --> 00:17:41,460 And that is small, which is one reason why we're not use equilibrium theory. 153 00:17:42,480 --> 00:17:49,980 Well, the good thing about that is that it allows for a w k b solution for that wave equation. 154 00:17:51,090 --> 00:17:54,870 So what I want to do is not even worry about the tidal forcing. 155 00:17:54,870 --> 00:18:00,660 Just now I want to look at the free waves, the free wave solutions of my tidal wave equation. 156 00:18:00,960 --> 00:18:11,610 So I put phi equals zero and then my variable g is just basically proportional to the tidal displacement, which is what I'm interested in calculating. 157 00:18:12,150 --> 00:18:16,610 And then I look for good old w kb wave solutions of this form. 158 00:18:16,740 --> 00:18:22,410 It's two dimensional and you see that less frequently in the literature. 159 00:18:23,220 --> 00:18:27,720 But it's the techniques that are used actually extend fairly naturally. 160 00:18:27,960 --> 00:18:38,100 And being a theorist, I stick in Epsilon just for fun because I want to keep track of my orders in a perturbation calculation. 161 00:18:38,850 --> 00:18:42,160 So I won't cover the boards with equations. 162 00:18:42,180 --> 00:18:46,139 I will simply show you what emerges from that kind of calculations. 163 00:18:46,140 --> 00:18:50,070 And it's actually starting with something that looks pretty hairy. 164 00:18:50,640 --> 00:18:55,710 It's pretty simple. What emerges in that kind of an analysis, the first, 165 00:18:56,070 --> 00:19:05,060 the largest turns the one over epsilon squared terms grouped together to form a wave dispersion relation. 166 00:19:05,070 --> 00:19:11,129 And lo and behold, it's the universal dispersion relation for all of physics. 167 00:19:11,130 --> 00:19:14,630 As far as I can tell, it's the scalar fields. 168 00:19:14,640 --> 00:19:22,590 It's plasma waves. It's pendulums with springs connected to one another. 169 00:19:22,920 --> 00:19:27,390 Omega squared is something times K squared plus a constant. 170 00:19:27,930 --> 00:19:33,150 So that's very good. We're on familiar ground. These are called long waves. 171 00:19:33,720 --> 00:19:37,750 You may wonder what happened to that really messy bit with the grab this cross. 172 00:19:37,770 --> 00:19:41,639 Grab that. That's actually another wave branch which is there. 173 00:19:41,640 --> 00:19:50,400 But I won't be using here called Rossby Waves, which are very important in geophysics and for, among other things, weather patterns. 174 00:19:50,760 --> 00:19:56,760 But they're not important for the kind of problem that I'm dealing with here at the kind of tidal frequencies I'm interested in study. 175 00:19:57,930 --> 00:20:04,979 Now, the next bit is kind of interesting so that if you think in terms of optics, that was geometrical optics. 176 00:20:04,980 --> 00:20:09,300 This is now physical optics, what dynamics this would call wave action. 177 00:20:09,600 --> 00:20:18,989 So if I group the next order turns, I get something that it's kind of fun because on your paper it kind of spews all over the place, 178 00:20:18,990 --> 00:20:23,660 but it folds up nicely into a divergence. So the divergence of g. 179 00:20:23,670 --> 00:20:30,780 G is just a constant, the earth's gravity. But I keep it with h h a squared amplitude squared divided by this resonant 180 00:20:30,780 --> 00:20:37,799 denominator d times the wave number k group velocity is just proportional to K. 181 00:20:37,800 --> 00:20:41,640 So this is a sort of conservation of wave action. 182 00:20:42,240 --> 00:20:45,420 And what it tells me is that if I have a channel, 183 00:20:45,480 --> 00:20:52,440 a simple channel of depth H with W and I'm working at a latitude lambda that the 184 00:20:52,440 --> 00:20:58,050 wave energy A squared is proportional to the grouping that you see in front of you. 185 00:20:58,060 --> 00:21:09,720 Cosine lambda lambda is now the latitude not it's not the not the co latitude theta latitude up from the equator w the width and h to the one half. 186 00:21:11,010 --> 00:21:20,250 And we'll be making use of this and estimating what will be going on under different paleo geographic constructions. 187 00:21:23,090 --> 00:21:27,829 Just to orient you, the local group velocity of shallow water, tidal waves. 188 00:21:27,830 --> 00:21:32,900 V is about due to the one half times this cosine factor. 