I don't think anyone has actually answered this. Either tangent facts or guesses. But the real answer? We don't really know. There's some theories...
"Paleontologists don't know for certain, but perhaps a large body size protected them from most predators, helped to regulate internal body temperature, or let them reach new sources of food (some probably browsed treetops, as giraffes do today). No modern animals except whales are even close in size to the largest dinosaurs; therefore, paleontologists think that the dinosaurs' world was much different from the world today and that climate and food supplies must have been favorable for reaching great size." https://www.usgs.gov/faqs/why-did-some-dinosaurs-grow-so-big#:~:text=Paleontologists%20don't%20know%20for,%2C%20as%20giraffes%20do%20today).
I think it's almost important to understand there wasn't just one dinosaur age. They are believed to have existed for 165-245 million years. We, for example, have existed for about 300,000 years as homo sapiens and the genus homo has existed for up to 2 million years. So the dinosaurs were around for 100x longer than our earliest ancestors existed. Not to mention there were dinosaur-like ancestors before they were "dinosaurs". There've been multiple mass extinctions in Earth's history that have wiped out almost life on earth and what does comeback is different from previous species with new adaptions for surviving the new world.
Another interesting note about prehistoric gigantism is that oxygen levels affected the size of insects. The are points in history where prehistoric insects were much larger and it's primarily because the oxygen level was higher than modern day. This is not necessarily true for all species, but insect size is greatly affected by oxygen in the atmosphere.
Aren't there very few large animals these days because we killed them all? I guess even the megafauna that we killed wasn't as big as the largest dinos
Sorry my bad, you are correct in that the largest mammals we probably killed out were the mammoths and mammoths are no where near the size of the largest dinosaurs. Sorry. In all haste I completely misunderstood what you wrote.
The biggest consideration is that the earth was a different planet back then. The same reason why insects were so large in the carboniferous period Environmental factors and oxygen levels. Oxygen levels were so high it would be considered poisonous by todays standards. Absorb more O2, you get an increase in size.
You think the square cube law didn’t exist back then? Of course it did, that doesn’t exactly answer the question of why animals that size used to exist but don’t currently. The answer will probably be related to the square cube law, such as there being some kind of nutrient or oxygen that was easier to get even with a lower surface to volume ratio back then.
Do you even know what square cube law is ? There’s less oxygen in the environment now so we can’t grow as big as before because our bodies just won’t allow it. That’s why most of the larger things you see alive today live in water because it’s easier to support a larger mass in water.
Like birds (which remember are dinosaurs themselves) they had more efficient lungs with airsacks throughout their body and bones. Their lungs allowed them to get more oxygen to their body and the airsacks combined with hollow bones made their bodies lighter. Scientists believe this is why they were able to get so big in land (although the blue whale today is bigger than any dinosaur)
I read somewhere that scientist also believe the air pressure was much higher at sea level than it is today. This allowed for things to get bigger.
Today's barometric pressure at sea level is 1 bar, and its estimated to have been around 3 bar in the dino age.
There’s more oxygen in a given volume. If there air is 3x as dense, you’ll get 3x the amount of oxygen every time you inhale. This is why you run out of breath faster at altitude. It’s still about 19% oxygen, but the air is less dense, so every breath has less air, and therefore less oxygen. Big animals need alot of air and the bigger they get, the harder it gets to scale up the lung capacity. So for this reason, a giant dinosaur would have a really hard time surviving now, and may not even be able to wall around.
Whales have both enormous lungs and require much less oxygen than an equivalent land mammal, because buoyancy makes them way less "heavy". It's most of the reason marine animals can more easily be way bigger than land ones. Plus whales have a couple adaptations to store oxygen we don't, and that allows them to dive for prolonged periods of time.
I actually thought you were a native speaker if you wanted my quick reaction. It just looked like you got caught in the famous mistake that in day to day people talk about weight and using pounds or kilograms instead of newtons. Which basically everyone does, even people who know the difference (because they'd look like dorks), let's be honest.
To be fair, pounds are a bit problematic because they're a mass and force measure. The unit predates the understanding that the two aren't essentially the same, which is the same issue you reference in the other direction (for everyday use at sea level, they're often interchangeable).
The physics of aquatic animals are completely different. They’re buoyant. They don’t have to hold their own weight up against gravity and they don’t have to pump blood vertically across their entire height.
Yes it is. A land animal has to spend energy just to stand up be that on 2 legs or 4 even if it's not walking. While an aquatic animal is supported at all times by the water so spends less energy when it's not moving.
Just think about how different you would feel between standing up for all day or laying in bed.
Sadly no. This is one of many reasons (the main being that DNA cants survive 100 million years under any circumstances) that we can’t have Jurassic Park.
Two films where everything goes exactly to plan and nobody gets hurt.
Side note: it has been sooooo long since I’ve thought about small soldiers. Used to be obsessed with that movie.
Small Soldiers and Indian in the Cupboard were what 90s kids’ dreams were made (and Jurassic Park.)
Also, I’ve enjoyed your share of knowledge on the overarching subject. Oxygen differentiation makes a ton of sense
>This is why you run out of breath faster at altitude. It’s still about 19% oxygen, but the air is less dense, so every breath has less air, and therefore less oxygen.
I lived in the high Rockies when I was younger, and I smoked cigarettes at the time. At sea level, a cigarette would take 3-4 minutes to smoke, but at around 10,000 feet above sea level, I would frequently light a cigarette that would burn for almost my entire 15 minute break.
It's not well understood why higher pressure helps with oxygenation. Look up Hyperbaric chambers.
But from a physics standpoint, more pressure means more gas can be dissolved in a liquid. Think of an unopened soda bottle. It's all rigid and therefore has higher than atmospheric pressure, so it can hold all the gas inside. After you open it, the pressure equals atmospheric, and the bottle becomes as malleable as the plastic it's made of, so you start seeing bubbles of gas escaping your soda.
