Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: Geoff on 02/10/2009 09:30:02
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Geoff asked the Naked Scientists:
Why did the dinosaurs grow so big?
Was it anything to do with the atmosphere? There must have been a lot of feed for them so the plants must have been very big also to grow dinosaurs that would graze and then become food to sustain the meat-eating dinosaurs.
Why did they grow so big?
What do you think?
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Its a popular misconception that all dinosaurs were huge. The fact is that the vast majority of the land dinosaurs were not. The Epidexipteryx for example was about 25 cms long and the Microraptor weighed in at about 1 kg.
The Tyrannosaurus weighed around 7 - 9 tons, a modern African bull Elephant weighs in at around 7.5 ton.
The real biggies, such as the Brachiosaurus at around 25 m. from head to tail end and weighing in at a massive 30+ tons, needed support for their huge bulk, and so tended to live part submerged in lakes. These huge reptiles may have grown to such a size out of the need to reach high growing vegetation.
A good modern example of this can be seen in the Galapagos giant tortoises. Each island has it's own distinct species of giant tortoise. Most look like you would expect:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.yale.edu%2Ftri%2Ffellows2006%2Fimages%2Fsullivan.jpg&hash=6062ac71a76d8aa419bcc24c7e16dbc8)
Geochelone nigrita(Picture: Yale Education)
But Lonesome George, the last surviving giant tortoise from Pinta Island looks very different:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.britishcheloniagroup.org.uk%2Fappeals%2Fimages%2Flonesome1.jpg&hash=7b41b87fa3f44512fff0b9e25b4cdf85)
Geochelone nigra abingdoni(Lonesome George - Picture: BCG)
The reason for the difference between these two tortoises is food. Geochelone nigrita lives on an island where the tortoise grazes at ground level, Geochelone nigra abingdoni grazes on very much higher growing plants. The two species have evolved according to the food availability of their habitat.
The herbivore dinosaurs probably evolved to their different sizes so some would feed at ground level, others some 10's of cms above ground level and some at many meters above ground level. This may have been in order to reach specific parts of a tree, as in the case of the modern Giraffe.
Their size was, I suspect, part of their undoing. The largest dinosaurs would probably have fallen victim to the mass extinction the earliest.
All of this, however, does not detract from the fact that the largest animal ever to exist on Earth is the modern Blue Whale Balaenoptera musculus weighing in at a humongous 170+ tons.
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From a google search, apparently Brachiosaurus was fully terrestrial.
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I remain to be convinced on Brachiosaurus and similar sized dinosaurs being fully terrestrial.
The perceived placement of the nostrils above eye level, did lead to the misconception that the animal spent its time submerged. That was probably wrong. But I am still of the opinion that these largest of herbivore dinosaurs would have spent a good deal of time part submerged. Say to around the lower 1/4 - 1/3rd of the body.
The effort required for such an animal to remain upright would have been very considerable and to lie down would also be a great strain. To get up again would probably have left the animal exhausted. Part submersion in water would have been the better option.
There is also the point of drinking. These animals would have required a good deal of water. Part submersion in water would have made drinking far easier.
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From what I read, their feet would not have been good for distributing weight or moving around on the muddy bottoms of lakes etc. Also they would have had difficulty breathing due to water pressure decreasing the volume of their lungs, and this is important for such huge animals.
I wonder if anyone has actually worked out the forces involved and whether their musculoskeletal systems could actually cope with all that mass.
Dont African elephants rarely lie down?
The hydration requirement could be solved by proximity to but not submersal in water.
Life reconstructions I've seen place the actual nostrils closer to the tip of the upper jaw. The space between the nostrils and the openings in the skull could have been taken up by a cavity used for sound communication or something else.
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I think land based giants are feasible for several reasons. The atmosphere in the late Jurassic was both richer in oxygen and much higher in carbon dioxide. so lush, abundant, fast growing plant life on land would abound, making giant plant eaters both possible and ecologically beneficial. Also. the sheer bulk of giant cold-blooded dinosaurs would allow them to stay warmer for a longer time and let them feed during the night. To support such size they would have had to eat a lot, even if they were cold blooded.
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From what I read, their feet would not have been good for distributing weight or moving around on the muddy bottoms of lakes etc.
This, I must admit, is a 'fly ointment'. But could it be that, similar to the elephant, they had an extremely soft fleshy foot, to spread the weight? Such a soft fleshy foot, might not leave any fossilized trace.
The hydration requirement could be solved by proximity to but not submersal in water.
The problem with hydration, as I see it, is the long neck many of these animals had. A giraffe must stoop down, spreading it's front legs, to drink. It's neck, at around 2.5 to 3.0 m long means it cannot drink for very long. What problems, then, would be presented to a Brachiosaurus with a neck of around 8 m and at some 16 m height?
Part submersion in water would have alleviated some of their problems.
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I think land based giants are feasible for several reasons. The atmosphere in the late Jurassic was both richer in oxygen and much higher in carbon dioxide. so lush, abundant, fast growing plant life on land would abound, making giant plant eaters both possible and ecologically beneficial. Also. the sheer bulk of giant cold-blooded dinosaurs would allow them to stay warmer for a longer time and let them feed during the night. To support such size they would have had to eat a lot, even if they were cold blooded.
These are good points, and would certainly be a factor in the dinosaurs being able to attain such size and bulk. But, nonetheless, most dinosaurs were not the giants depicted in Hollywood films.
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Perhaps they got most of the water they need from the moisture in the leaves
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The last I read about sauropod feet was that they were not elephant-style; there was no big pad behind the toes. We do have sauropod footprints and they are not disproportionally wide, to my knowledge.