189 00:21:33,590 --> 00:21:38,030 So if we take the average depth of the oceans 4.2 kilometres, 190 00:21:38,030 --> 00:21:45,200 we get something which is about the speed of a jet aircraft and that's about the velocity that tsunamis travel at. 191 00:21:45,650 --> 00:21:49,370 Important thing to take home. We don't even need to know the numbers. 192 00:21:49,850 --> 00:21:55,610 We get a faster response to external driving for deeper H. 193 00:21:55,670 --> 00:22:06,170 For bigger h. Finally, something I'm going to come back to and it could be potentially very important is the possibility of resonances. 194 00:22:07,100 --> 00:22:16,909 So if I use the dispersion relation to calculate the wave number for a given frequency, I get a very simple looking equation. 195 00:22:16,910 --> 00:22:28,459 The first one, K is the square root of D over g h and if I set that equal to n plus one half pi over L for a resonant condition, 196 00:22:28,460 --> 00:22:36,530 where does that come from? Well, that would be the resonant condition for a channel which is closed on one end and open on the other end. 197 00:22:37,200 --> 00:22:44,600 It corresponds to flapping at the open end, and I'm going to be interested in that particular configuration. 198 00:22:44,870 --> 00:22:50,330 So I show that to you now. We'll come back to it in application later. 199 00:22:52,490 --> 00:22:55,580 What does the actual tidal tidal forcing look like? 200 00:22:56,480 --> 00:23:03,889 Well, this is what the forcing would look like. This is the nominal equilibrium tide is what I'm calculating. 201 00:23:03,890 --> 00:23:08,270 But it's proportional. It's basically proportional to the forcing potential. 202 00:23:08,960 --> 00:23:14,620 Here is the result for tidal forcing with Devonian parameters without the moon. 203 00:23:14,630 --> 00:23:18,800 So this is in centimetres. This is an ours. 204 00:23:18,800 --> 00:23:22,250 The whole thing is about five months. Each of these is about 20 days. 205 00:23:23,180 --> 00:23:34,620 It's very boring without the moon. If the moon were where it was, but or if the moon had basically half the diameter in the sky than it does now, 206 00:23:34,620 --> 00:23:39,200 but the same density, I'd get something a little bit more interesting, but not very much. 207 00:23:39,800 --> 00:23:44,390 And if I put in my best estimate for the real realistic Devonian parameters, 208 00:23:44,750 --> 00:23:51,829 I get something which is really quite sculpted and really shows strong modulation from one. 209 00:23:51,830 --> 00:23:58,399 You can be imagine a a tidal height coming in here and then you notice the problem. 210 00:23:58,400 --> 00:24:04,850 If you're a fish is not up here, the problem is on the sides because here you might get another day to get yourself out of a mess. 211 00:24:05,300 --> 00:24:10,510 But if you're caught on the side up here, then it can be a long time before you get replenishment. 212 00:24:11,840 --> 00:24:22,070 Anyway, I'm kind of tipping my my cards a little bit because the question I wanted to pose was what are the consequences of this for our friends here? 213 00:24:22,760 --> 00:24:29,960 So I like to introduce some people, some creatures I've been spending some time with. 214 00:24:31,130 --> 00:24:37,940 You see a contest, Dega contest. Dega is one of the early tetrapods and one of the first body fossils ever discovered goes back. 215 00:24:38,150 --> 00:24:43,400 Excuse me, this is Ichthyosaur Staker, which was discovered in the 1930s, 216 00:24:43,400 --> 00:24:50,420 a contest agar which has just been recently rediscovered in terms of getting much more complete fossils. 217 00:24:50,750 --> 00:24:55,910 And these are very early tetrapods. You can just see the beginnings of fins turning into feats here. 218 00:24:56,270 --> 00:25:08,540 This is pandemic, these pandemic. These is a fish with tetrapod like characteristics and occupies kind of a cusp position in the evolutionary tree. 219 00:25:08,840 --> 00:25:19,700 It's probably very closely related to when to stagger, which is a very early tetrapod and with whom it shares very many structural features. 220 00:25:20,180 --> 00:25:33,350 So the question is, can we learn anything about why the way that tides behave and how that might relate to the appearance of these kinds of creatures? 