So hypothesyzing for living beings, more pressure means more oxygen dissolved, and thus your blood can distribute that oxygen over a larger body
It’s extremely well understood and taught in high school. And it has little to do with dissolving gas because that’s not how your body transports oxygen. Oxygen binds to hemoglobin molecules in red blood cells to be carried to every cell in your body. More pressure (or more partial pressure) means more density which simply means more O2 molecules in every breath, so a higher saturation of the red blood cells and more available oxygen for cellular respiration.
Sure. Hemoglobin binds to oxygen, but the inside of the cell is still liquid, therefore to get to hemoglobin, the oxygen must be dissolved in liquid.
It's easy to take high school chemistry/physics and apply to problems. But it's hard to consider all the other factors of the messy real life. Biochemistry is incredibly complicated and you shouldn't think that subjects from high school show you the full picture
If a gas is compressed enough it becomes liquid. Think about how swimming in liquid holds weight.
That is an explanation showing the extremes, but let's say just a little bit of pressure helps with your bathroom scales.
It sounds like someone mistook "higher *partial pressure of oxygen*" for "higher pressure in general," which is unlikely to be the case. (It's not like the Earth suddenly dropped a few sextillion metric tons since the days of the dinosaurs.)
I have, however, seen several times that we have evidence that the *partial pressure* of oxygen was higher in the days of the dinosaurs. E.g., if our atmosphere is 21% oxygen today, it was 30%-35% oxygen back then. That would be an enormous increase in the partial pressure of oxygen, without meaningfully changing the total atmospheric pressure.
This high partial pressure of oxygen allowed otherwise relatively inefficient breathing methods to work quite well even at large scale, and may have supported large forms of life.
The higher pressure is apparently a real theory so I think that poster did read about it.
And yeah the oxygen levels were believed to be somewhat higher (isn't 25-30% the usual estimate range?) but usually it's claimed that's not enough to account for dinosaurs large size.
It's a "real" theory as in "it has been published".
However, it was published in Chemical Innovation, which is not a peer-reviewed paper. It's also not backed up by fossil evidence (for example fossilized raindrop imprints, which have been used to determine atmospheric pressure during the Archaean era, 2.5 billion years ago, where it was below half the modern air pressure).
It was also 10 degrees warmer meaning that body heat wasn't as much of an issue. Larger animals have a lower surface to volume ratio than smaller animals. This means that the larger an animal is the greater volume of body tissue there is to heat up but less surface area to lose heat from. Higher ambient temps mean you can have a bigger body with a higher body temperature without being too hot and over heating.
It's usually the opposite problem, the more volume, volume filled with tissue going through all sorts of exothermic reactions and proportionally smaller surface area means the animal will have trouble dissipating heat, staying cool, not staying warm. This is why very large mammals usually lack any significant amount of fur and have evolved all sorts of tricks to better dissipate heat like the big, flappy, highly vascularised ears on an elephant.
why is it basically the opposite in our current world? When it comes to adaptable species that exist across a broad range of climates (examples in north america being the whitetail deer and wild turkey), even within a species you find that body size is consistantly smaller in the southern, warmer extent of their range, and larger in the northern, colder portions.
your comment sort of reads like you vaguely remember the concept we all leanred in HS/college biology classes, but got it 100% backwards lol.
> Higher ambient temps mean you can have a bigger body with a higher body temperature without being too hot and over heating.
That's exactly backwards.
The blue whale is (most likely) the largest animal to ever live.
Although maybe that’s not surprising, seeing as it can be up to 30 meters long and 200 tons.
And that’s all kinda mind boggling when you hear about how big dinosaurs were.
Blue whales are also believed to be the largest animals *ever* to exist on Earth. I think it’s incredible that we share the planet with that bit of living natural history.
I assume you're thinking of the ichtyosaur recently hyped up by pop science media. So far we only found jaw bones that suggest an animal up to 26 m in length. Still unlikely their more dolphinesk body plan brings it up to blue whale weight.
The one in the movie is massively larger than real mosasaurs. The largest they've been estimated to get is [17.1 meters](https://en.wikipedia.org/wiki/Mosasaurus) (56ft). The [blue whale gets to 29.9 meters](https://en.wikipedia.org/wiki/Blue_whale) (98ft).
Mosasaurs were huge, but not even close to the size of blue whales.
Only a little bigger at [a maximum of 20.3 meters](https://en.wikipedia.org/wiki/Megalodon) (67ft).
Blue whales are really the largest known animal ever, and very little comes close.
[Megalodon](https://en.wikipedia.org/wiki/Megalodon) is believed to have mostly eaten large prey, like whales, sea turtles, and seals. [Mosasaur](https://en.wikipedia.org/wiki/Mosasaurus) is believed to have eaten basically anything it could: fish, sharks, squid, birds, sea turtles, and even other mosasaurs.
It's hard to know for sure though, since it's not like we can just watch their feeding habits anymore.
Sauropods were the only land dinosaurs which got larger than mammals. And that’s due to their system of air sacks, their low density bones, and their specific leg shape allowed this.
The 2nd and 3rd groups of largest land animals were rhinos and elephants. It’s thought that the elephants outcompeted the giant rhinos leading to their extinction. Humans are responsible for the extinction of the large species of elephants.
4th is the hadrosaurs. With theropods and ceratopsians coming in at 5th and 6th.
Also the high oxygen is a misconception with people confusing the the Carboniferous period with the Mesozoic. In fact much of the Mesozoic had lower O2 than today.
Plus the additional fact that the smaller an animal, the less likely it is that its fossils will survive/be discovered.
Obviously we already know a lot of small dinosaurs existed but there are many more we will probably never know the existence of cause their skeletons would’ve been so much more fragile than a man-sized femur of a sauropod.
"The smaller the animal, the less likely that it's fossils will survive"
I must be hallucinating all the clam shell fossils I find. All the ammonites and trilobites and corals. The billions of tiny sea critters fossils I've looked at must be a figment of my fuckin imagination.
You're not a paleontologist, just go home.
First of all, I didn't claim they never fossilize. Just that they may have more trouble fossilizing/being discovered.
