Why were dinosaurs so large?

[Moontanman]

Is it known when reptiles split (mammals and dinosaurs) and the reason why?

 

 

Yes, mammals and dinosaurs evolved at about the same time,(during the Triassic age, about 200 million years ago) Dinosaurs were superior to mammals (at least in the ecological niches for large animals) and became the dominant animals until they were wiped out by a large asteroid hit about 65 million years ago, this allowed mammals, which had evolved to occupy the ecological niches for smaller animals during the domination of the dinosaurs, to evolve into large animals and eventually dominate. I don't understand what you mean by "the reason why" ...

[belovelife]

i'm late to chime in on this conversation, but if the earth gains so much matter a year, then after billions of years the earth is much larger, then at the the time of pangea and the dinosaurs wouldn't the earth be smaller, hence less gravity, this would have allowed for the extreme size of them right?

[Moontanman]

i'm late to chime in on this conversation, but if the earth gains so much matter a yea, then after billions of bears the earth is much larger, then at the the time of pangea and the dinosaurs wouldn't the earth be smaller, hence less gravity, this would have allowed for the extreme size of them right?

 

 

The Earth doesn't gain matter that fast, the time of the dinosaurs was only 200 million years ago to 65 million years ago, not billions of years.

[dduckwessel]

I don't understand what you mean by "the reason why" ...

 

What was different about the earth then (not happening now) that caused dinosaurs to grow so large?

[Moontanman]

What was different about the earth then (not happening now) that caused dinosaurs to grow so large?

 

 

Well first of all at that time dinosaurs were no larger than other animals, I have read many ideas around why they became successful the most reasonable of them pertains to the superior respiratory system (similar to birds) and an upright stance. The earth is thought to have been somewhat warmer then but really dinosaurs just won the luck of the draw, they exploited the environment better than any other creatures of the time. Mammals were stuck exploiting smaller body plans while dinosaurs got larger. Mammalian body plans do not seem to exploit large size as well as dinosaurs did but there have been some really large mammals since the demise of the dinosaurs but dinosaurs were in control a lot longer than larger mammals have been. Dinosaurs dominated the planet for something like a 150 million years, mammals have been dominant less than one third of that time if humans allow it mammals may yet give us creatures as large as dinosaurs.

[CraigD]

Well first of all at that time dinosaurs were no larger than other animals, ...

Huh?! :confused: What your source for this claim, MTM? It contradicts nearly everything I’ve heard and read.

 

As reconstructed from the fossil record, the largest land animals that ever existed were sauropod dinosaurs that lived about 150,000,000 years ago (late Jurassic). Though no single species was largest by all common size criteria – length, height, and mass – the largest of these four-legged animals were about 34 m from nose to tip of tail, 17 m from ground to top of head, and massed on the order of 100,000 kg. (Based on a very partial fossil skeleton, it’s been suggesting that late (70,000,000 y) sauropod, Bruhathkayosaurus, was perhaps 240,000 kg, but this remains controversial and speculative)

 

Mesozoic (251,000,000 to 65,500,000 years ago) animals were of many sizes, some very large, some very small, but none appear to have ever been as large as the sauropods.

 

Dinosaurs dominated the planet for something like a 150 million years, mammals have been dominant less than one third of that time if humans allow it mammals may yet give us creatures as large as dinosaurs.

This has already happened, at least for the mass category.

 

The saruopods were the largest land animals ever. The largest sea animal species that ever lived, and still lives, is the blue whale, which appeared about 20,000,000 years ago.

 

The most massive blue whale ever measured massed about 190,000 kg and 30 m long, the longest one 33.3 m. Assuming, as I think most do, that nothing as large as the suggested Bruhathkayosaurus actually existed, this makes the blue whale about twice as massive as the largest dinosaur.

 

Because Sauropods had long thin tails and necks, vs. whales’ more compact forms, it’s possible that the longest ones were perhaps a meter or two longer than the longest blue whale, but practically, the longest whales are about as long as the longest dinosaurs were.