Perhaps this would interest you? http://news.bbc.co.uk/2/hi/science/nature/3310531.stm
A brachiosaurus' neck is much longer than its entire height from foot to shoulder. It would have less difficulty reaching water to drink than a giraffe would. Since their necks were carried so vertically, this means they were browsing tall trees. Otherwise they wouldn't need their heads to be so high. So they would have had to be able to bring their heads down to drink, excluding Madidus_Scientia's suggestion. Also, if you are referring to the blood pressure problem, giraffes have a regulation system, so why not sauropods?
For the sauropods with more horizontal neck carriage, drinking water would have been even less problematic.
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Perhaps they got most of the water they need from the moisture in the leaves
This was long thought to be the case with tortoises, but in recent years it has been found to be untrue. Tortoises will walk into puddles, ponds and the like up to the point where the plastron and carapace meet to drink. They cannot get sufficient hydration from plants alone, not even the succulents many are so fond of.
Thanks for that link Stefan, I had not seen this before. It seems to support both possibilities to some degree. T'is food for thought.
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Yes, although these parts were particularly salient to me:
"It looks to me like sauropods liked dry land and didn't spend a lot of time in water. We do find their footprints along shorelines, so they must have gone there to drink and so on.
"But we also find them inland at fossil sites like the Morrison Formation, which millions of years ago would have been semi-arid and without much water," Professor Lockley told BBC News Online.
At the very least this shows that they did not need water to support their weight.
This theory took a blow in the late 1940s when a US researcher proposed that if a sauropod were submerged under several metres of water, the pressure would have collapsed its lungs and airways, killing it.
How deep would Braciosaurus have needed to submerge itself to drink more easily? At a certain point brachiosaurus's thorax/abdomen would become innefficient at drinking water due to the pressure. Was this point of difficulty involved in drinking less or more difficult than having to lower the neck to the water while on ground? Was being knee deep enough?
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The picture I have in mind of Braciosaurus is this:
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With the water level somewhere around the underside of the tail (around mid thigh level of the forelimb), the torso would probably not be submerged more than around 2 meters. This would not exert any undue pressure on the abdomen or thoracic region. It would not put the animal at risk of capsizing either. But it would give sufficient buoyancy to give the animal a reasonable amount of support and could have reduced the angle of incline of the neck by up to 50% when stooping to drink.
Such behavior would explain the sauropod tracks found in the beds of lakes.
The fact that tracks have also been found in arid and semi-arid areas, might even suggest that there were two distinct species of Braciosaurus and other such sauropods, identical in all but their habitat.
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I can happily agree to that scenario where drinking is concerned [:)]
I would need to be persuaded further that they actually needed water for support. Even if there were multiple species, their morphologies as far as I know are not dramatically different, so I cannot see how that would change whether they needed to support themselves with water. If the "desert" forms did not need to, why not the "submerged" ones?
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The fact that tracks have also been found in arid and semi-arid areas, might even suggest that there were two distinct species of Braciosaurus and other such sauropods, identical in all but their habitat.
This was something which just came into my mind, so I posted it.
.... If the "desert" forms did not need to, why not the "submerged" ones?
Quite so. It was a ridiculous suggestion, which I shall gladly retract.
I must remember to switch on my brain before I put my mouth into gear!
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Hehehe
So where do you still stand now? [:D]
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ALL RIGHT ALREADY! I, Cartman are here. You will obey my au-tho-rit-I!!
Many paleontologist think that sauropods did not live in water due to the analysis if the structural characteristics of their necks. The necks are designed like the segments of a gantry crane with interlocking tubercales and processes that, with the ligaments can easily support the very small head. It is even speculated that they fed on the tops of the trees, the most tender part.
Sorry Don [>:(]
AS TO THE ORIGINAL QUESTION - Oxygen levels may or may not have an influence on the size of the dinosaurs. The Great Permian Extinction was marked by a drastic drop in oxygen levels. (See reconstruction of atmospheric oxygen saturation - upper right corner http://en.wikipedia.org/wiki/Great_Oxygenation_Event) This event happened at the major peak about 250 MYA. The dinosaurs developed after this time of maximum oxygen saturation.
So - sorry but no banana on this one, folks. There is no direct correlation.
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Er well, JimBo, if you don't mind me a'sayin so, that's complete bollockos. Everyone knows them dang serapods is still swimmin' round in that there Lake Ness or Lock Cess or whatever them Icelanders calls it.
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That is Pterosaurs Geezer, them fly--
Ok, wait -is it Tyrannosaur
no - Hmmmmm Plesiosaur !! Yeh, that's the ticket!
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I wonder whether the recently discovered footprints from the Jura http://news.bbc.co.uk/1/hi/world/europe/8294425.stm (http://news.bbc.co.uk/1/hi/world/europe/8294425.stm) will cast further light on the sauropod question.
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Thar y'go. Jura is really close to that there Lake Ness!
(Oh! You mean the other Jura [:I])
http://en.wikipedia.org/wiki/Jura,_Scotland
and the nearby monster whirlpool
http://en.wikipedia.org/wiki/Gulf_of_Corryvreckan
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Ahh, yes Sorry I was not clear - Jura as in moutains and Jurassic, rather than Jura as in the land of bog and rock where half decent whisky comes from. [^]
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Right, there Mazurka. The best whiskey is Irish
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You will smart for that remark JB!
I was going to send you some of this, but now it's going to go to Neil.
http://www.spencerfieldspirit.com/sheepdip.html