Okay. Good afternoon, everybody, and welcome to this week's Physics Colloquium.
So it's a great pleasure today to welcome Steve Forbes to give the talk.
Now, Steve is by now quite well known to many of us,
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.
He was a professor at Virginia, and then later at the Annexe in Paris.
And in 2012, he moved here as the as the new civilian professor.
He's he's he's famous for his work in astrophysical fluids in the general area of astrophysical fluid,
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,
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,
on, on the website, it's really quite something, but that's just by way of a little bit of entertainment.
So today Steve is going to talk to us about where we all came from.
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.
Thank you, John. Sure I'm on. All right.
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.
I think one of the charms of doing astronomy, at least for me, is that there are profound truths to be had,
and they often begin with the most innocent astronomical observations.
Classic example is Olbers Paradox. It gets dark at night.
It shouldn't, at least in the simplest models of the universe, an eternal Euclidean universe would not.
The universe had to have had some kind of the beginning.
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.
They have the same angular size, about half a degree, 30 minutes.
So in most textbooks, if it's mentioned at all, this is just it's written off, just a coincidence.
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,
at their locations that they have, does something besides make nice eclipses.
Their gravitational fields, of course, exert tidal forces on the earth.
You all remember what a tide is. I don't need to go into the details, I think, for this audience.
But there's basically a equilibrium between the centres of mass.
In this case, I'm just looking at the moon and the earth. So if I go to a point on the surface,
it's not actually an absolutely precise force balance because the centre of mass points
perpendicular to this vector are the actual forces slightly off pointing to the moon.
And it's this difference which represents the tidal force. It's not difficult to work this out.
It's a sort of an elementary calculation. The tidal potential turns out to be proportional to the P to spherical harmonic.
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.
It points inward along the U and V acts UN vaccines, and it's repulsive along the w axis.
Key thing is that it is proportional to one over are cubed,
not a one over r squared force because it's the difference between one over r squared terms.
It's a one over ah cubed in its spatial dependence.
There are two tidal bulges, one on either side the first time one sees it.
It's a little counterintuitive. You can't understand why the far side is in kind of sucked along toward the moon.
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.
This is in orbit. So there's a centrifugal force. The centrifugal force is correspondingly weaker.
As I move away, the centrifugal force gets a little bit bigger and the gravitational force diminishes,
producing at least leading order a symmetric bulge.
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,
when you calculate the tidal forces from the sun in the moon, they are they're almost the same.
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,
and you can see in the numerator you have terms which differ by eight orders of magnitude.
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.
Well, the matching angular size and the matching tides are not two different coincidences.
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.
And I write the masses as some average density times a diameter cubed or proportional to that.
Then you can quickly see with one line of calculation that if I write that relationship in terms of angular size,
the angular size of the moon and the angular size of the sun are equal to one another, except for this factor here.
And if F is slightly bigger than one, the density of the sun being a gas and the moon is slightly less than one.
And it comes into this one third power and it is never a number which differs very much from unity.
So the point is, is that even a rough agreement in the tidal strength produces a very close agreement.
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.
If you're anywhere near being in focus, then your position is very tightly constrained and you can see this graphically as well.
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.
So the real importance, I think, of the angular size argument is that it tells us an Isaac Newton, this isn't anything new.
This is something even argued. It's not a difficult calculation.
This is something that Isaac Newton was able to use.
In fact, he has a he has a well-known,
ingenious argument for why the density of the moon has to be bigger than the density of the sun based on tidal forces,
the same angular size and the ratio of spring tides to need tides.
So this is hundreds of years old. But somehow it kind of gets lost.
It's not something that is widely appreciated.
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.
Right. Dynamics have content and we can begin to ask serious questions about what the consequences of the equal tidal forces are.
So is there something important to us being here now that is related to the sun and moon having comparable tidal forces?
In other words, is there some sort of an anthropic basis for this?
And I will try to make the case that there may well be.
So if we want to calculate tidal forces the way we do this is we put the.
We go back to Ptolemy. We put the Earth at the centre of the universe.
Then the sun goes around in the ecliptic. Here's the celestial equator, the north celestial pole, south celestial pole.
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.
And the moon is also on the celestial sphere, roughly speaking, not exactly, but roughly speaking.