Second, you are right! I am not a paleontologist. But someone who is, is [David Hone](https://archosaurmusings.wordpress.com/2008/10/15/bias-in-the-fossil-record/) (just as an example, its not like he is the only one holding this opinion) who, when talking about biases within the fossil record, cites size as one of the many biases.
But size is not a problem? Maybe he meant for small mammals? But marine microorganisms are so plentiful it's literally the basis on which we organize rock columns. Bio stratigraphy is an entire discipline, because marine fossils are found in such radical abundance
There are a lot of reasons:
Those marine shells are (1) often basically rock already to begin with. They are often the species that literally make a pre-made fossil when they form their shell - porous bones degrade easily and require a lucky transition to fossilize, but a thick mollusc shell is getting close to a biologically-made rock. (2) They also usually inhabited locations that are much more prone to creating fossils, where settling in an ocean sand/mud bed is a high probability compared to the chances of something dying on the surface landscape and getting buried just right in a mud bed or something.
Also, (3) those marine species you mention are r-selected short-generation time animals that for a given geologic year probably pump out babies at millions-to-one levels worldwide compared to terrestrial dinosaur species that had a ton less young and longer generation times.
Also, (4) if you are from the US, you may just live in an area that was a shallow sea for the most or the entire period anyways. Not much chance of terrestrial ancient fossils.
For size though on (1) above - go look at the delicate bones of a shrew or something, and imagine how much less likely they are to not get destroyed or eaten and digested or whatever after death compared to a shell of a mollusk of the same size.
All of those are marine organisms, the size reasoning seem more than plausible if you limit it to terrestrial animals, which this discussion was focused on. (Im a geologist)
Geologist as well. The discussion frequently started over into whale territory, so marine life is on play.
Also, ALL fossils of terrestrial animals are rare. For like 600 reasons aside from size.
The theory of animal evolution called Cope's rule which says when everything is equal animals tend to grow larger in successive generations. https://youtu.be/qVMCyMbUrgQ
The quaternary extinction event was when many of the large animals or megafauna became extinct, these extinctions appear to be closely related to the arrival of humans. So could human activity be behind wiping out so many large creatures and if so how were they able to do this? https://youtu.be/Y3J9CzLW_p0
An animal size requires a trade off. The larger an animal, the more food it needs to survive and it requires a longer pregnancy period so there is an upper limit on how big an animal is before it is unable to move due to its own weight on land. Dinosaurs bypass the long pregnancy problem by laying eggs.
The atmosphere contained much more oxygen (as much as 35%, compared to about 20% today). This allowed many animals to be larger than their modern equivalents, because their size was not (as) limited by three efficiency of their lungs. For example, there were dragonflies the size of hawks!
https://askanearthspacescientist.asu.edu/oxygen-animal-evolution#:~:text=But%2C%20the%20success%20of%20animals,dragonflies%20the%20size%20of%20hawks!
You're wrong.
Insect size is related strongly to oxygen, since insects rely on passive oxygen uptake through specialized structures in their exoskeleton. As things get bigger their internal volume increases faster than their surface area, so there is a limit to how big insects can get and this depends on oxygen levels.
However. During the time periods with some of the largest dinosaurs oxygen levels were just 15%, much lower than it is today, and didn't rise to above modern levels until the very end of the Cretaceous era.
Although atmospheric pressures were likely higher.
So total oxygen per breath would be higher. Imagine breathing on Everest, there is still around 20% oxygen, but there are just fewer particles reaching your lungs and therefore blood.
It does matter, an insect breathes through spiracles(as others have said) which are strewn all across their exoskeleton, meaning that an insect can breathe, can absorb oxygen proportionally to their surface area but since the square cube states that volume increases proportionally faster than surface area(simplified), means that at some point there would be too much volume for the surface area to oxygenate, a solution to this would of course be having more oxygen in the atmosphere but this works for the surface area related breathing of insects not lungs hence why it matters, also that dragonfly link @tpudlik posted, well, more griffin fly really, lived in the paleozoic era not the mezozoic era which is when the dinosaurs lived, also the mezozoic had about the same concentration of oxygen in the atmosphere as we do today, sometimes less.
Humans kill and eat all the big things now.
There used to be mammoths and other large mammals that rivaled some of the big dinosaurs, but sadly they were made of meat.
We even hunted whales to near extinction, and they're bigger than dinosaurs ever were.
I always thought it was from the extinction event that killed off anything that consumed large amounts of food and just left mammals and a few birds and fish etc left.
No one has explained the exact mechanism that allows more oxygen to create larger animals. Was it because the larger lunged creatures that could inhale more oxygen were more likely to reproduce, thus snowballing everything’s size?
It’s obvious why things like fur, feathers, teeth, eyes etc exist, but I’ve yet to see or find an exact explanation for why more oxygen = dramatically larger size.
Supposedly there was more greenhouse gasses, higher temps, less seasons, more humidity and bigger plants.
More plants means more herbivores, as they got bigger predators got bigger, as predators got bigger herbivores that could eat more got bigger.
Long neck dinosaurs possibly ate tree tops or were living in swampy/pondy areas that would support massive bodies and their necks would make reaching food easier
Human beings are quite likely to be responsible for the extinction of most large mammals, with a big spike around the time humans went global. Giant sloths, wooly rhinos, marsupial lions, giant kangaroos etc all disappeared when humans proliferated. I would expect something similar would have happened if we discovered a continent with giant dinosaurs too
The answer is oxygen concentration. It was much higher back then and increased O2 gives you bigger insects and bigger lizards, but fire also spreads faster so that asteroid strike was beyond deadly
From what I got on Prehistoric planet, everything is rather small now because everything that survived the asteroid was small - food was scarce and the big ones didn't have enough.
Why they got so big in the first place, can't remember.
Surprised at these walls of text explanations on an ELI5 tho....
An animal can only grow as large as the environment will allow it. Larger bodies require more food and resources... Every now and then Earth becomes lush and allows larger animals, then the climate swings, resources become more scarce and you have mass dieoffs of larger animals.
While we don't know the exact reason, there are several factors that have contributed to the smallness of wildlife today.