[Moontanman]

There were aquatic reptiles that approached the size of whales but they were not dinosaurs. I often wonder what if in scenarios like this and my what if is what if mammals had dominated the dinosaurs would we even recognize them as mammals? This was the direction mammal like reptiles were going before dinosaurs took over

 

http://en.wikipedia.org/wiki/Dimetrodon

 

http://en.wikipedia.org/wiki/Edaphosaurus

 

Living in the shadows of the giant reptiles changed mammals forever and without that is unlikely mammals as we know them would have ever existed...

[Rade]

The proposed reptile ancestor (Marasuchus) that evolved into the 1000+ species we call dinosaurs was a relatively small vertebrate, a few pounds weight and less than two foot long--see this link:

 

http://dinosaurs.about.com/od/predinosaurreptiles/p/marasuchus.htm

 

And, the very early dinosaur fossils are from South America, here is one of the first recognized dinosaur fossils, which would have evolved from the Marsuchus reptile ancestor:

 

http://dinosaurs.about.com/od/carnivorousdinosaurs/p/eoraptor.htm

 

There were many different types of reptiles on the earth prior to the dinosaurs. What is clear from the fossil record is that the first dinosaurs were not large, but began relatively small, at the size of ancestral reptiles that lived on the earth before dinosaurs. There are many good explanations why some dinosaur species became very large, while others remained very small, with a wide range of sizes for the 1000+ dinosaur species. I suggest a visit to the Museum of Natural History in London, there you will see one of largest collections of many different sized dinosaur species.

[Moontanman]

Huh?! :confused: What your source for this claim, MTM? It contradicts nearly everything I’ve heard and read.

 

I think rade already answered this

 

 

 

As reconstructed from the fossil record, the largest land animals that ever existed were sauropod dinosaurs that lived about 150,000,000 years ago (late Jurassic). Though no single species was largest by all common size criteria – length, height, and mass – the largest of these four-legged animals were about 34 m from nose to tip of tail, 17 m from ground to top of head, and massed on the order of 100,000 kg. (Based on a very partial fossil skeleton, it’s been suggesting that late (70,000,000 y) sauropod, Bruhathkayosaurus, was perhaps 240,000 kg, but this remains controversial and speculative)

 

But these were not the first dinosaurs by any definition.

 

 

Mesozoic (251,000,000 to 65,500,000 years ago) animals were of many sizes, some very large, some very small, but none appear to have ever been as large as the sauropods.

 

Again Not the first dinosaurs.

[CraigD]

Well first of all at that time dinosaurs were no larger than other animals' date=' ...[/quote']

Huh?! :confused: What your source for this claim, MTM? It contradicts nearly everything I’ve heard and read.

... But these were not the first dinosaurs by any definition.

Ah, my bad. :embarass: I understand you now. I thought “that time” to which you were referring was the whole “time of dinosaurs”, about 230,000,000 to 65,000,000 years ago, not just its beginning, the “post Permian-Triassic extinction synapsid – archosaur takeover”.

 

I agree – the first dinosaurs, evolved from Permian archosaurs like 2 m long Protorosaurus, were likely about the same size as dinosaur-like synapsids, like 4 m long Dimetrodon. Lots of evolution had to occur before the record-setting 30 m + Cretacious (145,500,000 to 65,500,000 years ago) sauropods left their fossils, though 200,000,000 year old fossils like 10 m long Yizhousaurus sunae suggest that sauropods were getting pretty big early in their evolution.

 

So this thread’s title question, “why were dinosaurs so large?” seems to me to reduce to “why were the big dinosaurs evolutionary lines so successful?”

 

I think part of the answer – I’m pretty confident there were several, not just one, critical contributing causes – is, as you and others mentioned earlier in the thread, that dinosaurs, like their moderns surviving species, the birds, had really good (roughly twice as good as mammals’) respiratory systems. Like birds, they had excellent mass-reducing and supporting anatomies. These factors alone, I think, give a pretty good explanation for why dinosaurs, not synapsids-descended mammals, set the land-animal size records.

 

Why dinosaur descendents – the only remaining clade of which are the birds (this wikipedia graph image:

shows this prettily) – haven’t had very many successful big species when they still have the basic genes for it, has, I think, a more complicated explanation.