Also going around in a circular orbit, elliptical orbit, but circular orbit on the celestial sphere inclination angle of 28.6 degrees.
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.
So the moon orbits with a frequency. Well, let me do it.
I have it down on the slide in terms of frequency, a period of 27.3 days.
The sun would go around, obviously, 365 and a quarter days.
So these are slow. They proceed around slowly.
They don't whiz around on the sky. But of course, we perceive being on the earth much larger frequencies.
They're Doppler boosted by the one day period of the Earth's rotation.
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,
but not exactly because of the difference in the frequencies of their own particular orbits.
Well, we know what happens when two periodic functions superposed and have comparable amplitudes and nearly but not quite the same frequency.
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.
What this means physically is that the net tidal force and the resulting tidal displacement, the height of the sea, will be strongly modulated.
And that in addition to having this one day or twice a day frequency component,
in other words, from one high tide to another, the incoming sea level won't be the same.
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.
And over the course of time, depending upon circumstances and the precise layout of the land.
You can imagine a complex network of such tidal pools being created.
Schematically, there's a high tide, and then the next high tide could could quite easily leave an isolated tidal pool.
And that poor little fish may be stuck there for a month.
So how do we actually calculate the height of a tide? Let's call it Zeta.
A standard textbook way of doing it is basically to calculate how far and actually
potential surface is displaced by the presence of the moon or in more physical term.
Energy equals force times distance for your GCSE.
So memorise this equation g times zeta is potential energy.
So G is the earth's gravitational field. Z The tidal height.
P The potential energy. Well, that's fine, except hey, you know, we're talking about water and water doesn't do that.
Water sloshes around. So these are the so called the plus tidal equations.
And it's actually reasonably you can sort of get a handle on what's going on.
We have a Coriolis force term, Theta and Phi are just the theta and Phi coordinates on the surface of a sphere.
Theta is called Latitude Phi. As a smooth Zeta I can advance this.
Zeta is the height above the average sea level.
H is the actual depth of the sea positive number.
And so my equations two equations for f equals m e I include.
This is the pressure term with the little bulge of the wave on top, the Coriolis term,
the inertial derivative and phi is the forcing tidal potential so to equations
for the theta and PHI components of the velocity and mass conservation equation.
And G is a kind of convenient variable to work with.
G Zeta plus phi is how I define big g and the equilibrium tide,
which I just calculated before corresponds to g equals zero, v equals zero and no time derivative.
Just to remind you what spherical coordinates are.
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.
Although it's not too bad. So g remember has a piece of phi in it.
So it's a little more complicated than it looks because I have Phi on both sides of the equation.
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.
And D is a resonant denominator, omega squared, minus four omega.
I realise I'm going a little fast here. I'm assuming a harmonic dependence of time in the tidal potential since I was
linear ising I should say that linear ais in the equations it makes sense.
248 Decompose which is actually a pretty good approximation anyway.
48 decompose the force in tidal potential and I wind up with this equation to solve
and there's a resonant denominator at a frequency of two omega cosine theta with two.
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.
FIELD But if you did a purely hydrodynamic calculation for the tides where you got rid of the land,
you know, the spherical cow calculation and just took H to be a constant.
You can show that just the tidal forcing itself actually produces a result which is pretty unimpressive.
You get tides about 30 centimetres.
So clearly to get something more interesting, this function h the depth which will in general depend upon Theta and Phi obviously,
and the outline of the continents themselves are key players in this kind of a game.
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.
Shortly, we would need a theory of Devonian bathymetry, as it's called, and detailed numerical simulations.
So what's the difference between depth and bathymetry?
Well, bathymetry refers not only to the depths of the seas, but the height of the continents as well.
And when people do these kinds of calculations, they like to know whether there will be inland flooding.
And so they are careful to calculate the slop that comes over the mountains
and whether you can get so-called epi continental seas on the land as well.
And in fact, we are trying to do that as I speak. I have a group of people I'm working with,
and we're using reconstructions to try and get some idea of what the tidal effects really were like 370 million years ago.
But look, we're physicists and there was a time when physicists didn't go whining to their computers.
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?
Sorry, that's uncalled for, but I'm getting old in ten.
But look, there's a small parameter in this problem.
Yes. G h over four omega squared.
R squared, which appears in the equation. What is that?
That's basically the speed of the wave it will turn out compared to the rotation rate of the earth.