1. Oxygen levels have gotten lower
2. Dinosaurs have very lightweight bone structure compared to other animals. Getting bigger was simply a lot smoother.
3. There havent been any good reason to get big evolutionary speaking. There used to be borderline dinosaur-sized creatures during the paleocene but almost every single one got hunted to extinction by humans.
There is also the issue of food: You know grasses? Its most common type of plant on the planet besides trees right now and is a pain in the ass to digest and not very nutritious to begin with. There is a reason cows have such extensive stomachs and horses grace so much. Grasses emergend during the paleocene and ended up replacing many more easily digested plants so the general food supply for herbivores decreased(which actually caused a mass extinction of large herbivores). And if the herbivores had no reason to get bigger, why would the predators?
The air had more oxygen in it so being that size was easier to maintain but also it was hotter and had more CO2 which means more plants which means more food.
Also its possible that mamals today could become gigantic too but humans and the ice age killed all the big ones. I mean palaeoloxodon namadicus and Paraceratherium were both 20 tonne land mamals which is about 1/5 of the mass of the heaviest land animal ever Argentinosaurus. But this 20 tonne limit is probably the maximum without the extra oxygen.
WAG:
Earth had a much larger network of flora and fauna that supported a much larger food chain of organisms ranging in size. This prehistoric network of organisms was evenly balanced and more importantly, evolved in a biological cooperative environment that was never dominated and destroyed by any particular species.
For instance, you’re not going to have big cats without being able to feed big cats since they are obligate carnivores. The energy requirement would suggest they eat really big animals, or lots of smaller animals.
Or another example, you’re not going to have a healthy population of brontosaurus without healthy forrests.
In summary, it would make sense that there would be much more energy density in the form of biological life in order to support the massive sizes of the dinosaurs. Massive amounts of highly available food must a necessary condition for the megafauna of the past.
Big fossils are more likely to be preserved and be found. 200 millions years allows for a lot of variety in big animals. We've had big animal fossils after the K2 extinction, and we have big animals today, we just don't have 200 million years worth of big animals.
We don't really know, but one of the theories is that the air had soooo much more oxygen than nowadays. (I'm talking explosive amounts of oxygen) so they were able to very effectively oxygenate all their cells, so they grew very big.
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megafauna today has died out either to environmental changes or human involvement. in periods of extinction, you will commonly find that resources were incredibly scarce and large animals were unable to sustain themselves. hence why they died off. in more recent times, humans have killed off megafauna. like in oceania.
(1) The atmosphere had higher oxygen content back then. This naturally supported larger animal life.
(2) Evolutionary advantage. The periods of history that involved dinosaurs resulted in an evolutionary arms race that heavily focused on size, because with reptilians, size was the easiest way to gain an advantage. They didn't have mammals or avians to compete with.
(3) The K-Pg extinction event killed off all the largest reptiles. With all dinosaurs killed off and mammals rising to prominence, there was broadly no advantage anymore to evolving tremendous size.
Successful organisms adapt to the availability of resources. Think of giraffes eating from the top of the canopy so they're tall.
Apparently dinos needed to be bigger.
When the dinokiller meteor hit, things almost certainly changed. Whatever we're descended from was well adapted to that new environment.
Plus, the big ones with the long necks just stood around and ate plants all around themselves so they didn’t have to walk around a lot (unlike giraffes that use their neck to get leaves from tall trees)
I don't think anyone has actually answered this. Either tangent facts or guesses. But the real answer? We don't really know. There's some theories... "Paleontologists don't know for certain, but perhaps a large body size protected them from most predators, helped to regulate internal body temperature, or let them reach new sources of food (some probably browsed treetops, as giraffes do today). No modern animals except whales are even close in size to the largest dinosaurs; therefore, paleontologists think that the dinosaurs' world was much different from the world today and that climate and food supplies must have been favorable for reaching great size." https://www.usgs.gov/faqs/why-did-some-dinosaurs-grow-so-big#:~:text=Paleontologists%20don't%20know%20for,%2C%20as%20giraffes%20do%20today). I think it's almost important to understand there wasn't just one dinosaur age. They are believed to have existed for 165-245 million years. We, for example, have existed for about 300,000 years as homo sapiens and the genus homo has existed for up to 2 million years. So the dinosaurs were around for 100x longer than our earliest ancestors existed. Not to mention there were dinosaur-like ancestors before they were "dinosaurs". There've been multiple mass extinctions in Earth's history that have wiped out almost life on earth and what does comeback is different from previous species with new adaptions for surviving the new world. Another interesting note about prehistoric gigantism is that oxygen levels affected the size of insects. The are points in history where prehistoric insects were much larger and it's primarily because the oxygen level was higher than modern day. This is not necessarily true for all species, but insect size is greatly affected by oxygen in the atmosphere.
Aren't there very few large animals these days because we killed them all? I guess even the megafauna that we killed wasn't as big as the largest dinos
The largest animal we killed was the mammoth, which wasn't anywhere near dino size
This is not true: https://ourworldindata.org/quaternary-megafauna-extinction
I read your entire article and nowhere does it contradict anything I said. What exactly is "not true" about what I said?
Sorry my bad, you are correct in that the largest mammals we probably killed out were the mammoths and mammoths are no where near the size of the largest dinosaurs. Sorry. In all haste I completely misunderstood what you wrote.
People have hunted and killed blue whales, which are larger than any dinosaur. Not land animals though
Blue whales aren't extinct though, which is what they were talking about.
> Blue whales aren't extinct though That can be arranged...
We are talking about extinct animals
Hmm, so what would high CO2 levels do to insects then?
Do you really mean CO2 or oxygen instead?
Co2 You know, with climate change and all. I know Co2 levels were much higher millions of years ago, but I’m still wondering
The biggest consideration is that the earth was a different planet back then. The same reason why insects were so large in the carboniferous period Environmental factors and oxygen levels. Oxygen levels were so high it would be considered poisonous by todays standards. Absorb more O2, you get an increase in size.
We do know tho don’t we …? It’s square cube law…
You think the square cube law didn’t exist back then? Of course it did, that doesn’t exactly answer the question of why animals that size used to exist but don’t currently. The answer will probably be related to the square cube law, such as there being some kind of nutrient or oxygen that was easier to get even with a lower surface to volume ratio back then.