221 00:25:34,130 --> 00:25:40,340 So just to remind you of the basic story, there was a high tide and there will be a low tide. 222 00:25:41,600 --> 00:25:44,690 The next high tide isn't going to help very much. 223 00:25:44,990 --> 00:25:52,549 If you're stuck in an inland pool, then there is a low tide and good luck to the fish there. 224 00:25:52,550 --> 00:26:02,090 Who could be there for a month? This is a Paleo geographic reconstruction of the Devonian. 225 00:26:02,210 --> 00:26:05,480 Devonian was a time when we had two supercontinents. 226 00:26:05,480 --> 00:26:11,210 We had gone on to land in the south and then what's called your America, your America or Laurasia. 227 00:26:11,480 --> 00:26:14,750 I don't know why they're two names, but it's referred to in both ways in the top. 228 00:26:14,990 --> 00:26:16,680 So we had two supercontinents. 229 00:26:17,270 --> 00:26:25,400 And what is interesting here, you notice, is that there's a very broad opening in the Relic Ocean and then a narrowing of the straits. 230 00:26:25,850 --> 00:26:31,850 And that is exactly the kind of configuration which is interesting. 231 00:26:34,070 --> 00:26:40,340 From a title point of view, if I look at this now in isolation, 232 00:26:40,850 --> 00:26:48,500 here are three regions of the earth famous for their tides, the English Channel, which has enormous tidal currents, 233 00:26:48,890 --> 00:27:00,470 the Bristol Channel, which has famous sashes that run up in by Cardiff into the River Severn, and then the Bay of Fundy, which has enormous tides. 234 00:27:08,150 --> 00:27:12,710 And if you look at the configuration of the relic ocean, that looks very suggestive. 235 00:27:14,030 --> 00:27:17,930 You find a similar thing on the northwest coast of Australia. 236 00:27:18,530 --> 00:27:22,129 So don't put off be put off by all these kind of little funny islands. 237 00:27:22,130 --> 00:27:26,060 This is actually a continental shelf and this is only a very shallow sea. 238 00:27:26,060 --> 00:27:31,760 So you have a rather deep sea here and then you have a width which is narrowing 239 00:27:31,760 --> 00:27:37,370 gradually but dramatically over the course of a couple of thousand miles. 240 00:27:37,670 --> 00:27:44,120 And you get some of the highest tides in the world on the northwest coast of Australia up to nine or ten metres. 241 00:27:47,310 --> 00:28:00,630 Now in the late Devonian we just saw the western regions of Eurasian and gone one and landmasses contained a subcontinent sized bay. 242 00:28:01,410 --> 00:28:08,580 This is something not very different in size than India. Narrowing down to intercontinental shallow straits. 243 00:28:09,240 --> 00:28:13,560 This really is a greatly scaled up version of what we see in the Bay of Fundy, 244 00:28:14,280 --> 00:28:18,930 Bristol Channel, north west coast of Australia, other places famous for their tides. 245 00:28:20,070 --> 00:28:26,010 Well, just the square root of argument explains what's going on here. 246 00:28:26,670 --> 00:28:33,180 The eastern shallows, because each is so much less respond much more slowly to the deeper western water. 247 00:28:34,080 --> 00:28:39,480 What tends to happen is you get an impedance match surge of Western Bay water. 248 00:28:39,780 --> 00:28:45,599 Provided the coastline is in too irregular and it's dumped into a very constricted 249 00:28:45,600 --> 00:28:50,460 channel and the eastern tidal amplitude can become very large indeed. 250 00:28:51,420 --> 00:28:56,610 Scale up the Bay of Fundy. I may be cheating a bit because I think the Bay of Fundy has resonant interactions, 251 00:28:57,210 --> 00:29:09,100 but scale up these point the regions that you've seen by a factor of five or so and you can get an idea of really a very unusual condition. 252 00:29:10,110 --> 00:29:16,110 This is Planet Swamp. So was the Devonian, in fact, special? 253 00:29:18,000 --> 00:29:21,719 The Devonian really was special. The Devonian was heavily forested. 254 00:29:21,720 --> 00:29:24,870 The first trees. Archaeopteryx. 255 00:29:24,900 --> 00:29:35,250 Not Archaeopteryx. Archaeopteryx, one of the first trees ever could grow to a height of 30 metres, and it apparently loved wetlands. 256 00:29:35,700 --> 00:29:40,680 The first deciduous leaves appear. Devonian was a time when the climate was stress. 