And that is small, which is one reason why we're not use equilibrium theory.
Well, the good thing about that is that it allows for a w k b solution for that wave equation.
So what I want to do is not even worry about the tidal forcing.
Just now I want to look at the free waves, the free wave solutions of my tidal wave equation.
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.
And then I look for good old w kb wave solutions of this form.
It's two dimensional and you see that less frequently in the literature.
But it's the techniques that are used actually extend fairly naturally.
And being a theorist, I stick in Epsilon just for fun because I want to keep track of my orders in a perturbation calculation.
So I won't cover the boards with equations.
I will simply show you what emerges from that kind of calculations.
And it's actually starting with something that looks pretty hairy.
It's pretty simple. What emerges in that kind of an analysis, the first,
the largest turns the one over epsilon squared terms grouped together to form a wave dispersion relation.
And lo and behold, it's the universal dispersion relation for all of physics.
As far as I can tell, it's the scalar fields.
It's plasma waves. It's pendulums with springs connected to one another.
Omega squared is something times K squared plus a constant.
So that's very good. We're on familiar ground. These are called long waves.
You may wonder what happened to that really messy bit with the grab this cross.
Grab that. That's actually another wave branch which is there.
But I won't be using here called Rossby Waves, which are very important in geophysics and for, among other things, weather patterns.
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.
Now, the next bit is kind of interesting so that if you think in terms of optics, that was geometrical optics.
This is now physical optics, what dynamics this would call wave action.
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,
but it folds up nicely into a divergence. So the divergence of g.
G is just a constant, the earth's gravity. But I keep it with h h a squared amplitude squared divided by this resonant
denominator d times the wave number k group velocity is just proportional to K.
So this is a sort of conservation of wave action.
And what it tells me is that if I have a channel,
a simple channel of depth H with W and I'm working at a latitude lambda that the
wave energy A squared is proportional to the grouping that you see in front of you.
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.
And we'll be making use of this and estimating what will be going on under different paleo geographic constructions.
Just to orient you, the local group velocity of shallow water, tidal waves.
V is about due to the one half times this cosine factor.
So if we take the average depth of the oceans 4.2 kilometres,
we get something which is about the speed of a jet aircraft and that's about the velocity that tsunamis travel at.
Important thing to take home. We don't even need to know the numbers.
We get a faster response to external driving for deeper H.
For bigger h. Finally, something I'm going to come back to and it could be potentially very important is the possibility of resonances.
So if I use the dispersion relation to calculate the wave number for a given frequency, I get a very simple looking equation.
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,
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.
It corresponds to flapping at the open end, and I'm going to be interested in that particular configuration.
So I show that to you now. We'll come back to it in application later.
What does the actual tidal tidal forcing look like?
Well, this is what the forcing would look like. This is the nominal equilibrium tide is what I'm calculating.
But it's proportional. It's basically proportional to the forcing potential.
Here is the result for tidal forcing with Devonian parameters without the moon.
So this is in centimetres. This is an ours.
The whole thing is about five months. Each of these is about 20 days.
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,
but the same density, I'd get something a little bit more interesting, but not very much.
And if I put in my best estimate for the real realistic Devonian parameters,
I get something which is really quite sculpted and really shows strong modulation from one.
You can be imagine a a tidal height coming in here and then you notice the problem.
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.
But if you're caught on the side up here, then it can be a long time before you get replenishment.
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?
So I like to introduce some people, some creatures I've been spending some time with.
You see a contest, Dega contest. Dega is one of the early tetrapods and one of the first body fossils ever discovered goes back.
Excuse me, this is Ichthyosaur Staker, which was discovered in the 1930s,
a contest agar which has just been recently rediscovered in terms of getting much more complete fossils.
And these are very early tetrapods. You can just see the beginnings of fins turning into feats here.
This is pandemic, these pandemic. These is a fish with tetrapod like characteristics and occupies kind of a cusp position in the evolutionary tree.
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.
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?
So just to remind you of the basic story, there was a high tide and there will be a low tide.
The next high tide isn't going to help very much.
If you're stuck in an inland pool, then there is a low tide and good luck to the fish there.
Who could be there for a month? This is a Paleo geographic reconstruction of the Devonian.
Devonian was a time when we had two supercontinents.
We had gone on to land in the south and then what's called your America, your America or Laurasia.