 

Since birds descended from less gigantic theropod dinosaur line (the largest of had “only” around 20,000 kg mass and 18 m length), they don’t have the right body plan to rival the cretaceous sauropods, but something like 70,000,000 year old, 2,000 kg, 8 m Gigantoraptor erlianensis, not quite an bird, but not to genetically distant, might be possible. Gigantoraptor is only about half the mass of the largest living animal, the African Elephant, but with it’s light bird-ish anatomy, about the same length and height.

 

Yet, as best I can tell, the biggest true bird was the 400 kg, 3 m tall Aepyornis maximus elephant bird – 2.5 times the mass of the 160 kg Ostrich.

 

Here’s another nice wikipedia diagram comparing these 3:

 

I think the cause of the extinction of the elephant bird – believed to be humans, around 1000 AD, and the extinction of big flightless birds like the terror birds, which lived from about 60,000,000 to 2,000,000 years ago, which is hypothesized to be due to unknown, likely small mammal egg-stealers, might be key clues to no elephant or greater size bird/dinosaurs species appear to have succeeded since the C-T extinction 65,500,000 years ago: the Triassic ecosystem’s mammal-dominated predation, is critically different than the Cretaceous’s.

 

In short, I suspect we mammals have a knack for wiping out giant animals that the previous dinosaur occupants of our ecological niches lacked. We’re in some way more voracious than they were.

[athinker]

I think that sensory input plays a part in many results of reproductive success. Some results, for instance, birds with mating rituals that emphasize the raising of tail feathers rasults in the exagerated raising in some specis, peacocks for instance. While the beginnings of tail feather raising began small it was a neccesary part of flight a sensory identification of mates who could do it well was a reproductively advantageous trait. Later, however, the sensory indentification mechanism may have become fooled so that the raising of tail feathers alone in an exagerated way, like a peacock, became in itself a reproductively succesful trait in some specis in spite of any degradation in its actual usefulness in flight.

 

Many large animals have fighting of each other in their mating rituals. Becomming large is only one path an animal may take in survival. When an animal becomes large enough due to many factors for its size itself to become a major factor in its reproductive success then then evolution to even larger size tends to dominate. The ones that win mating combat tend to be bigger. In that case, as with birds, the ones that look bigger may tend be be reproductively successful. Even if the merely only look bigger. We see many examples of animals that can make themselves merely look bigger without actually being bigger.

 

Dinosaurs did not start out big with large compost heaps in their bellies. That dinosaurs had compost heaps in their bellies was a more result of their need for large amounts of food to support large size. With the ability to eat lower quality food with lower nutrition value they had more food stocks to draw on.

 

The fact that they survived so long seems agreed by most experts to be due to the stability of the environment rather than any particulay survival value conferred by large size. In fact many experts posit that dinosaurs were declining for tens of millions of years before the extiction event due to rather small chances in the environment.

 

After the extiction event the animals that needed large supplies of food died as the plants died. The ones small enough to live on seeds burried in the ground, sprouts early after the extiction event and other small food supplies survived. The differance could have been as little as a few months from the time the large plant food supplies were exhausted and the new sprouts became big enough to support large animals. They just came up too late.

[Moontanman]

I think that sensory input plays a part in many results of reproductive success. Some results, for instance, birds with mating rituals that emphasize the raising of tail feathers rasults in the exagerated raising in some specis, peacocks for instance. While the beginnings of tail feather raising began small it was a neccesary part of flight a sensory identification of mates who could do it well was a reproductively advantageous trait. Later, however, the sensory indentification mechanism may have become fooled so that the raising of tail feathers alone in an exagerated way, like a peacock, became in itself a reproductively succesful trait in some specis in spite of any degradation in its actual usefulness in flight.

 

I'm not sure what this has to do with some few dinosaurs being large.

 

Many large animals have fighting of each other in their mating rituals. Becomming large is only one path an animal may take in survival. When an animal becomes large enough due to many factors for its size itself to become a major factor in its reproductive success then then evolution to even larger size tends to dominate. The ones that win mating combat tend to be bigger. In that case, as with birds, the ones that look bigger may tend be be reproductively successful. Even if the merely only look bigger. We see many examples of animals that can make themselves merely look bigger without actually being bigger.

