Why did the big beasties go away?

[Santalum]

This is almost certainly true, it can be demonstrated by fish populations as well.

 

 

 

You are aware that dinosaurs are no longer though to have been exothermic aren't you? Some are considered to have been gigantotherms (sp?) but then so are some mammals, the larger stomachs thing is valid and that would require larger predators like allosaurus or T-Rex but dinosaurs, like birds are thought to have been endothermic

 

 

I thought the general consensus was that they were not a homogeneous group of animals as far as warm/cold bloodedness. Some were largely cold blooded and some were incompletely warm blooded. In either case I would presume large size and heat retention would still be of some advantage.

 

As they evolved they became increasingly efficient endotherms until we ended up with the bird like and mammal like reptiles.

Edited January 17, 2012 by Santalum

[Moontanman]

I thought the general consensus was that they were not a homogeneous group of animals as far as warm/cold bloodedness. Some were largely cold blooded and some were incompletely warm blooded. In either case I would presume large size and heat retention would still be of some advantage.

 

Which one were though to be largely exothermic? Dinosaurs were too active and large to be exothermic. Many mammals are somewhat less than warm blooded but we still classify them as mammals and as endothermic... btw, warm/cold blooded is not an accurate description and may be the root of our miss-communication...

 

As they evolved they became increasingly efficient endotherms until we ended up with the bird like and mammal like reptiles.

 

And none of these would have been dinosaurs.... mammals and dinosaurs are distinct groups, that evolved from reptiles, once they became mammals and dinosaurs there were no long part of those groups. Bats are mammals the same way birds are dinosaurs, there weren't bird like reptiles, anymore than there were bat like reptiles, there were bird like dinosaurs...

[Santalum]

Which one were though to be largely exothermic? Dinosaurs were too active and large to be exothermic. Many mammals are somewhat less than warm blooded but we still classify them as mammals and as endothermic... btw, warm/cold blooded is not an accurate description and may be the root of our miss-communication...

 

 

 

And none of these would have been dinosaurs.... mammals and dinosaurs are distinct groups, that evolved from reptiles, once they became mammals and dinosaurs there were no long part of those groups. Bats are mammals the same way birds are dinosaurs, there weren't bird like reptiles, anymore than there were bat like reptiles, there were bird like dinosaurs...

 

Well I think to begin with that the sail of Dimetrodon and the plates of Stegosaurus were heat exchange devices of some sort. So that would indicate that they were either cold blooded or incompletely warm blooded.

 

Velocoraptor and related animals had at least partial covering of feathers so that would indicate they were probably well on the way to endothermy if not there.

 

Don't know precisely what the latest on the sauropods and other large theropods is but perhaps they were some where in between exothermy and endothermy.

[Moontanman]

Well I think to begin with that the sail of Dimetrodon and the plates of Stegosaurus were heat exchange devices of some sort. So that would indicate that they were either cold blooded or incompletely warm blooded.

 

Velocoraptor and related animals had at least partial covering of feathers so that would indicate they were probably well on the way to endothermy if not there.

 

Don't know precisely what the latest on the sauropods and other large theropods is but perhaps they were some where in between exothermy and endothermy.

 

 

Dimetrodon was not a dinosaur and stegosaurus were more than big enough to be gigantotherms, theropods are thought to have been endotherms, saurpods were big enough to generate their internal heat just from being large as well.

[Santalum]

Dimetrodon was not a dinosaur and stegosaurus were more than big enough to be gigantotherms, theropods are thought to have been endotherms, saurpods were big enough to generate their internal heat just from being large as well.

 

 

Dimetrodon was on the same evolutionary branch as dinosaurs.

 

Actually, what is it prcisely that determines if an animal is a dinosaur or not a dinosaur?

 

Was just thinking about the debate about how tiny sauropod babies grew to such enormous adults and that some believe them to have been full on endotherms. But if you consider some of the larger goanna species, nile crocodiles and salt water crocodiles, their babies are rather tiny compared to the adults but those that survive clearly have a fairly rapid growth rate under the optimum environmental conditions. So why is it such a mystery as to how the sauropods did it - the earth was supposed to be a great deal warmer back then.

[Moontanman]

Dimetrodon was on the same evolutionary branch as dinosaurs.

