MadSci Network: Evolution
Query:

Re: Is it possible that Dinosaurs were large because of genetic defects?

Date: Mon Jan 8 11:44:55 2001
Posted By: Brian Foley, Post-doc/Fellow Molecular Genetics
Area of science: Evolution
ID: 978924040.Ev
Message:

> I was wondering if it is possible that the reason dinosaurs 
> grew so large may have been because they had some sort of 
> genetic problem that kept them from being able to stop 
> growing in size? 

	I don't think so.  That type of genetic "problem"
would result in animals that just kept growing until they 
died, so you would find fossils of all sizes of animals
of a particular type.  For example you would find T-rex 
fossils everywhere from one ton size up to 200 tons or
something like that.  Instead we find fossils of jevenile
T-rex that are smaller, but all the adult T-rex are roughly
the same size, indicating that they stopped growing at
adult size.

> And do scientists have any idea as to how fast they 
> grew? 

	I'm not sure, but I doubt it.  You can tell
roughly what percentage of the total lifespan
is spent growing up, vs what percenatge is spent as
a full-sized adult, by studying what percentage of 
fossils semm to be juveniles.  But it is very difficult
to tell exactly how long individuals lived before dying
to then find out how fast they grew.  I would bet that
there are good estimates of how fast they grew, but the
estimates would be based on some assumptions.
	

> It seems that even the biggest animals on earth 
>  (such as whales) take a very long time to reach their 
> size, which is dwarfed by the size of certain dinosaurs, 

	I wouldn't say that great blue whales are 
"dwarfed" by huge dinosaurs.  Nor would I say that
they grow very slowly.  Whales and elephants are
huge compared to ants and mice, and the further
difference in size between an elephant and the largest
dinosaur is not as big as the difference in size between a
mouse and an elephant.  An elephant might be 80,000 times
larger than a mouse (I'm making a rough guess) and I
doubt that any dinosaur was more than 200 to 1,000 times
bigger than an elephant (guessing again).

> such as the brachiosaurus or diplodicus. This would suggest 
> that an individual animal could live as long as 200 or 
> 300 years before dying of old age. 

	Life spans of 200 to 1000 years might be reasonable.
It seems like a long life, but is still only 3 to 15 times
as long as humans live now.  Humans live about 23 times
as long a mice (70 years vs 3 years), but about 1/4 as long
as sea turtles.  

> It seems like animals 
> that live this long and grow unbounded would be genetically 
> inferior from a species of animals that had a relatively 
> shorter life and did not grow unbounded.

	That would depend on what you consider to be "inferior".
All life forms are "different" and humans tend to think
of many things as "better" or "worse" than other things, when
in fact the measurement of how "good" something is depends very
strongly on the opinions of the person doing the measuring.
A polar bear is genetically superior to a panda for living on 
the ice, but a panda is better than a polar bear for living
in bamboo forests.
	If two species have DNA that mutates at exactly the
same rate, then the species that reproduces most often can
evolve slightly faster (by giving more chances for selection
on the same number of mutations).  Mice tend to evolve
a bit faster than larger mammals, partly by reproducing more
often.  Reproduction has to be balanced by death, or the
planet (or the portion of the planet that is good habitat
for the species) soon would  become overcrowded with that
one species (as has been happening with humans over the
last 2,000 years or so).
	In a stable, unchanging environment, slow evolution
is usually a good thing.  In a rapidly changing environment, 
faster evolution is good, up to some limit.

> I once read that when the extinction of the dinosaurs 
> occured, nothing over 50 kilograms survived. 

	Nothing over 50 Kg lived on land.  Many large organisms
survived in the oceans and largest lakes.  And there may have
been factors other than size that mattered.  Purhaps things
like fur, warm-blooded metabolism, and the ability to 
hybernate favored mammals, and the mammals of the day just
happened to also be small.

> Is it possible that the reason that mammals 
> and smaller organisms survived the meteor impact was 
> because of shorter life spans and ability to not grow 
> unbounded because the dinosaurs would not have been able 
> to find enough food to sustain themselves after the 
> meteor impact blocked out the sun, but the smaller animals 
> didn't need as much?

	It's possible.  But I would suspect that many different
factors contributed to survival.  
	With things like a meteor impact, the environment changes
far faster than evolution can keep up.  Any organism
that does not have pre-existing skills (such as hibernation
or living underground in tunnels, or living in the deep ocean)
won't survive.  There is no time to evolve to keep up with the
changing environment.  So short lifespan and fast evolution would
not help on the day the meteor hit, but would help in the
2000 to 200,000 years or so afterward as the survivors went out to
fill empty niches left by the species that died in the impact.

Brian



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