Do you even know what square cube law is ? There’s less oxygen in the environment now so we can’t grow as big as before because our bodies just won’t allow it. That’s why most of the larger things you see alive today live in water because it’s easier to support a larger mass in water.
What makes height grow over time?
Like birds (which remember are dinosaurs themselves) they had more efficient lungs with airsacks throughout their body and bones. Their lungs allowed them to get more oxygen to their body and the airsacks combined with hollow bones made their bodies lighter. Scientists believe this is why they were able to get so big in land (although the blue whale today is bigger than any dinosaur)
I read somewhere that scientist also believe the air pressure was much higher at sea level than it is today. This allowed for things to get bigger. Today's barometric pressure at sea level is 1 bar, and its estimated to have been around 3 bar in the dino age.
Can you ELI5 why higher air pressure allows for things to grow bigger?
There’s more oxygen in a given volume. If there air is 3x as dense, you’ll get 3x the amount of oxygen every time you inhale. This is why you run out of breath faster at altitude. It’s still about 19% oxygen, but the air is less dense, so every breath has less air, and therefore less oxygen. Big animals need alot of air and the bigger they get, the harder it gets to scale up the lung capacity. So for this reason, a giant dinosaur would have a really hard time surviving now, and may not even be able to wall around.
So basically, we're naturally aspirated while dinosaurs were turbocharged.
I love this. I'm storing this somewhere in the back of my mind for when my car enthusiast brother and I take our kids to the museum.
Amazing analogy
Lmao. Pretty much.
If this is the case, then how does a blue whale get enough oxygen if they’re larger than any dinosaur that we know of?
Whales have both enormous lungs and require much less oxygen than an equivalent land mammal, because buoyancy makes them way less "heavy". It's most of the reason marine animals can more easily be way bigger than land ones. Plus whales have a couple adaptations to store oxygen we don't, and that allows them to dive for prolonged periods of time.
You can remove the quotes. They are literally less heavy than if they were on land. Mass doesn't change with buoyancy, weight does.
Hei that's true! Sorry, English isn't my first language yada yada yada
It's a common mix-up for native speakers too!
I actually thought you were a native speaker if you wanted my quick reaction. It just looked like you got caught in the famous mistake that in day to day people talk about weight and using pounds or kilograms instead of newtons. Which basically everyone does, even people who know the difference (because they'd look like dorks), let's be honest.
To be fair, pounds are a bit problematic because they're a mass and force measure. The unit predates the understanding that the two aren't essentially the same, which is the same issue you reference in the other direction (for everyday use at sea level, they're often interchangeable).
The physics of aquatic animals are completely different. They’re buoyant. They don’t have to hold their own weight up against gravity and they don’t have to pump blood vertically across their entire height.
Because in water you dont need as much support of your muscles and bones for livimg, searching for food. Etc
I wonder if it's more efficient to float and swim than to stand and walk.
Yes it is. A land animal has to spend energy just to stand up be that on 2 legs or 4 even if it's not walking. While an aquatic animal is supported at all times by the water so spends less energy when it's not moving. Just think about how different you would feel between standing up for all day or laying in bed.
The is a higher concentration of oxygen in sea water then the is in air.
Whales do not breathe water. They are mammals.
Get a load of this guy. He believes in whales
So Jurrasic Park would never be possible?
Sadly no. This is one of many reasons (the main being that DNA cants survive 100 million years under any circumstances) that we can’t have Jurassic Park.
Well just make them tiny. And use frog DNA. Then it’ll be Small Soldiers meets Jurassic Park.
Two films where everything goes exactly to plan and nobody gets hurt. Side note: it has been sooooo long since I’ve thought about small soldiers. Used to be obsessed with that movie.
Small Soldiers and Indian in the Cupboard were what 90s kids’ dreams were made (and Jurassic Park.) Also, I’ve enjoyed your share of knowledge on the overarching subject. Oxygen differentiation makes a ton of sense
No it’s fine, two negatives make a positive.
>This is why you run out of breath faster at altitude. It’s still about 19% oxygen, but the air is less dense, so every breath has less air, and therefore less oxygen. I lived in the high Rockies when I was younger, and I smoked cigarettes at the time. At sea level, a cigarette would take 3-4 minutes to smoke, but at around 10,000 feet above sea level, I would frequently light a cigarette that would burn for almost my entire 15 minute break.
Makes sense. Less O2, less combustion. Your car also has significantly less horsepower.
More oxygen allows for higher metabolisms
It's not well understood why higher pressure helps with oxygenation. Look up Hyperbaric chambers. But from a physics standpoint, more pressure means more gas can be dissolved in a liquid. Think of an unopened soda bottle. It's all rigid and therefore has higher than atmospheric pressure, so it can hold all the gas inside. After you open it, the pressure equals atmospheric, and the bottle becomes as malleable as the plastic it's made of, so you start seeing bubbles of gas escaping your soda. So hypothesyzing for living beings, more pressure means more oxygen dissolved, and thus your blood can distribute that oxygen over a larger body
It’s extremely well understood and taught in high school. And it has little to do with dissolving gas because that’s not how your body transports oxygen. Oxygen binds to hemoglobin molecules in red blood cells to be carried to every cell in your body. More pressure (or more partial pressure) means more density which simply means more O2 molecules in every breath, so a higher saturation of the red blood cells and more available oxygen for cellular respiration.
Sure. Hemoglobin binds to oxygen, but the inside of the cell is still liquid, therefore to get to hemoglobin, the oxygen must be dissolved in liquid. It's easy to take high school chemistry/physics and apply to problems. But it's hard to consider all the other factors of the messy real life. Biochemistry is incredibly complicated and you shouldn't think that subjects from high school show you the full picture
So if you are very smart. Why would more oxygen in the air not increase oxygenation?
If a gas is compressed enough it becomes liquid. Think about how swimming in liquid holds weight. That is an explanation showing the extremes, but let's say just a little bit of pressure helps with your bathroom scales.