 

Again, I'm not sure why this would have made some dinosaurs as large as they were.

 

Dinosaurs did not start out big with large compost heaps in their bellies. That dinosaurs had compost heaps in their bellies was a more result of their need for large amounts of food to support large size. With the ability to eat lower quality food with lower nutrition value they had more food stocks to draw on.

 

I can see how this might have had an effect, elephants and other large mammalian herbivores do indeed use the size of their stomachs to allow for "compost heaps" in their stomachs as well.

 

The fact that they survived so long seems agreed by most experts to be due to the stability of the environment rather than any particulay survival value conferred by large size. In fact many experts posit that dinosaurs were declining for tens of millions of years before the extiction event due to rather small chances in the environment.

 

This is simply not true, dinosaurs did not live in a particularly stable environment and they faced and survived more than one extinction event.

 

After the extiction event the animals that needed large supplies of food died as the plants died. The ones small enough to live on seeds burried in the ground, sprouts early after the extiction event and other small food supplies survived. The differance could have been as little as a few months from the time the large plant food supplies were exhausted and the new sprouts became big enough to support large animals. They just came up too late.

 

If this scenario is true then why did the small dinosaurs also become extinct?

[athinker]

I'm not sure what this has to do with some few dinosaurs being large.

This has to do with the OP question about visual cues being involved.

 

Again, I'm not sure why this would have made some dinosaurs as large as they were.

This has to do with what large living animals have in common. Fighting each other, the big one wins.

 

 

 

This is simply not true, dinosaurs did not live in a particularly stable environment and they faced and survived more than one extinction event.

 

It was stable as far as glacial advance. There were not as great glacial advances as now.

 

 

If this scenario is true then why did the small dinosaurs also become extinct?

If you mean at the KT boundry event, all of them didn't. Some survived to become birds. Some went extinct later. Some that depended on niches provided by large dinosaurs went. But the large dinosaurs went due to starvation.

 

I hope you're not into that whole "fireball planet" thing. Meteor setting fire to everything and only those that lived in burrows survived.

 

First of all there's no evidence of burned stuff all over the world. 2, animals that didn't live in burrows also survived. 3 why did animals that lived in the ocean die? The way that one is usually answered is that smoke from the fires blocked out the sun so the plant base of the food chain in the ocean died. Well the volacnic cauldera(s?) opened by the meteor(s?) provided enough ash to do the job on both the land and ocean plant base without invoking an unsupported planet wide firestorm. And there's plenty of evidence of volcanic ash all over the place. In fact the volcanic ash is the sole cause of the extiction. That a meteor struck at the same time was known due to the iridium in the KT layer but it was not known how it figured in to the event until the Chixilube crater was identified and dated. Then to, even before the identification of Chixilube, it was known that large granite intrusions in the granite of Iceland had the same iridium signature as the KT layer. A meteor struck Iceland at the same time as KT producing a larger cauldera than Chiixilube. A cauldera that even today is still active. However, while there is no meteor crater in Iceland due to all the vocanic activity there, the iridium signature in the rock is convincing to some. The iridium bearng rock intrusions must have been molton when the iridium was deposited and the concentration of iridium in those rocks is higher than anywhere else on Earth. That's convincing. Not a smoking gun but definitely powder burns.

[Moontanman]

This has to do with the OP question about visual cues being involved.

This has to do with what large living animals have in common. Fighting each other, the big one wins.

 

Then why don't we see Elephants or rhinos as big as sauropods?

 

It was stable as far as glacial advance. There were not as great glacial advances as now.

 

Glacial advances and retreats had nothing to do with the extinction of dinosaurs, they were endothermic and lived as close if not closer to the North and South poles as mammals do.

 

If you mean at the KT boundry event, all of them didn't. Some survived to become birds.

 

No, birds were already extant at the time of the dinosaurs, they were not dinosaurs that went on to become birds after the K/T extinction.

 

Some went extinct later.

 

Which ones?