257 00:29:41,310 --> 00:29:45,299 A plant took a long time to get the idea that it should evolve live. 258 00:29:45,300 --> 00:29:51,300 And then in the Devonian it decides that it should get rid of the leaves, that it just spent all this time and energy. 259 00:29:51,810 --> 00:29:54,810 Why is that? It was expensive to keep a leaf in the wintertime. 260 00:29:54,900 --> 00:30:01,260 The idea of dormant plants, trees losing their leaves arose in the Devonian. 261 00:30:02,040 --> 00:30:11,219 There is fossil evidence in the form of black shales for sea level changes and increased organic marine deposits, 262 00:30:11,220 --> 00:30:15,540 i.e. all those leaves fall falling in these heavy forests. 263 00:30:15,930 --> 00:30:20,400 They didn't just stay there or didn't always just they got washed into that narrow channel. 264 00:30:22,920 --> 00:30:28,770 What also seems to have happened is that there was widespread anoxia. 265 00:30:28,890 --> 00:30:32,890 Anoxia is depleted oxygen in the waters. 266 00:30:32,910 --> 00:30:41,790 And it's something that we see today when we're not careful with organic with dumping of organic materials into enclosed ponds. 267 00:30:42,720 --> 00:30:46,200 The last such event, when there seems to have been this large in wash, 268 00:30:46,200 --> 00:30:52,710 is known as the Hanging Berg event was general and widespread and one of the great mass extinctions of all time, 269 00:30:53,400 --> 00:31:01,500 and it decimated the shallow water loving, lobed fin fish, basically the ancestral stock of the tetrapods. 270 00:31:03,060 --> 00:31:08,130 So this just doesn't happen every day. The Devonian was an unusual period. 271 00:31:08,640 --> 00:31:12,510 Let's look at paleo geography a little bit more carefully. 272 00:31:14,010 --> 00:31:17,140 Here is the slurry in the period just before the Devonian. 273 00:31:17,160 --> 00:31:20,030 You can see things happening, but we don't have a narrow channel. 274 00:31:20,040 --> 00:31:28,920 We have a rather equally distributed broad strait between Laurasia and gone one inland and gone into the early Devonian. 275 00:31:28,980 --> 00:31:35,210 Things are already starting to happen, or Asia is descending in the east, constricting it. 276 00:31:35,220 --> 00:31:43,440 The ocean is still open. When we get to the late Devonian, we are in extremis now. 277 00:31:43,440 --> 00:31:48,360 We have very narrow straits here and a big broad opening in the West. 278 00:31:51,840 --> 00:31:54,959 In the early carboniferous we've been cut off. 279 00:31:54,960 --> 00:31:59,130 The wreck ocean has all but disappeared and we're starting to close off. 280 00:32:00,000 --> 00:32:06,300 We don't have anywhere near the sort of tidal receptivity that we had in the Devonian, the late Carboniferous. 281 00:32:07,440 --> 00:32:15,300 We are just about to form Pangaea. If we see the formation actually of the Appalachian Mountains in the Atlas Mountains in Morocco, 282 00:32:15,720 --> 00:32:23,820 as these two continents come together, and then by the Permian, we are one supercontinent. 283 00:32:26,380 --> 00:32:31,180 Let's go back to the late Devonian and think about what was going on. 284 00:32:31,630 --> 00:32:36,760 Recall w kb theory for depth h channel width w latitude lambda. 285 00:32:37,180 --> 00:32:40,510 The wave energy was proportional to the combination you see there. 286 00:32:40,930 --> 00:32:47,080 Cosine lambda over w h to the one half. So to amplify a wave, what do you do? 287 00:32:47,110 --> 00:32:55,450 One Travel toward the equator. If you're not near the equator, that'll boost you with cosine and lambda to travel in a narrowing. 288 00:32:55,450 --> 00:33:03,730 Channel three travel into ever more shallow water, which is exactly what that Relic Seaway would force the waves to do. 289 00:33:04,120 --> 00:33:13,510 If you put in numbers, you find 30 centimetres swells which are just there for the taking can turn into in principle, 14 metre waves. 290 00:33:15,370 --> 00:33:19,690 Well, I think all of this suggests a new line of support for an old idea. 291 00:33:20,410 --> 00:33:29,649 Dating from the 1930 is a classic argument due to the eminent palaeontologist Alfred S Romer, who said, excuse. 292 00:33:29,650 --> 00:33:35,799 I'm sort of lapsing into jargon a little bit. I'll try and be careful to explain weird words like charity and weight bearing. 