 

No Dimetrodon was a mammal like reptile an off shoot of the direct linage from reptile to mammal, it was not on the tree of reptile to dinosaur.

 

Actually, what is it prcisely that determines if an animal is a dinosaur or not a dinosaur?

 

It has to do with the skull and teeth, if you look at dimetrodon you will notice it already has dentation more typical of mammals, dimetrodon even had canine teeth, the teeth of dinosaurs were more reptilian and the shape of the skulls are different, if wiki was up it would be easy to find illustrations, i miss wiki.

 

Was just thinking about the debate about how tiny sauropod babies grew to such enormous adults and that some believe them to have been full on endotherms. But if you consider some of the larger goanna species, nile crocodiles and salt water crocodiles, their babies are rather tiny compared to the adults but those that survive clearly have a fairly rapid growth rate under the optimum environmental conditions. So why is it such a mystery as to how the sauropods did it - the earth was supposed to be a great deal warmer back then.

 

Dinosaurs grew fast like mammals, not slow like reptiles, the growth rings in the bones of dinosaurs and the blood supply to those bones indicates very fast juvenile growth like mammals then a slow down like mammals. I think this shows up even in the sauropod dinosaurs... It is also true, that how that growth could have been maintained in the sauropods is puzzling...

Edited January 19, 2012 by Moontanman

[JohnB]

With the megafauna versus contemporary mammals one theory it that hunting pressure from humans has resulted in dwarfing as humans nearly always target larger animals because the meat goes further. Over generations this puts selective pressure against the largest of the individuals of a species and for smaller individuals.

Also larger animals require more food and this is an evolutionary disadvantage in hard times such as droughts etc. This has certainly also been a factor in the dwarfing of Australian mammals.

 

Santalum, even the megafauna didn't reach the sizes that were around only 30 million years ago. Also on the timespans I'm talking about humans don't matter as until recently we were more prey than predator. I can't see the size/drought thing as too valid either since extremely large critters survived for tens of millions of years which means they survived droughts.

 

I don't know that I've been expressing myself well on what I've observed so I'll try a rephrase.

 

Life on this planet evolved in a series of phases separated by extinction events. After each of the extinctions life came back and very large animals were common. In each stage the tendency was for the animals to get bigger. After the dino-killer event life did this again. Even thought he mammals were ruling, they were getting very big and the same trend was apparent as was in earlier periods.

 

Around (as a guesstimate) 25 million years ago the trend reversed and smaller became more evolutionarily successful. Multiple extremely large species of Rhino became the 4 much smaller species we have today. "Lucy"s people were only 4 foot tall yet they survived. To me the megafauna were the last, dying gasp of the evolutionary concept of "huge land animals" and even they didn't make it to the size of their predecessors.

 

So the question is, I suppose "What changed?". Why has small become more successful than big when you consider just how successful big was and for how long. (Successful in this case being defined as "Continuing to live")

 

We've had O2 levels halve, SuperContinents and separate ones, shallow seas and deep ones, a phenomenal range of differing climatic conditions for life to survive under and "big" was always successful, but not for the last 30 million years and very certainly not for the last 2 or 3.

 

This is why I thought the pressure idea interesting. Insects are limited in their size due to the method they use to breathe so I have to wonder if there is a similar limit for animal lung capacity. Perhaps there is a threshold limit somewhere. Similarly it seems odd that increasing the O2 levels by 50%, from 21% now to the 35% of the Carboniferous Era is enough to allow the insects to grow as large as they did. A 50% increase in O2 for a 1,000% increase in size seems a bit much. But if we add in higher pressure then it becomes quite reasonable. (I may not know much evolutionary biology, but from Scuba diving I do know the effects of gasses under pressure on organisms.)

 

Another way of phrasing the question would be to say that for all of Earths history, evolution has allowed animals to grow as big as they could possibly be. Assuming the basic rules of evolution don't change then why is "As big as it can possibly be" smaller now than it was then?

[Santalum]

Dinosaurs grew fast like mammals, not slow like reptiles, the growth rings in the bones of dinosaurs and the blood supply to those bones indicates very fast juvenile growth like mammals then a slow down like mammals. I think this shows up even in the sauropod dinosaurs... It is also true, that how that growth could have been maintained in the sauropods is puzzling...