I don't know why they're two names, but it's referred to in both ways in the top.
So we had two supercontinents.
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.
And that is exactly the kind of configuration which is interesting.
From a title point of view, if I look at this now in isolation,
here are three regions of the earth famous for their tides, the English Channel, which has enormous tidal currents,
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.
And if you look at the configuration of the relic ocean, that looks very suggestive.
You find a similar thing on the northwest coast of Australia.
So don't put off be put off by all these kind of little funny islands.
This is actually a continental shelf and this is only a very shallow sea.
So you have a rather deep sea here and then you have a width which is narrowing
gradually but dramatically over the course of a couple of thousand miles.
And you get some of the highest tides in the world on the northwest coast of Australia up to nine or ten metres.
Now in the late Devonian we just saw the western regions of Eurasian and gone one and landmasses contained a subcontinent sized bay.
This is something not very different in size than India. Narrowing down to intercontinental shallow straits.
This really is a greatly scaled up version of what we see in the Bay of Fundy,
Bristol Channel, north west coast of Australia, other places famous for their tides.
Well, just the square root of argument explains what's going on here.
The eastern shallows, because each is so much less respond much more slowly to the deeper western water.
What tends to happen is you get an impedance match surge of Western Bay water.
Provided the coastline is in too irregular and it's dumped into a very constricted
channel and the eastern tidal amplitude can become very large indeed.
Scale up the Bay of Fundy. I may be cheating a bit because I think the Bay of Fundy has resonant interactions,
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.
This is Planet Swamp. So was the Devonian, in fact, special?
The Devonian really was special. The Devonian was heavily forested.
The first trees. Archaeopteryx.
Not Archaeopteryx. Archaeopteryx, one of the first trees ever could grow to a height of 30 metres, and it apparently loved wetlands.
The first deciduous leaves appear. Devonian was a time when the climate was stress.
A plant took a long time to get the idea that it should evolve live.
And then in the Devonian it decides that it should get rid of the leaves, that it just spent all this time and energy.
Why is that? It was expensive to keep a leaf in the wintertime.
The idea of dormant plants, trees losing their leaves arose in the Devonian.
There is fossil evidence in the form of black shales for sea level changes and increased organic marine deposits,
i.e. all those leaves fall falling in these heavy forests.
They didn't just stay there or didn't always just they got washed into that narrow channel.
What also seems to have happened is that there was widespread anoxia.
Anoxia is depleted oxygen in the waters.
And it's something that we see today when we're not careful with organic with dumping of organic materials into enclosed ponds.
The last such event, when there seems to have been this large in wash,
is known as the Hanging Berg event was general and widespread and one of the great mass extinctions of all time,
and it decimated the shallow water loving, lobed fin fish, basically the ancestral stock of the tetrapods.
So this just doesn't happen every day. The Devonian was an unusual period.
Let's look at paleo geography a little bit more carefully.
Here is the slurry in the period just before the Devonian.
You can see things happening, but we don't have a narrow channel.
We have a rather equally distributed broad strait between Laurasia and gone one inland and gone into the early Devonian.
Things are already starting to happen, or Asia is descending in the east, constricting it.
The ocean is still open. When we get to the late Devonian, we are in extremis now.
We have very narrow straits here and a big broad opening in the West.
In the early carboniferous we've been cut off.
The wreck ocean has all but disappeared and we're starting to close off.
We don't have anywhere near the sort of tidal receptivity that we had in the Devonian, the late Carboniferous.
We are just about to form Pangaea. If we see the formation actually of the Appalachian Mountains in the Atlas Mountains in Morocco,
as these two continents come together, and then by the Permian, we are one supercontinent.
Let's go back to the late Devonian and think about what was going on.
Recall w kb theory for depth h channel width w latitude lambda.
The wave energy was proportional to the combination you see there.
Cosine lambda over w h to the one half. So to amplify a wave, what do you do?
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.
Channel three travel into ever more shallow water, which is exactly what that Relic Seaway would force the waves to do.
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.
Well, I think all of this suggests a new line of support for an old idea.
Dating from the 1930 is a classic argument due to the eminent palaeontologist Alfred S Romer, who said, excuse.
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.
Charity and limbs. Charity and limbs. Physicists know that chirality refers to fingers, handedness, limbs that are digitised,
that have fingers were, according to Romer, an adaptation by lobed fin fish.