293 00:33:35,800 --> 00:33:44,470 Charity and limbs. Charity and limbs. Physicists know that chirality refers to fingers, handedness, limbs that are digitised, 294 00:33:44,470 --> 00:33:51,820 that have fingers were, according to Romer, an adaptation by lobed fin fish. 295 00:33:51,820 --> 00:33:59,860 Lobed fin fish are very important. The bones in their fins are homologues to our own humerus radius and ulna. 296 00:34:00,220 --> 00:34:03,460 You can trace it back 370 million years. 297 00:34:05,290 --> 00:34:15,220 So it's these low fin fish driven by what he viewed and what I view is an obvious selection pressure of being able to flail seaward 298 00:34:16,360 --> 00:34:24,760 and remember you're going to have progressively deeper puddles as you get closer to the sea when their own puddle turned inhospitable. 299 00:34:25,900 --> 00:34:30,340 It's inevitable. I like to call them. Sorry about this. Romer's Romer's. 300 00:34:32,380 --> 00:34:42,670 Now, Romer argued that the red clay sediments in which these early tetrapods were fossilised was typical of arid conditions. 301 00:34:42,940 --> 00:34:46,810 And I think at the time most people agreed and they said, Well, great, that's a story. 302 00:34:47,890 --> 00:34:51,850 Somehow you get a pool of water in the desert, never mind that part. 303 00:34:51,850 --> 00:34:59,530 But that's what they thought. The pools dry, the stranded fish that can manage some degree of out of water navigation will live, 304 00:34:59,710 --> 00:35:03,490 impress their mates with their stories and have more baby fish. 305 00:35:06,010 --> 00:35:12,550 Well, subsequently it was understood that these red clay sediments need not reflect arid conditions, 306 00:35:13,030 --> 00:35:19,270 nor are they always representative of the sediments in which the organism in fact was laid down. 307 00:35:20,590 --> 00:35:28,810 So they threw the tetrapod out with the bath water and gave up on the whole idea, which I think was premature. 308 00:35:29,350 --> 00:35:40,840 And in fact there was much fruitless and in my view, very silly wrangling about the real precise reason why it is good to have legs. 309 00:35:42,640 --> 00:35:46,150 Well, there are obvious advantages to having paddles on your body. 310 00:35:46,150 --> 00:35:53,890 I think if you live in choked, swampy, intertidal wetland with a vast network of available pools, 311 00:35:54,580 --> 00:36:02,469 there need not be one dominant reason for the development of tetrapod morphology from the French. 312 00:36:02,470 --> 00:36:07,090 Of course, the best suggestion yet that it was for better sex. 313 00:36:08,440 --> 00:36:19,420 I recommend this particular reference because it even comes with diagrams, a sort of Kamasutra for a contest stages. 314 00:36:23,300 --> 00:36:30,800 Bottom line, this is a family presentation, so I'm going to stick with my black and white slides. 315 00:36:31,430 --> 00:36:34,540 There are all kinds of reasons to try to leave. 316 00:36:34,550 --> 00:36:43,550 Maybe you can think of some as shallow, disgusting, fetid, slimy, brackish puddle, even under non arid conditions. 317 00:36:44,780 --> 00:36:48,320 Especially. Especially under nine. 318 00:36:48,560 --> 00:36:54,620 Because that's when you're going to find another puddle. If you get out of a puddle in a desert, you're going to die. 319 00:36:58,370 --> 00:37:09,259 So the argument was always, I think, much better and much more likely to be applied under conditions where it is, in fact, very humid. 320 00:37:09,260 --> 00:37:16,460 And we see this today in Southeast Asia. There are so-called mud skippers and climbing perches, which in fact, 321 00:37:16,640 --> 00:37:22,910 in fact flap around and they're very much kind of a ro marian ideal in their behaviour. 322 00:37:23,180 --> 00:37:32,180 And they do precisely this. They go from one puddle when it starts to get brackish, moving to another one and there are lots of them around. 323 00:37:33,590 --> 00:37:41,060 And so apologies to the bard we many we happy many we banned of Roemer's Roemer's. 324 00:37:41,600 --> 00:37:51,260 We are all tetrapods now. So imagine my delight when I read about the relatives, a family in Trackways. 325 00:37:53,060 --> 00:38:01,400 What, you ask, are the Australian trackways. Tetrapod Trackways Extra x three. 326 00:38:01,400 --> 00:38:06,410 Read all about it from the early middle Devonian period of Poland. 