[nassar]

Nassar asked the Naked Scientists:

My daughter has asked me. Why were dinosaurs so large compared to the animals we have now.

Thanks

What do you think?

[dentstudent]

I think that there may be (at least) 3 reasons:

The first is that there was a greater percentage of oxygen in the atmosphere then, which meant that the animals, especially I believe the insects, were able to grow larger, whereas the atmospheric oxygen content of today is a limiting factor.

The second would be that there is some bias in the collection of animals, in that a larger proportion of the larger animals are easier to find as fossils, and it's easier to find a big fossil than a small one.

The third would be that it's sexier to have books about great big animals than it is to have books about small ones, and so I think that there is also some "advertising" bias in what we perceive the real size range to be.

I'm sure there are plenty of other reasons too!

[Karen W.]

I wonder if it was to do with how much space there was in their environment?  A small feeder fish will grow to a very large fish in a large environment, without too much clutter and other fish! Perhaps the same is true of Dinosaurs.. I imagine the lack of civilization and the abundant sources of food and surroundings could have contributed to this.. That is only my guess so maybe some one who knows more may come to tell me I am full of hot air? LOL..It does seem to be plausible or maybe even possibly a single factor along with DNA etc..

[Madidus_Scientia]

Temperature may have been a factor too. Larger animals can stay warmer in a cold climate because of their lower surface area to volume ratio.

[Don_1]

The second would be that there is some bias in the collection of animals, in that a larger proportion of the larger animals are easier to find as fossils, and it's easier to find a big fossil than a small one.

The larger the fossil the more likely it is to be found. While there were many large dinosaurs, they were vastly outnumbered by the smaller ones. As dentstudent says, it is also 'sexier' to talk about T. Rex and Diplodocous than the smaller Microrapter (about 40cms long) the Compsognathus (about 60cms) and the Heterodontosaurus, a skull of which was recently found which is just about 5cms long.

Lets not forget that the largest animal ever is the still living (JUST!) Blue Whale.

[DoctorBeaver]

It's simple - they always "went large" at McDonald's  []

[RD]

The first is that there was a greater percentage of oxygen in the atmosphere ...

In the Jurassic period the level of  CO₂ was several times higher than todays level, O₂ was only about 25% higher.

There has historically been much more CO2 in our atmosphere than exists today. For example, during the Jurassic Period (200 mya), average CO2 concentrations were about 1800 ppm or about 4.8 times higher than today.

http://ff.org/centers/csspp/library/co2weekly/2005-08-18/dioxide.htm

More CO₂ would permit larger amounts of vegetation than today, allowing the existence of very large herbivores.

[that mad man]

I have also read that the oxygen content may have been higher but am very sceptical about that one and how that would work. The plant life at the time was also gigantic and yet the plants rely on C02 to grow.

Could it be down to the metabolic rate of the animal? Under the ice caps in the deep sea the marine animals tend to grow very large and live a lot longer than their shallow sea counterparts. On average it seems that the smaller the land animal the faster the metabolic rate and its lifespan normally shorter.

Edit:

The post by RD make sense to me.

[Alandriel]

Simple!

Competition.


BIGGER IS BETTER (in this case apparently at least)

 []

It's kind of difficult to bring down e.g. a huge mammoth plus being huge allows for easier storage of energy in the form of fat which is a definite survival bonus during hard times.

But I think in the end the real answer is







NOBODY KNOWS  [] []


because knowbody has seen - or rather, nobody has been able to study live dinosaurs.
You see, some of the most interesting dinosaur bitts sadly don't fossilize.

[DoctorBeaver]

You see, some of the most interesting dinosaur bitts sadly don't fossilize.

I won't ask!  []