 

 

Like I pointed out salt water crocodile etc babies have a fairly rapid growth rate in ideal conditions.

 

Females usually reach maturity at the length of 2.3 metres total length and it takes them a minimum of 12 years to reach that size. Males mature at around 3.35 metres and it will take around 16 years. In some crocodile farms it will only be 3 metres to mature, and females around 2.1 metres and that may take only 6 to 7 years.

 

A crocodile farm is the optimum conditions for a crocodile with an unlimited supply of food. And 6 years from hatchling to 2.1m long is comparable to many mammal species. And crocodiles are 100% ectothermic.

 

So if the sauropods were partially endothermic (giagantothermic when mature), and there was ideal greenhouse earth conditions, then surely it is not all that remarkable that tiny suaropod hatchlings grew into giant suaropods?

[Moontanman]

Like I pointed out salt water crocodile etc babies have a fairly rapid growth rate in ideal conditions.

 

 

 

A crocodile farm is the optimum conditions for a crocodile with an unlimited supply of food. And 6 years from hatchling to 2.1m long is comparable to many mammal species. And crocodiles are 100% ectothermic.

 

So if the sauropods were partially endothermic (giagantothermic when mature), and there was ideal greenhouse earth conditions, then surely it is not all that remarkable that tiny suaropod hatchlings grew into giant suaropods?

 

 

Six to twelve years to reach maturity which is not maximum size is not typical for mammals, deer for instant reach maturity and pretty much maximum size with in a couple years, dogs within a year, Humans are odd in that we mature slowly but elephants take several years as well but not as many as humans. I'm not sure why you would say 6 and 12 years to maturity is common in mammals...

[questionposter]

So the question is, I suppose "What changed?". Why has small become more successful than big when you consider just how successful big was and for how long. (Successful in this case being defined as "Continuing to live")

 

 

Back when dinosaurs were around, the average temperatures were much much warmer, some tropical regions even averaged 120 degrees. Because of this, animals such as reptiles could direct little or no energy towards generating their own metabolic energy and could instead direct it towards growing and maintaining a bigger body. Now, average temperatures are lower and there's less vegetation. However, in the regions that do mimic the dinosaur eras, such as along the equator in south America and in Indonesia, reptiles are very large compared to other animals, and there is more vegetation and food.

Of course, it's not just one factor, but this is a major factor.

Edited January 20, 2012 by questionposter

[granpa]

http://en.wikipedia.org/wiki/Antarctica#Neogene_Period_.2823.E2.80.930.05_mya.29

 

The cooling of Antarctica occurred stepwise, as the continental spread changed the oceanic currents from longitudinal equator-to-pole temperature-equalizing currents to latitudinal currents that preserved and accentuated latitude temperature differences.

 

Africa separated from Antarctica around 160 Ma, followed by the Indian subcontinent, in the early Cretaceous (about 125 Ma). About 65 Ma, Antarctica (then connected to Australia) still had a tropical to subtropical climate, complete with a marsupial fauna. About 40 Ma Australia-New Guinea separated from Antarctica, so that latitudinal currents could isolate Antarctica from Australia, and the first ice began to appear. During the Eocene-Oligocene extinction event about 34 million years ago, CO2 levels have been found to be about 760 ppm and had been decreasing from earlier levels in the thousands of ppm. Around 23 Ma, the Drake Passage opened between Antarctica and South America, resulting in the Antarctic Circumpolar Current that completely isolated the continent. Models of the changes suggest that declining CO2 levels became more important. The ice began to spread, replacing the forests that then covered the continent. Since about 15 Ma, the continent has been mostly covered with ice, with the Antarctic ice cap reaching its present extension around 6 Ma.

 

In 1986, Peter Webb and a team of paleontologists at Ohio State University discovered the remains of an extensive temperate forest that flourished 640 kilometres (400 mi) from the South Pole 3 million years ago

[Santalum]

Six to twelve years to reach maturity which is not maximum size is not typical for mammals, deer for instant reach maturity and pretty much maximum size with in a couple years, dogs within a year, Humans are odd in that we mature slowly but elephants take several years as well but not as many as humans. I'm not sure why you would say 6 and 12 years to maturity is common in mammals...