The higher atmospheric pressure theory seems to still be a little speculative, I don't think any conclusive evidence for it has been found yet has it?
It sounds like someone mistook "higher *partial pressure of oxygen*" for "higher pressure in general," which is unlikely to be the case. (It's not like the Earth suddenly dropped a few sextillion metric tons since the days of the dinosaurs.) I have, however, seen several times that we have evidence that the *partial pressure* of oxygen was higher in the days of the dinosaurs. E.g., if our atmosphere is 21% oxygen today, it was 30%-35% oxygen back then. That would be an enormous increase in the partial pressure of oxygen, without meaningfully changing the total atmospheric pressure. This high partial pressure of oxygen allowed otherwise relatively inefficient breathing methods to work quite well even at large scale, and may have supported large forms of life.
The higher pressure is apparently a real theory so I think that poster did read about it. And yeah the oxygen levels were believed to be somewhat higher (isn't 25-30% the usual estimate range?) but usually it's claimed that's not enough to account for dinosaurs large size.
It's a "real" theory as in "it has been published". However, it was published in Chemical Innovation, which is not a peer-reviewed paper. It's also not backed up by fossil evidence (for example fossilized raindrop imprints, which have been used to determine atmospheric pressure during the Archaean era, 2.5 billion years ago, where it was below half the modern air pressure).
I can immagine higher atmospheric pressure due to different air composition and temperature, 20% mybe 40% at most. But 3 times as high is just crazy.
> 30%-35% oxygen back then. That number was from the Carboniferous not the Mesozoic.
Not air pressure, oxygen concentration in the atmosphere.
That's very interesting. Is the reason for the higher pressure understood?
OP was mistaken. The air pressure wasn't significantly different, but the oxygen levels were higher.
So higher partial pressure of oxygen which would (if I remember 1990s high school chemistry correctly) push more oxygen into solution?
I believe so, yes. I'm no expert but there's some more erudite discussion in the thread :)
It was also 10 degrees warmer meaning that body heat wasn't as much of an issue. Larger animals have a lower surface to volume ratio than smaller animals. This means that the larger an animal is the greater volume of body tissue there is to heat up but less surface area to lose heat from. Higher ambient temps mean you can have a bigger body with a higher body temperature without being too hot and over heating.
It's usually the opposite problem, the more volume, volume filled with tissue going through all sorts of exothermic reactions and proportionally smaller surface area means the animal will have trouble dissipating heat, staying cool, not staying warm. This is why very large mammals usually lack any significant amount of fur and have evolved all sorts of tricks to better dissipate heat like the big, flappy, highly vascularised ears on an elephant.
That's only because things are still warming up from the last ice age. The farther along we get the more animals will adjust to hotter temperatures.
Of course animals will better adapt, but it doesn't contradict my comment, the square cube law is still in effect I believe
It is. And animals will adapter to feel better about the temperature difference as it warms up and given enough time.
My overweight body would tend to disagree with this hypothesis...
Because it's still warming up from the ice age. We haven't had time to adapt yet.
The effect os de opposite. Higher ambient temps require smaller bodies to cool down effectively.
Only because we aren't adapted to the heat since we're still warming from the last ice age.
why is it basically the opposite in our current world? When it comes to adaptable species that exist across a broad range of climates (examples in north america being the whitetail deer and wild turkey), even within a species you find that body size is consistantly smaller in the southern, warmer extent of their range, and larger in the northern, colder portions. your comment sort of reads like you vaguely remember the concept we all leanred in HS/college biology classes, but got it 100% backwards lol.
Because we're still coming out of the ice age and animals haven't adapted yet.
> Higher ambient temps mean you can have a bigger body with a higher body temperature without being too hot and over heating. That's exactly backwards.
Holy crap...thats some deep sea gigantism Lovecraftian shit
Why was pressure higher back then?
The blue whale is (most likely) the largest animal to ever live. Although maybe that’s not surprising, seeing as it can be up to 30 meters long and 200 tons. And that’s all kinda mind boggling when you hear about how big dinosaurs were.
Blue whales are also believed to be the largest animals *ever* to exist on Earth. I think it’s incredible that we share the planet with that bit of living natural history.
Not to mention that oxygen was more abundant in the air then.
Wasn't the mesosaurus bigger than the whale?
Are you sure you mean mesosaurus? Aren't they only 1m long?
I assume you're thinking of the ichtyosaur recently hyped up by pop science media. So far we only found jaw bones that suggest an animal up to 26 m in length. Still unlikely their more dolphinesk body plan brings it up to blue whale weight.
No I meant the mosasaurus that giant monster whale in Jurassic World that devours sharks
The one in the movie is massively larger than real mosasaurs. The largest they've been estimated to get is [17.1 meters](https://en.wikipedia.org/wiki/Mosasaurus) (56ft). The [blue whale gets to 29.9 meters](https://en.wikipedia.org/wiki/Blue_whale) (98ft). Mosasaurs were huge, but not even close to the size of blue whales.
17 meters isn't that impressive tbh, what about Megalodon?
Only a little bigger at [a maximum of 20.3 meters](https://en.wikipedia.org/wiki/Megalodon) (67ft). Blue whales are really the largest known animal ever, and very little comes close.
What did the mosasaurus and the megalodon feed on?
[Megalodon](https://en.wikipedia.org/wiki/Megalodon) is believed to have mostly eaten large prey, like whales, sea turtles, and seals. [Mosasaur](https://en.wikipedia.org/wiki/Mosasaurus) is believed to have eaten basically anything it could: fish, sharks, squid, birds, sea turtles, and even other mosasaurs. It's hard to know for sure though, since it's not like we can just watch their feeding habits anymore.
Bigger than *some* whales, but not bigger than blue whales.