327 00:38:06,950 --> 00:38:11,000 This was definitely four star RAF material. 328 00:38:14,570 --> 00:38:24,480 What was discovered in 2010 was that 18 million years before they should be present before there was any evidence of body fossils. 329 00:38:25,190 --> 00:38:32,330 There are beautiful footprints clumping through the mud, walking. 330 00:38:33,020 --> 00:38:39,740 And I do mean walking. No body dragging. These are highly developed and highly specialised. 331 00:38:39,860 --> 00:38:44,450 Fully Ceridian. That is to say digitised penta dactyl. 332 00:38:44,690 --> 00:38:48,649 Five, five toes, limbs or five fingers. 333 00:38:48,650 --> 00:38:55,870 I guess these were thought to be the analogies, the hand, the four pads, the four. 334 00:38:56,210 --> 00:39:09,620 You know what I mean? The tetrapods. The tetrapods and the their closely related cousins lived extensively for tens of millions of years. 335 00:39:09,620 --> 00:39:15,650 And this came as a real surprise. There is an actual picture. 336 00:39:15,710 --> 00:39:20,570 You can get a sense one, two, three, four, five. 337 00:39:21,740 --> 00:39:29,899 And they've actually have. Now, I have learned from the palaeontologists involved with this discovery have a remarkably 338 00:39:29,900 --> 00:39:39,200 detailed reconstruction of what the foot morphology look like and is surprisingly advanced. 339 00:39:39,200 --> 00:39:43,610 That had obviously been evolving for a long time before the footprint was made. 340 00:39:44,330 --> 00:39:46,630 Where did they find these? Where were these trackers? 341 00:39:46,640 --> 00:39:53,299 The if fairly and by the way, simply refers to the period within the Devonian when the time period, 342 00:39:53,300 --> 00:39:57,620 when the discovery was made, it was found right near the title Sweet Spot. 343 00:40:02,670 --> 00:40:09,480 So the discovery of the stallion trackways in what is now Poland in shallow seas 344 00:40:09,900 --> 00:40:14,820 has prompted yet another scenario for why tetrapods may have left the water. 345 00:40:15,030 --> 00:40:20,250 So the idea here put forth by the eminent palaeontologist Jenny Clark, 346 00:40:20,760 --> 00:40:27,330 is that the tetrapods are not the only ones who are going to get stuck in these isolated seas. 347 00:40:27,750 --> 00:40:32,700 You will have less fortunate fish that are also stuck in these tidal pools. 348 00:40:33,150 --> 00:40:40,920 And so it's like a sushi bar for these tetrapods who can go around and help themselves to fish, who can't do anything about it. 349 00:40:40,920 --> 00:40:49,410 So this was another motivation for why growing feet would be evolutionarily advantageous. 350 00:40:50,700 --> 00:41:02,130 My emphasis here is the word intertidal zone, because when you see a description of what is going on with these early tetrapods and their habitats, 351 00:41:03,450 --> 00:41:06,930 the word tidal comes up again and again and again. 352 00:41:07,950 --> 00:41:11,520 Let's talk a little bit more about tetrapod habitats. 353 00:41:13,350 --> 00:41:17,969 Now, remember a contest, Ortega and Nick Theo Staker, the body fossils. 354 00:41:17,970 --> 00:41:20,640 They were the first ones found back in the 1930s. 355 00:41:21,270 --> 00:41:28,170 They happened to be associated with non marine inland basins in East Greenland, what is now East Greenland. 356 00:41:28,560 --> 00:41:34,530 And for many years people thought that the first amphibians is what they called them. 357 00:41:34,530 --> 00:41:42,000 That's actually a bad word to use because these tetrapods are no more related to amphibians than they are to us. 358 00:41:42,300 --> 00:41:45,900 But these early tetrapods were, in fact, freshwater. 359 00:41:46,350 --> 00:41:51,570 And that sort of set their the way of thinking about their origins for many years. 360 00:41:52,200 --> 00:42:01,769 But in fact, in the last two decades, a huge amount of fossil finds have completely altered the thinking on this with a host of not just tetrapods, 361 00:42:01,770 --> 00:42:06,660 but key transitional species in marginal tidal environments. 362 00:42:07,050 --> 00:42:10,110 These really are the creatures from the Black Lagoon. 363 00:42:12,210 --> 00:42:19,440 The assumption origin of freshwater origin for Tetrapod has strongly been strongly challenged, writes Jenny Clarke. 