 

 

 

 

 

I hope you're not into that whole "fireball planet" thing. Meteor setting fire to everything and only those that lived in burrows survived.

 

That is a well accepted idea.

 

First of all there's no evidence of burned stuff all over the world.

 

Yes there is....

 

2, animals that didn't live in burrows also survived.

 

While debatable i will not say that only animals that lived in burrows survived, but animals that did or could have lived in burrows did indeed survive at higher rates than those that did or could not have.

 

3 why did animals that lived in the ocean die? The way that one is usually answered is that smoke from the fires blocked out the sun so the plant base of the food chain in the ocean died. Well the volacnic cauldera(s?) opened by the meteor(s?) provided enough ash to do the job on both the land and ocean plant base without invoking an unsupported planet wide firestorm.

 

If the effect is the same I see no reason to debate whether or not the entire surface of the planet burned in it's entirety. The sun was almost certainly cut off from the surface for a long period of time. Both volcanic ash and ash from burning forests no doubt contributed.

 

 

And there's plenty of evidence of volcanic ash all over the place. In fact the volcanic ash is the sole cause of the extiction. That a meteor struck at the same time was known due to the iridium in the KT layer but it was not known how it figured in to the event until the Chixilube crater was identified and dated. Then to, even before the identification of Chixilube, it was known that large granite intrusions in the granite of Iceland had the same iridium signature as the KT layer. A meteor struck Iceland at the same time as KT producing a larger cauldera than Chiixilube. A cauldera that even today is still active. However, while there is no meteor crater in Iceland due to all the vocanic activity there, the iridium signature in the rock is convincing to some. The iridium bearng rock intrusions must have been molton when the iridium was deposited and the concentration of iridium in those rocks is higher than anywhere else on Earth. That's convincing. Not a smoking gun but definitely powder burns.

 

 

I'm not sure what you are saying with this, are you saying the asteroid strike did not contribute the demise of the dinosaurs or that is was just one of the factors that resulted in the K/T extinction?

[athinker]

Then why don't we see Elephants or rhinos as big as sauropods?

 

Metabolic rates and available food supplies are limiting factors. Dinosaurs had a lower metabolic rate therefore could build larger bodies on the same amount of food as mammals. Mammals spend a larger amount of their food energy maintaining body heat than dinosaurs did. Speaking only of vegeterian land animals. Due to differances in the type of food and the energy requrements. aquatic animals have gotten bigger than land animals. The largest animal of the "dinosaur age" was an aquatic "reptile". The largest animal of today and of all time is mammal. Both were carnivores, higher energy food than plants.

 

There are a lot of causes of animals being a certain size, shape, color behavior pattern etc.

 

Bringing the topic back to the OP of the "why are dinosaurs so large" thread. Among the causes are visual cues. So I agree with the OP's suggestion.

 

 

 

Further discussion of the KT extinction in "discussion-of-kt-extiction-fireball-earth-dispute"

 

http://scienceforums.com/topic/23549-discussion-of-kt-extiction-fireball-earth-dispute/

[Moontanman]

Metabolic rates and available food supplies are limiting factors. Dinosaurs had a lower metabolic rate

 

Do you have any evidence of this?

 

The habits and habitats of Dinosaurs indicates very strongly they were endothermic. mammals are endothermic as well, dinosaurs had metabolisms as high as and almost certainly many cases higher than mammals, some of them might have been gigantotherms as a source of their endothermy but they were endothermic.

[athinker]

Do you have any evidence of this?

 

You mean besides the facts that they were reptile, the brain structure involved in regulating metabolism is not developed enough to do it in them, and that other structures and organs for endothermism are incompetent or nonexistant in them? Not to mention, of course, that the environment was warm and stable enough for internal heat production to be unneccesary.

 

It has recently been discovered that some reptiles of today and then have some behaviors in common with warm blodded animals such as caring for their young, hunting in groups, "sustained" high speed locomotion. There is some evidence that they had some degree of internal heat generation cpacity. But that is not saying that they had as high levels of metabolic demand as mammals. No where near it in fact. To extend these discoveries into a fact that they were as endothermic as mammals or even gigantotherms is pure pop science fiction.