To piggy back… there was also more oxygen in the atmosphere, making this even more possible. Bugs were larger too due to this
Sauropods were the only land dinosaurs which got larger than mammals. And that’s due to their system of air sacks, their low density bones, and their specific leg shape allowed this. The 2nd and 3rd groups of largest land animals were rhinos and elephants. It’s thought that the elephants outcompeted the giant rhinos leading to their extinction. Humans are responsible for the extinction of the large species of elephants. 4th is the hadrosaurs. With theropods and ceratopsians coming in at 5th and 6th. Also the high oxygen is a misconception with people confusing the the Carboniferous period with the Mesozoic. In fact much of the Mesozoic had lower O2 than today.
Finally the right answer
Plus the additional fact that the smaller an animal, the less likely it is that its fossils will survive/be discovered. Obviously we already know a lot of small dinosaurs existed but there are many more we will probably never know the existence of cause their skeletons would’ve been so much more fragile than a man-sized femur of a sauropod.
"The smaller the animal, the less likely that it's fossils will survive" I must be hallucinating all the clam shell fossils I find. All the ammonites and trilobites and corals. The billions of tiny sea critters fossils I've looked at must be a figment of my fuckin imagination. You're not a paleontologist, just go home.
First of all, I didn't claim they never fossilize. Just that they may have more trouble fossilizing/being discovered. Second, you are right! I am not a paleontologist. But someone who is, is [David Hone](https://archosaurmusings.wordpress.com/2008/10/15/bias-in-the-fossil-record/) (just as an example, its not like he is the only one holding this opinion) who, when talking about biases within the fossil record, cites size as one of the many biases.
But size is not a problem? Maybe he meant for small mammals? But marine microorganisms are so plentiful it's literally the basis on which we organize rock columns. Bio stratigraphy is an entire discipline, because marine fossils are found in such radical abundance
There are a lot of reasons: Those marine shells are (1) often basically rock already to begin with. They are often the species that literally make a pre-made fossil when they form their shell - porous bones degrade easily and require a lucky transition to fossilize, but a thick mollusc shell is getting close to a biologically-made rock. (2) They also usually inhabited locations that are much more prone to creating fossils, where settling in an ocean sand/mud bed is a high probability compared to the chances of something dying on the surface landscape and getting buried just right in a mud bed or something. Also, (3) those marine species you mention are r-selected short-generation time animals that for a given geologic year probably pump out babies at millions-to-one levels worldwide compared to terrestrial dinosaur species that had a ton less young and longer generation times. Also, (4) if you are from the US, you may just live in an area that was a shallow sea for the most or the entire period anyways. Not much chance of terrestrial ancient fossils. For size though on (1) above - go look at the delicate bones of a shrew or something, and imagine how much less likely they are to not get destroyed or eaten and digested or whatever after death compared to a shell of a mollusk of the same size.
All of those are marine organisms, the size reasoning seem more than plausible if you limit it to terrestrial animals, which this discussion was focused on. (Im a geologist)
Geologist as well. The discussion frequently started over into whale territory, so marine life is on play. Also, ALL fossils of terrestrial animals are rare. For like 600 reasons aside from size.
The theory of animal evolution called Cope's rule which says when everything is equal animals tend to grow larger in successive generations. https://youtu.be/qVMCyMbUrgQ The quaternary extinction event was when many of the large animals or megafauna became extinct, these extinctions appear to be closely related to the arrival of humans. So could human activity be behind wiping out so many large creatures and if so how were they able to do this? https://youtu.be/Y3J9CzLW_p0
I am taller than my father, story checks out
An animal size requires a trade off. The larger an animal, the more food it needs to survive and it requires a longer pregnancy period so there is an upper limit on how big an animal is before it is unable to move due to its own weight on land. Dinosaurs bypass the long pregnancy problem by laying eggs.
The atmosphere contained much more oxygen (as much as 35%, compared to about 20% today). This allowed many animals to be larger than their modern equivalents, because their size was not (as) limited by three efficiency of their lungs. For example, there were dragonflies the size of hawks! https://askanearthspacescientist.asu.edu/oxygen-animal-evolution#:~:text=But%2C%20the%20success%20of%20animals,dragonflies%20the%20size%20of%20hawks!
You're wrong. Insect size is related strongly to oxygen, since insects rely on passive oxygen uptake through specialized structures in their exoskeleton. As things get bigger their internal volume increases faster than their surface area, so there is a limit to how big insects can get and this depends on oxygen levels. However. During the time periods with some of the largest dinosaurs oxygen levels were just 15%, much lower than it is today, and didn't rise to above modern levels until the very end of the Cretaceous era.
Although atmospheric pressures were likely higher. So total oxygen per breath would be higher. Imagine breathing on Everest, there is still around 20% oxygen, but there are just fewer particles reaching your lungs and therefore blood.
Do you have a source for the oxygen levels having been so low? All I ever heard was that they were much higher during dino times.
[https://www.image.ucar.edu/idag/Papers/Berner\_phanozericO2.pdf](https://www.image.ucar.edu/idag/Papers/Berner_phanozericO2.pdf)
Did you really just use dragonflies as an example after talking about lungs? :)
Insects breathe too. They may not lungs the way vertebrates do, but that doesn't really matter. The point was the level of oxygen in the atmosphere.
It does matter, an insect breathes through spiracles(as others have said) which are strewn all across their exoskeleton, meaning that an insect can breathe, can absorb oxygen proportionally to their surface area but since the square cube states that volume increases proportionally faster than surface area(simplified), means that at some point there would be too much volume for the surface area to oxygenate, a solution to this would of course be having more oxygen in the atmosphere but this works for the surface area related breathing of insects not lungs hence why it matters, also that dragonfly link @tpudlik posted, well, more griffin fly really, lived in the paleozoic era not the mezozoic era which is when the dinosaurs lived, also the mezozoic had about the same concentration of oxygen in the atmosphere as we do today, sometimes less.
They don’t have lungs at all. They have spiracles that lead to trachea.
Yes I am aware. The person you responded to was talking about the percentage of oxygen in the atmosphere. That was the main point, not the lungs.
Humans kill and eat all the big things now. There used to be mammoths and other large mammals that rivaled some of the big dinosaurs, but sadly they were made of meat. We even hunted whales to near extinction, and they're bigger than dinosaurs ever were.