Lobed fin fish are very important. The bones in their fins are homologues to our own humerus radius and ulna.
You can trace it back 370 million years.
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
and remember you're going to have progressively deeper puddles as you get closer to the sea when their own puddle turned inhospitable.
It's inevitable. I like to call them. Sorry about this. Romer's Romer's.
Now, Romer argued that the red clay sediments in which these early tetrapods were fossilised was typical of arid conditions.
And I think at the time most people agreed and they said, Well, great, that's a story.
Somehow you get a pool of water in the desert, never mind that part.
But that's what they thought. The pools dry, the stranded fish that can manage some degree of out of water navigation will live,
impress their mates with their stories and have more baby fish.
Well, subsequently it was understood that these red clay sediments need not reflect arid conditions,
nor are they always representative of the sediments in which the organism in fact was laid down.
So they threw the tetrapod out with the bath water and gave up on the whole idea, which I think was premature.
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.
Well, there are obvious advantages to having paddles on your body.
I think if you live in choked, swampy, intertidal wetland with a vast network of available pools,
there need not be one dominant reason for the development of tetrapod morphology from the French.
Of course, the best suggestion yet that it was for better sex.
I recommend this particular reference because it even comes with diagrams, a sort of Kamasutra for a contest stages.
Bottom line, this is a family presentation, so I'm going to stick with my black and white slides.
There are all kinds of reasons to try to leave.
Maybe you can think of some as shallow, disgusting, fetid, slimy, brackish puddle, even under non arid conditions.
Especially. Especially under nine.
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.
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.
And we see this today in Southeast Asia. There are so-called mud skippers and climbing perches, which in fact,
in fact flap around and they're very much kind of a ro marian ideal in their behaviour.
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.
And so apologies to the bard we many we happy many we banned of Roemer's Roemer's.
We are all tetrapods now. So imagine my delight when I read about the relatives, a family in Trackways.
What, you ask, are the Australian trackways. Tetrapod Trackways Extra x three.
Read all about it from the early middle Devonian period of Poland.
This was definitely four star RAF material.
What was discovered in 2010 was that 18 million years before they should be present before there was any evidence of body fossils.
There are beautiful footprints clumping through the mud, walking.
And I do mean walking. No body dragging. These are highly developed and highly specialised.
Fully Ceridian. That is to say digitised penta dactyl.
Five, five toes, limbs or five fingers.
I guess these were thought to be the analogies, the hand, the four pads, the four.
You know what I mean? The tetrapods. The tetrapods and the their closely related cousins lived extensively for tens of millions of years.
And this came as a real surprise. There is an actual picture.
You can get a sense one, two, three, four, five.
And they've actually have. Now, I have learned from the palaeontologists involved with this discovery have a remarkably
detailed reconstruction of what the foot morphology look like and is surprisingly advanced.
That had obviously been evolving for a long time before the footprint was made.
Where did they find these? Where were these trackers?
The if fairly and by the way, simply refers to the period within the Devonian when the time period,
when the discovery was made, it was found right near the title Sweet Spot.
So the discovery of the stallion trackways in what is now Poland in shallow seas
has prompted yet another scenario for why tetrapods may have left the water.
So the idea here put forth by the eminent palaeontologist Jenny Clark,
is that the tetrapods are not the only ones who are going to get stuck in these isolated seas.
You will have less fortunate fish that are also stuck in these tidal pools.
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.
So this was another motivation for why growing feet would be evolutionarily advantageous.
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,
the word tidal comes up again and again and again.
Let's talk a little bit more about tetrapod habitats.
Now, remember a contest, Ortega and Nick Theo Staker, the body fossils.
They were the first ones found back in the 1930s.
They happened to be associated with non marine inland basins in East Greenland, what is now East Greenland.
And for many years people thought that the first amphibians is what they called them.
That's actually a bad word to use because these tetrapods are no more related to amphibians than they are to us.
But these early tetrapods were, in fact, freshwater.
And that sort of set their the way of thinking about their origins for many years.
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,
but key transitional species in marginal tidal environments.
These really are the creatures from the Black Lagoon.
The assumption origin of freshwater origin for Tetrapod has strongly been strongly challenged, writes Jenny Clarke.