 

Yeah well there is considerably less of a size difference between a baby dear and its parent than between a crocodile hatchling and its parent.

 

A mother dear could certainly not carry its baby in her mouth as a mother crocodile can.

 

That's the whole point of being a placental mammal in sporning larger off spring that have a greater chance of survival.

 

Apart from that the growth rates between many mammals and reptiles are probably quite similar I suspect.

[michel123456]

Santalum, even the megafauna didn't reach the sizes that were around only 30 million years ago. Also on the timespans I'm talking about humans don't matter as until recently we were more prey than predator. I can't see the size/drought thing as too valid either since extremely large critters survived for tens of millions of years which means they survived droughts.

 

I don't know that I've been expressing myself well on what I've observed so I'll try a rephrase.

 

Life on this planet evolved in a series of phases separated by extinction events. After each of the extinctions life came back and very large animals were common. In each stage the tendency was for the animals to get bigger. After the dino-killer event life did this again. Even thought he mammals were ruling, they were getting very big and the same trend was apparent as was in earlier periods.

 

Around (as a guesstimate) 25 million years ago the trend reversed and smaller became more evolutionarily successful. Multiple extremely large species of Rhino became the 4 much smaller species we have today. "Lucy"s people were only 4 foot tall yet they survived. To me the megafauna were the last, dying gasp of the evolutionary concept of "huge land animals" and even they didn't make it to the size of their predecessors.

 

So the question is, I suppose "What changed?". Why has small become more successful than big when you consider just how successful big was and for how long. (Successful in this case being defined as "Continuing to live")

 

We've had O2 levels halve, SuperContinents and separate ones, shallow seas and deep ones, a phenomenal range of differing climatic conditions for life to survive under and "big" was always successful, but not for the last 30 million years and very certainly not for the last 2 or 3.

 

This is why I thought the pressure idea interesting. Insects are limited in their size due to the method they use to breathe so I have to wonder if there is a similar limit for animal lung capacity. Perhaps there is a threshold limit somewhere. Similarly it seems odd that increasing the O2 levels by 50%, from 21% now to the 35% of the Carboniferous Era is enough to allow the insects to grow as large as they did. A 50% increase in O2 for a 1,000% increase in size seems a bit much. But if we add in higher pressure then it becomes quite reasonable. (I may not know much evolutionary biology, but from Scuba diving I do know the effects of gasses under pressure on organisms.)

 

Another way of phrasing the question would be to say that for all of Earths history, evolution has allowed animals to grow as big as they could possibly be. Assuming the basic rules of evolution don't change then why is "As big as it can possibly be" smaller now than it was then?

 

You are right, something changed.

[Santalum]

Santalum, even the megafauna didn't reach the sizes that were around only 30 million years ago. Also on the timespans I'm talking about humans don't matter as until recently we were more prey than predator. I can't see the size/drought thing as too valid either since extremely large critters survived for tens of millions of years which means they survived droughts.

 

I don't know that I've been expressing myself well on what I've observed so I'll try a rephrase.

 

Life on this planet evolved in a series of phases separated by extinction events. After each of the extinctions life came back and very large animals were common. In each stage the tendency was for the animals to get bigger. After the dino-killer event life did this again. Even thought he mammals were ruling, they were getting very big and the same trend was apparent as was in earlier periods.

 

Around (as a guesstimate) 25 million years ago the trend reversed and smaller became more evolutionarily successful. Multiple extremely large species of Rhino became the 4 much smaller species we have today. "Lucy"s people were only 4 foot tall yet they survived. To me the megafauna were the last, dying gasp of the evolutionary concept of "huge land animals" and even they didn't make it to the size of their predecessors.

 

So the question is, I suppose "What changed?". Why has small become more successful than big when you consider just how successful big was and for how long. (Successful in this case being defined as "Continuing to live")

 

We've had O2 levels halve, SuperContinents and separate ones, shallow seas and deep ones, a phenomenal range of differing climatic conditions for life to survive under and "big" was always successful, but not for the last 30 million years and very certainly not for the last 2 or 3.