364 00:42:19,770 --> 00:42:27,510 Among more recent finds of early tetrapods are many that are now known to be tidal, marginal marine, 365 00:42:27,570 --> 00:42:38,010 brackish waters, one of the richest sources of tetrapods and so called tetrapod or more for fossils. 366 00:42:38,310 --> 00:42:42,420 These are fossils of fish that have tetrapod like characteristics. 367 00:42:42,930 --> 00:42:52,530 There's a huge tidal delta in what is now modern Latvia, Lithuania, Russia, Estonia, the so-called Baltic Devonian Delta. 368 00:42:54,720 --> 00:42:59,520 It was around 30 million years. It was a Delta plane and it was graded. 369 00:43:00,840 --> 00:43:08,040 It's tilted slightly. And it shows direct fossil evidence for tidal intermittency in the sedimentation rates. 370 00:43:08,580 --> 00:43:13,130 Sedimentation in the upper plane were sporadically interrupted by spring tides. 371 00:43:13,440 --> 00:43:19,560 It was very much an it not only a tidal environment, but an environment that was dominated by intermittent tides. 372 00:43:20,880 --> 00:43:30,540 To get an idea of what it's like. The authors use this analogy with a river delta in Indonesia, the so-called McKim River Delta. 373 00:43:31,800 --> 00:43:36,000 And I can't even pronounce that East Collie Mountain and Cowley Mountain. 374 00:43:37,410 --> 00:43:38,459 It had a span. 375 00:43:38,460 --> 00:43:47,010 It has this is currently it has a span of 50 kilometres and the Baltic River Delta has been likened in its morphology to this structure. 376 00:43:47,220 --> 00:43:54,510 But you have to imagine a region that was ten times the size of the comparable between, say, here and Edinburgh. 377 00:43:54,900 --> 00:44:03,060 Huge. Well, there's a huge number of transitional species that have been found there. 378 00:44:03,270 --> 00:44:08,730 I won't even bother going through the names except to note that in addition to the trackways, 379 00:44:09,090 --> 00:44:16,560 there are early tetrapods and the ancestral stock that is believed to have given rise to the tetrapods. 380 00:44:16,830 --> 00:44:21,930 And they're all found in this huge tidal region. 381 00:44:22,470 --> 00:44:33,750 So it's very compelling evidence that there's something special about the way these creatures evolved and this particular tidal habitat. 382 00:44:36,030 --> 00:44:42,990 Moreover, there is a consensus that the Devonian was indeed a special time for other reasons as well. 383 00:44:43,830 --> 00:44:52,500 Writing in a collection, Devonian Events and Correlations, a collection of scientific papers. 384 00:44:53,040 --> 00:45:01,500 The lead paper starts by saying a combination of climactic plate tectonic magmatic and still poorly. 385 00:45:01,560 --> 00:45:10,290 Understood. Apparently, oceanographic factors cause the recurrent sudden perturbation of what were 386 00:45:10,320 --> 00:45:16,980 stable ecological conditions by short term global events of variable magnitude. 387 00:45:17,760 --> 00:45:22,410 What could these poorly understood Paleo Oceanographic features be? 388 00:45:23,760 --> 00:45:28,920 Allow me to make a suggestion. What about a global tidal resonance? 389 00:45:29,850 --> 00:45:35,160 Let's go back to our formula. Now I'm cherry picking here, but it was easy to do. 390 00:45:35,730 --> 00:45:42,090 If you look at the map, you get a length of a little over 5000 kilometres for the Seaway. 391 00:45:42,090 --> 00:45:51,540 Seaway. If you take sort of a best guess of about one and a half kilometres, something a little bit more shallow than the average in an ocean. 392 00:45:52,290 --> 00:45:57,000 The period that's associated with the Devonian Lunar Tidal period, the moon was of course, 393 00:45:57,240 --> 00:46:04,320 a bit closer in the Devonian 365,000 kilometres, it's believed, instead of 390, which is where it is now. 394 00:46:05,010 --> 00:46:14,579 Then, lo and behold, the calculated wave number that is associated with those parameters is in fact 395 00:46:14,580 --> 00:46:19,590 equal to the and equal to one resonance in the formula that I have here. 396 00:46:19,980 --> 00:46:24,840 So the conclusion is not that those numbers are accurate and that that is what happened. 397 00:46:25,050 --> 00:46:29,730 But remember, the Ric Seaway was around for 30 million years at least. 398 00:46:31,110 --> 00:46:36,749 What's the timescale associated with Continental Drift, as it used to be called? 