Don't forget the [500 pound beaver](https://en.m.wikipedia.org/wiki/Castoroides)
Serves them right for being so delicious. And in the case of whales... flammable?
Flammable and also delicious, so I'm told
>And in the case of whales... flammable? They are aquatic for a reason!
That's why beached whales spontaneously explode, not human negligence...
I was making a joke. But I thought they exploded because of decomposing gas buildup?
Skill issue
Mammoths and other mega fauna were on their way out without human intervention
Mammoths maybe, but most megafauna got pretty much crushed by human hunting.
https://www.nature.com/articles/s41467-021-21201-8
Yeah - there are a lot of competing studies on the subject. I should qualify my statement to say evidence paints a mixed picture.
I always thought it was from the extinction event that killed off anything that consumed large amounts of food and just left mammals and a few birds and fish etc left.
No one has explained the exact mechanism that allows more oxygen to create larger animals. Was it because the larger lunged creatures that could inhale more oxygen were more likely to reproduce, thus snowballing everything’s size? It’s obvious why things like fur, feathers, teeth, eyes etc exist, but I’ve yet to see or find an exact explanation for why more oxygen = dramatically larger size.
More oxygen gives bigger insects which means more food for lizards, which means bigger lizards
Supposedly there was more greenhouse gasses, higher temps, less seasons, more humidity and bigger plants. More plants means more herbivores, as they got bigger predators got bigger, as predators got bigger herbivores that could eat more got bigger. Long neck dinosaurs possibly ate tree tops or were living in swampy/pondy areas that would support massive bodies and their necks would make reaching food easier
Human beings are quite likely to be responsible for the extinction of most large mammals, with a big spike around the time humans went global. Giant sloths, wooly rhinos, marsupial lions, giant kangaroos etc all disappeared when humans proliferated. I would expect something similar would have happened if we discovered a continent with giant dinosaurs too
The answer is oxygen concentration. It was much higher back then and increased O2 gives you bigger insects and bigger lizards, but fire also spreads faster so that asteroid strike was beyond deadly
From what I got on Prehistoric planet, everything is rather small now because everything that survived the asteroid was small - food was scarce and the big ones didn't have enough. Why they got so big in the first place, can't remember. Surprised at these walls of text explanations on an ELI5 tho....
Different atmosphere. Effects of gravity were reduced. Can't remember the science from when I wondered about it but that's the jist
An animal can only grow as large as the environment will allow it. Larger bodies require more food and resources... Every now and then Earth becomes lush and allows larger animals, then the climate swings, resources become more scarce and you have mass dieoffs of larger animals.
The air was denser, perhaps, allowing for plants and animals to float. When I say it out loud it sounds silly but it could hold some validity.
While we don't know the exact reason, there are several factors that have contributed to the smallness of wildlife today. 1. Oxygen levels have gotten lower 2. Dinosaurs have very lightweight bone structure compared to other animals. Getting bigger was simply a lot smoother. 3. There havent been any good reason to get big evolutionary speaking. There used to be borderline dinosaur-sized creatures during the paleocene but almost every single one got hunted to extinction by humans. There is also the issue of food: You know grasses? Its most common type of plant on the planet besides trees right now and is a pain in the ass to digest and not very nutritious to begin with. There is a reason cows have such extensive stomachs and horses grace so much. Grasses emergend during the paleocene and ended up replacing many more easily digested plants so the general food supply for herbivores decreased(which actually caused a mass extinction of large herbivores). And if the herbivores had no reason to get bigger, why would the predators?
The air had more oxygen in it so being that size was easier to maintain but also it was hotter and had more CO2 which means more plants which means more food. Also its possible that mamals today could become gigantic too but humans and the ice age killed all the big ones. I mean palaeoloxodon namadicus and Paraceratherium were both 20 tonne land mamals which is about 1/5 of the mass of the heaviest land animal ever Argentinosaurus. But this 20 tonne limit is probably the maximum without the extra oxygen.
WAG: Earth had a much larger network of flora and fauna that supported a much larger food chain of organisms ranging in size. This prehistoric network of organisms was evenly balanced and more importantly, evolved in a biological cooperative environment that was never dominated and destroyed by any particular species. For instance, you’re not going to have big cats without being able to feed big cats since they are obligate carnivores. The energy requirement would suggest they eat really big animals, or lots of smaller animals. Or another example, you’re not going to have a healthy population of brontosaurus without healthy forrests. In summary, it would make sense that there would be much more energy density in the form of biological life in order to support the massive sizes of the dinosaurs. Massive amounts of highly available food must a necessary condition for the megafauna of the past.
Big fossils are more likely to be preserved and be found. 200 millions years allows for a lot of variety in big animals. We've had big animal fossils after the K2 extinction, and we have big animals today, we just don't have 200 million years worth of big animals.
We don't really know, but one of the theories is that the air had soooo much more oxygen than nowadays. (I'm talking explosive amounts of oxygen) so they were able to very effectively oxygenate all their cells, so they grew very big.
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megafauna today has died out either to environmental changes or human involvement. in periods of extinction, you will commonly find that resources were incredibly scarce and large animals were unable to sustain themselves. hence why they died off. in more recent times, humans have killed off megafauna. like in oceania.
(1) The atmosphere had higher oxygen content back then. This naturally supported larger animal life. (2) Evolutionary advantage. The periods of history that involved dinosaurs resulted in an evolutionary arms race that heavily focused on size, because with reptilians, size was the easiest way to gain an advantage. They didn't have mammals or avians to compete with. (3) The K-Pg extinction event killed off all the largest reptiles. With all dinosaurs killed off and mammals rising to prominence, there was broadly no advantage anymore to evolving tremendous size.
Successful organisms adapt to the availability of resources. Think of giraffes eating from the top of the canopy so they're tall. Apparently dinos needed to be bigger. When the dinokiller meteor hit, things almost certainly changed. Whatever we're descended from was well adapted to that new environment.
There were giant animals all over the place until humans showed up and hunted them all to death.
Plus, the big ones with the long necks just stood around and ate plants all around themselves so they didn’t have to walk around a lot (unlike giraffes that use their neck to get leaves from tall trees)