 

This is why I thought the pressure idea interesting. Insects are limited in their size due to the method they use to breathe so I have to wonder if there is a similar limit for animal lung capacity. Perhaps there is a threshold limit somewhere. Similarly it seems odd that increasing the O2 levels by 50%, from 21% now to the 35% of the Carboniferous Era is enough to allow the insects to grow as large as they did. A 50% increase in O2 for a 1,000% increase in size seems a bit much. But if we add in higher pressure then it becomes quite reasonable. (I may not know much evolutionary biology, but from Scuba diving I do know the effects of gasses under pressure on organisms.)

 

Another way of phrasing the question would be to say that for all of Earths history, evolution has allowed animals to grow as big as they could possibly be. Assuming the basic rules of evolution don't change then why is "As big as it can possibly be" smaller now than it was then?

 

Apart from human hunting pressure, as I understand it food supply(related to climate) and level of competition among large populations of various species determines the optimum upper limits to the size of any given species.

 

Harsh climate and limited food supply or high levels of of comeptition for a given food supply tends to select for smaller size. Smaller animals can get by on less food.

 

But there is no reason to assume that in a future epoch very large animals will not again befavoured.

 

Sexual selection often promotes large size of males that can then more easily over power their rivals for example.

[Moontanman]

The most obvious thing that changed between now and the time of the dinosaurs is the CO2 content of the air, it was much warmer then, although not during the entire age of dinosaurs, I'd like to see some evidence it was 120 on average at the equator but it still got cold near the poles. Not cold enough to support permanent ice caps but it was very cold in the winter, cold enough for ice and snow, and yes dinosaurs flourished there as well.

 

I'm not sure about the air pressure being 2X what we have now, that would be difficult to explain, but I would be open to mechanisms for this drop in pressure.

 

As for mammals, it should be noted that dinosaurs had much longer to evolve large land forms than mammals have and that dinosaurs seem to have been pre-adapted for large size to some extent due to the way they are put together and hollow bones and a more efficient respiratory systems might have helped as well.

 

I don't think it can be argued that humans have not had a huge negative impact on large animals around the earth, both by killing larger animals and by introducing animals and their diseases outside the native range of these animals. I think that some of this effect also had to do with minor extinction events like super volcanoes and meter impacts as well but the science on their effects is not clear, it could have just been humans in the last 35,000 years or so or it could have been a just natural disasters or more likely it was a synergy of both...

 

here are some links, some supportive and some not so supportive...

 

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

 

Climate

 

The Triassic was generally dry, a trend that began in the late Carboniferous, and highly seasonal, especially in the interior of Pangaea. Low sea levels may have also exacerbated temperature extremes. With its high specific heat capacity, water acts as a temperature-stabilizing heat reservoir, and land areas near large bodies of water—especially the oceans—experience less variation in temperature. Because much of the land that constituted Pangaea was distant from the oceans, temperatures fluctuated greatly, and the interior of Pangaea probably included expansive areas of desert. Abundant evidence of red beds and evaporites such as halite support these conclusions.

Sea levels began to rise during the Jurassic, which was probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) more than today, which flooded coastal areas. Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the Tethys Sea. Temperatures continued to increase and began to stabilize. Humidity also increased with the proximity of water, and deserts retreated.

The climate of the Cretaceous is less certain and more widely disputed. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10°C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20°C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as "black shale".

Not all of the data support these hypotheses, however. Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of polar ice caps and glaciers, but there is no evidence of either. Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient.[citation needed]

The amount of oxygen in the Mesozoic atmosphere varied widely, but was significantly higher on average (~26%) than in today's atmosphere (20 to 21%).[6]

[edit]

 

http://www.dinodatabase.com/dinoage.asp

 

http://pubs.acs.org/subscribe/archive/ci/30/i12/html/12learn.html

 

http://www.lakepowell.net/sciencecenter/paleoclimate.htm

[Santalum]

more efficient respiratory systems might have helped as well.

 

Interesting!. I have never come across anything like that before. How was it more efficient?

[Moontanman]

Interesting!. I have never come across anything like that before. How was it more efficient?

 

 

Dinosaurs had respiratory systems like birds, they do not draw air in and right back out like mammals and reptiles do. Air is circulated around and out instead of in and out, it means that oxygen better extracted and CO2 is exhausted.

 

http://people.eku.edu/ritchisong/birdrespiration.html