399 00:46:36,750 --> 00:46:42,540 It's centimetres per year over timescale of 30 million years. 400 00:46:42,570 --> 00:46:50,420 That gives you a thousand kilometres to play with. It would be amazing if you didn't pass through a tidal resonance under those circumstances. 401 00:46:50,430 --> 00:47:00,170 Right? So I think it is not at all far fetched to imagine that these kinds of occurrence could easily have been relevant. 402 00:47:03,570 --> 00:47:11,129 I think a synthesis emerges of a truly special Devonian period, where a confluence of highly modulated tides, 403 00:47:11,130 --> 00:47:21,240 a receptive geography, led to a vast network of isolated pools and planetary scale swamps, heavily forested. 404 00:47:21,780 --> 00:47:29,339 As it was, there were few herbivores on the land, nothing to eat, the leaves full of deciduous trees. 405 00:47:29,340 --> 00:47:32,820 And we also now know dispersed seeds. 406 00:47:33,060 --> 00:47:41,370 Hard seeds were invented during the Devonian period as well and allowed trees to colonise vast areas. 407 00:47:42,360 --> 00:47:51,670 The accumulating plant debris in swamps produces a background of slowly degrading, anoxic waters. 408 00:47:51,690 --> 00:48:00,269 This is not my idea. This was put forth already 15 years ago in an influential and well-known paper. 409 00:48:00,270 --> 00:48:11,130 So it was a time when you had, in addition to everything else that's going on, a massive in wash of the Earth's first cache of organic material, 410 00:48:11,520 --> 00:48:18,000 including organic soils, because there were no organic soils before there was this extensive plant cover. 411 00:48:21,270 --> 00:48:37,170 A subclass, however of SARS-CoV-2 rigid read lobe fin fish evolves limbs probably as a navigational aid for both aquatic and semi-aquatic habitats. 412 00:48:38,160 --> 00:48:47,040 These tetrapods apparently coexist for at least 20 million years, 18 million years with their elitist posterior cousins. 413 00:48:47,040 --> 00:48:52,260 Those are the fish that gave rise the ancestral stock to the tetrapods. 414 00:48:52,830 --> 00:48:57,959 They coexisted without being dominant until the shallow, 415 00:48:57,960 --> 00:49:05,790 water loving lobed fins fish were eventually decimated by encroaching anoxia from an overwhelming, 416 00:49:06,090 --> 00:49:08,850 perhaps tidally resonant in wash, 417 00:49:09,150 --> 00:49:19,260 resulting in the Hanging Berg event which marked the end of the Devonian and the elimination of a huge range, a huge number of species. 418 00:49:20,070 --> 00:49:30,360 However, the offshoot tetrapod line has options under these circumstances more attainable niches in hard times. 419 00:49:31,380 --> 00:49:35,790 In other words, Romer's romer's were the ones that managed to thrive. 420 00:49:37,230 --> 00:49:40,770 So let me offer the following closing perspective. 421 00:49:43,570 --> 00:49:47,980 The Sun and the moon, which is where we started, appear to be the same size. 422 00:49:48,640 --> 00:49:52,090 I prefer to view this as my fossil. 423 00:49:52,150 --> 00:49:57,340 This is the astronomy fossil. It's telling us something important if we know how to read it. 424 00:49:59,200 --> 00:50:09,460 It's telling us that there is an important role in evolutionary biology and for us being here for sizeable, significant, highly modulated tides. 425 00:50:11,050 --> 00:50:22,720 I think this idea is strongly supported by the fossil record and that it is deeply intellectually dissatisfying to write it off as a mere coincidence. 426 00:50:23,650 --> 00:50:32,469 By contrast, if we adopt it as a working hypothesis, it organises several strands of thought, 427 00:50:32,470 --> 00:50:42,340 several classic strands of thought that had been there for ages, but more or less untethered, strands of thought that really have been shared by many. 428 00:50:42,700 --> 00:50:52,270 It mutually reinforces these strands, and it also suggests something to do a particularly fruitful and fruitful direction, I think of inquiry, 429 00:50:52,750 --> 00:51:00,850 namely the development of a more rigorous theory of paleo tides and what their effect on paleo habitats might have actually been. 430 00:51:02,320 --> 00:51:05,830 And I think on that note, I will stop. 431 00:51:06,130 --> 00:51:06,940 Thank you very much.