Among more recent finds of early tetrapods are many that are now known to be tidal, marginal marine,
brackish waters, one of the richest sources of tetrapods and so called tetrapod or more for fossils.
These are fossils of fish that have tetrapod like characteristics.
There's a huge tidal delta in what is now modern Latvia, Lithuania, Russia, Estonia, the so-called Baltic Devonian Delta.
It was around 30 million years. It was a Delta plane and it was graded.
It's tilted slightly. And it shows direct fossil evidence for tidal intermittency in the sedimentation rates.
Sedimentation in the upper plane were sporadically interrupted by spring tides.
It was very much an it not only a tidal environment, but an environment that was dominated by intermittent tides.
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.
And I can't even pronounce that East Collie Mountain and Cowley Mountain.
It had a span.
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.
But you have to imagine a region that was ten times the size of the comparable between, say, here and Edinburgh.
Huge. Well, there's a huge number of transitional species that have been found there.
I won't even bother going through the names except to note that in addition to the trackways,
there are early tetrapods and the ancestral stock that is believed to have given rise to the tetrapods.
And they're all found in this huge tidal region.
So it's very compelling evidence that there's something special about the way these creatures evolved and this particular tidal habitat.
Moreover, there is a consensus that the Devonian was indeed a special time for other reasons as well.
Writing in a collection, Devonian Events and Correlations, a collection of scientific papers.
The lead paper starts by saying a combination of climactic plate tectonic magmatic and still poorly.
Understood. Apparently, oceanographic factors cause the recurrent sudden perturbation of what were
stable ecological conditions by short term global events of variable magnitude.
What could these poorly understood Paleo Oceanographic features be?
Allow me to make a suggestion. What about a global tidal resonance?
Let's go back to our formula. Now I'm cherry picking here, but it was easy to do.
If you look at the map, you get a length of a little over 5000 kilometres for the Seaway.
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.
The period that's associated with the Devonian Lunar Tidal period, the moon was of course,
a bit closer in the Devonian 365,000 kilometres, it's believed, instead of 390, which is where it is now.
Then, lo and behold, the calculated wave number that is associated with those parameters is in fact
equal to the and equal to one resonance in the formula that I have here.
So the conclusion is not that those numbers are accurate and that that is what happened.
But remember, the Ric Seaway was around for 30 million years at least.
What's the timescale associated with Continental Drift, as it used to be called?
It's centimetres per year over timescale of 30 million years.
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.
Right? So I think it is not at all far fetched to imagine that these kinds of occurrence could easily have been relevant.
I think a synthesis emerges of a truly special Devonian period, where a confluence of highly modulated tides,
a receptive geography, led to a vast network of isolated pools and planetary scale swamps, heavily forested.
As it was, there were few herbivores on the land, nothing to eat, the leaves full of deciduous trees.
And we also now know dispersed seeds.
Hard seeds were invented during the Devonian period as well and allowed trees to colonise vast areas.
The accumulating plant debris in swamps produces a background of slowly degrading, anoxic waters.
This is not my idea. This was put forth already 15 years ago in an influential and well-known paper.
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,
including organic soils, because there were no organic soils before there was this extensive plant cover.
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.
These tetrapods apparently coexist for at least 20 million years, 18 million years with their elitist posterior cousins.
Those are the fish that gave rise the ancestral stock to the tetrapods.
They coexisted without being dominant until the shallow,
water loving lobed fins fish were eventually decimated by encroaching anoxia from an overwhelming,
perhaps tidally resonant in wash,
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.
However, the offshoot tetrapod line has options under these circumstances more attainable niches in hard times.
In other words, Romer's romer's were the ones that managed to thrive.
So let me offer the following closing perspective.
The Sun and the moon, which is where we started, appear to be the same size.
I prefer to view this as my fossil.
This is the astronomy fossil. It's telling us something important if we know how to read it.
It's telling us that there is an important role in evolutionary biology and for us being here for sizeable, significant, highly modulated tides.
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.
By contrast, if we adopt it as a working hypothesis, it organises several strands of thought,
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.
It mutually reinforces these strands, and it also suggests something to do a particularly fruitful and fruitful direction, I think of inquiry,
namely the development of a more rigorous theory of paleo tides and what their effect on paleo habitats might have actually been.
And I think on that note, I will stop.
Thank you very much.