Naked Science Forum
Life Sciences => Plant Sciences, Zoology & Evolution => Topic started by: lunar11 on 04/05/2013 00:05:32
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Why were dinosaurs very large?
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My friend(he's a blue whale) says they were not that big . tee hee.
Maybe there were not so many good places to hide and so The big always ate the little and so the bigger ones always reproduced and so they evolved bigger and bigger until they were huuuuuuge. What I want to know is when people first knew about dinosaurs, surely they dug up bones in the past and knew they had existed ... i guess dragon legends could have been borne from this. Slay the t-rex and snag yourself a princess.
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While some dinosaurs were very large, even huge, there were a great many which were no bigger than your average family dog.
Take a look at these threads:
http://www.thenakedscientists.com/forum/index.php?topic=18167.msg204622#msg204622
http://www.thenakedscientists.com/forum/index.php?topic=42994.msg386158#msg386158
http://www.thenakedscientists.com/forum/index.php?topic=47207.0
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Yes, dinos came in all sizes.
To a large extent, there may have been an "arms race", with both predators and prey becoming more powerful. It likely would take an awful lot to take down a large adult sauropod.
Different dinosaurs roamed the Earth for hundreds of millions of years. Undoubtedly there were great climate fluctuations. But, much of the time was significantly warmer than it is today, and often described as a "tropical climate". Thus more food availability for the large species.
I have heard a comparison between snake size and temperature, with the higher the temperatures, the larger the snakes (in general), even today.
Likely the falling carbon dioxide levels and falling temperatures 65 million years ago caused a decrease in plant growth and the decline in the dinosaurs before the asteroid impact.
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Okay, so dinosaurs came in all sizes, but still, the largest land dinosaurs was much larger than the largest land mammals.
Is it because huge warm-blooded land mammals would have difficulty getting rid of the heat they would generate? Whales can grow to huge sizes because they are in water, and water is a good heat sink as well as being able to support them physically.
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....... more food availability for the large species.
I'm not sure I can entirely agree with you on that point Clifford. There are still many areas to this day where vegetation is 'lush' and could support dinosaur size animals.
There is some new evidence that dinosaurs may not have been wholly ectothermic. If this were the case, the falling CO2 levels may have caused a reduction in tree growth leading to reduced opportunity for larger dinosaurs to get shade, thus they may have overheated. Of course, that does nothing to explain the demise of the smaller dinosaurs, but again the CO2 levels could have played a part, or rather the rising oxygen level may have played a part. Too much oxygen can be just as damaging as too little.
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Okay, so dinosaurs came in all sizes, but still, the largest land dinosaurs was much larger than the largest land mammals.
Is it because huge warm-blooded land mammals would have difficulty getting rid of the heat they would generate? Whales can grow to huge sizes because they are in water, and water is a good heat sink as well as being able to support them physically.
You are overlooking the plesiosaurs, pliosaurs, ichthyosaurs and mosasaurs, the marine dinosaurs. Also the Archelon, though not a dinosaur, this 4m long 2 ton turtle also fell foul of the KT event. Archelon was a surface feeder and the marine dinosaurs would have had to remain close to the surface in order to breathe. What ever brought about the end of the dinosaur's reign, it was not a single condition, but a combination of changes, some of which preceeded the KT event. The KT event was perhaps the straw that broke the camel's back.
So why no giant mammals? Perhaps for no other reason than they did not need great size. Perhaps mutations which were leading to great size proved to be less efficient than their smaller counterparts, so they were not the ideal mate. Optimum size achieved, larger offspring fail the test of natural selection.
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So why no giant mammals? Perhaps for no other reason than they did not need great size.
Then why giant dinosaurs?
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Ok,
Here are some temperature and atmospheric gas estimates.
http://phl.upr.edu/library/notes/habitabilityofthepaleo-earthasamodelforearth-likeexoplanets
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fphl.upr.edu%2F_%2Frsrc%2F1312563142291%2Flibrary%2Fnotes%2Fhabitabilityofthepaleo-earthasamodelforearth-likeexoplanets%2FVPE_Temp-CO2-O2.png&hash=14b2035bc5e119d8c7db0327ef565a1f)
There was a moderate increase in Oxygen during the early Cretaceous period, but it leveled off by the end of the Cretaceous period.
For Animals, Oxygen ==> Energy. So, while a sudden shift in oxygen levels may be hard on a species, the species should be able to adapt to moderate changes.
However, it is noted that during much of the Jurassic and early Cretaceous period, the levels of oxygen were about at about 15%, significantly less than the 20% today.
Could it be that mammals, for example, would fare poorly with lower oxygen levels, even if adapted to them over millions of years. I.E. less speed, and less extreme bursts of energy. This might favor the larger animals.
Other improvements such as better lungs might also help smaller animals generate more energy, and compete better with the larger animals.
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I always wondered why insects didn't get bigger. Is an exoskeleton unstable beyond a certain size? Is their respiratory system not efficient enough to get oxygen to all the the tissues beyond a certain size? Well, anyway, I'm glad they didn't.
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Prehistoric bugs apparently were bigger too.
http://listverse.com/2013/01/14/10-prehistoric-bugs-that-could-seriously-mess-you-up/
http://www.cracked.com/article_18445_7-thankfully-extinct-giant-versions-modern-animals.html
http://www.lipstickalley.com/f389/pulmonoscorpius-gian-t-scorpion-481851/
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi2.wp.com%2Flistverse.com%2Fwp-content%2Fuploads%2F2013%2F01%2F61.jpg&hash=497163074c9f61f65230b68b10f224f6)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi.crackedcdn.com%2Fphpimages%2Farticle%2F6%2F2%2F0%2F19620.jpg&hash=c7da160b278e2c71e6075e8290f2628c)
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Flistverse.com%2Fwp-content%2Fuploads%2F2010%2F03%2F2285160839_0443ba1389-tm.jpg&hash=4ea33f79f74d698f8a1c9d1f80ca7b53)
Not including some of the sea dwelling insect relatives.
Would overpopulation be an issue? Larger animals tend to have longer lifespans. If the number of offspring aren't tightly regulated, they could easily overwhelm their food source.
Smaller animals, while potentially subject to more predatory species, tend to have a quicker generation time, more offspring, and there are more of them, so it is harder to wipe them out. And, more life cycles means the genes get spread out more.
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Eww. That is going to give me nightmares!
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I always wondered why insects didn't get bigger. Is an exoskeleton unstable beyond a certain size? Is their respiratory system not efficient enough to get oxygen to all the the tissues beyond a certain size? Well, anyway, I'm glad they didn't.
I think when they are scaled up , although the new dimensions can hold more weight , the forces required is cubed or something like that. Watched interesting tv program a while ago. Also glad here that the design does not scale up. An ant the size of a t-rex would rule the world, well if it had its buddies.
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I suspect that the same drivers that lead to the phenomenon of "Island Gigantism" were at play.
Given the fairly low preservation potential of most terrestial creatures and difficulties in dating sedimentary rocks with an understandable level of precision (+/- millions of years is easy to say but harder to think!) it is hard to reliably establish the geographic and temporal range of most dinosaurs.
Following the principal of uniformitarianism it follows that in many cases that populations could evolved on "islands" even if these are simply virtual rather than actual.