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Q: earth science/physics ( Answered 4 out of 5 stars,   0 Comments )
Subject: earth science/physics
Category: Science > Earth Sciences
Asked by: doright-ga
List Price: $10.00
Posted: 19 May 2004 19:16 PDT
Expires: 18 Jun 2004 19:16 PDT
Question ID: 349117
I would like to know the reason why no land animals of today approach
the size of those in "Jurassic Park". I read somewhere that the Earth
is continually increasing in mass (and so in gravity) due to an
enormous daily accretion of meteroite dust. Perhaps a brontosaurus of
70 million years ago could exist because of a lower gravity Earth. The
largest insects of that time also are outsized compared to today's
largest. The reason for the difference in upper size limits for land
animals is not addressed in the popular literature that I have seen.
This suggests to me that scientists are unsure of the cause, but
prefer not to admit it outside of their professional journals. If an
accepted scientific answer does exists, then of course I want it. If
not, then that will be an acceptable answer as well. In either case, I
need links to the sites which you use to confirm that judgement. By
the way, this not a homework question. Thank you.




 crush a brontosaurus
Subject: Re: earth science/physics
Answered By: digsalot-ga on 20 May 2004 04:54 PDT
Rated:4 out of 5 stars
Hello there

We can divide this into two parts, one dealing with the big bugs and
another with the dinosaurs.

First, the "oversized" insects.

We do know that dragonflies with wing spans as wide as a hawk?s and
cockroaches big enough to take on house cats were once very real.  We
also know that gravity was not a player in that gigantism.  It is
likely that oxygen was.

The giant insects and the dinosaurs didn't really overlap.  The big
bugs vanished just about the time the first dinosaurs came along.  And
no, the dinos didn't just eat 'em all up.

The answer is probably found in the way insects breathe.  There have
been studies on how respiratory physiology of modern insects affects
their body size.  The amount of oxygen limits insect body size because
of how the bug?s respiratory systems are made. Instead of lungs,
insects breathe with a network of tubes called tracheae. The distance
oxygen can travel down the tracheae depends on its concentration in
the air. If atmospheric oxygen is doubled, theory says that it should
be able to make it twice as far.

So how do we relate this theory to the giant ancient insects?

Today, the concentration of oxygen in the atmosphere is about 21%. 
But ancient soil testing has shown that about 300 million years ago,
the concentration of oxygen in the atmosphere was about 35%.  This
oxygen concentration stayed high for about 100 million years, then
suddenly it dropped to about 15%.

"Scientists think that the then-recent evolution of oxygen producing
land plants caused this oxygen peak. Interestingly, the rise and fall
of atmospheric oxygen also coincided with the evolution and extinction
of giant insects." - quote from "ASU Research E-Magazine: Big Ideas
About Big Bugs"

You may also enjoy: "Where giants roamed: The big bug theory."  - This
article from the International Herald Tribune also tells of the
connection between ancient higher oxygen levels and the rise of the
giant insects.

Now we can get to the dinosaurs.

1 - some dinosaurs were very large.  2 - they were lightweights   3 -
there are still living dinosaurs
Now here is the biggie - After decades of acrimony and debate it's
almost official:  dinosaurs are birds, not reptiles, they even had
feathers.  4 - today's birds are living dinosaurs.

Dinosaur bones are hollow and often made up of  'bony foam.'  So even
though many species of dino reached great size and weighed many times
more than a modern elephant, they were rather light weight is
relationship to their size.

Here is an article from the National Geographic supporting that claim.
 "The two animals were discovered in the rich fossil beds of China?s
Liaoning Province, the source of other major discoveries in recent
years. More than 120 million years old, they add considerable weight
to support the thinking of most paleontologists today: Birds are
dinosaurs." - Quote from the article -
- the article also has text links to related stories.

And this from CNN - Were dinosaurs really prehistoric birds?

And, the biggest dinosaurs didn't run around worrying about gravity,
they floated. - - "Sauropod dinosaurs, the largest terrestrial animals
ever to have lived on our planet, could float like corks in water,..."
- quote from Jennifer Viegas, Discovery News - "Discovery Channel ::
Study: Huge Dinosaurs Floated" - you
will find an interesting animation of a Brachiosaur bobbing merrily

More on floating dinos from BBC News:

There are many theories about increased gravity as the reason such
animals could not live today.  But if you check them closely, you will
find that they are "alternative science" or "alternative theory"
rather than mainstream, and often connected with creationist theory or
alien pyramid builders.

The Earth's mass is about 5.97 times 1024 kg. That's about
6,000,000,000,000,000,000,000 tons. It's measured, incidentally, by
measuring the periods and sizes of the Moon or Earth-orbitting
satellites. Isaac Newton's law of gravitation, along with his laws of
motion, show that you can "weigh" a planet or star just by measuring
the orbits of bodies in orbit around it.

50,000 tons of meteorites fall on the Earth each year and it's
certainly true that at least 200 tons of material strikes the Earth
every day, most of those meteors are very small and burn up in the
atmosphere. Between 1740 and 1990, the total mass of known meteorite
finds was around 500 tons. If we estimate that between 2 and, maybe, 5
tons of material lands on earth each year, the total in the last 60
million years would be 30,000,000,000 tons. Even if the figure were
50,000 tons per year, the accummulate mass over 60 million years would
only be 300,000,000,000,000 tons, about 20,000,000 times smaller than
the mass of the Earth.  As you can see, the Earth is many, many times
more massive than the total mass of meteorites which have struck it in
the 60 million years or so since the dinosaurs existed, so its gravity
hasn't really changed at all in that time, maybe by one twenty

The only reason there are no land animals as large as the largest
dinosaurs today is that none have evolved.  But then again, we now
know that some of the largest dinos were "floaters" and made only
short excursions onto land.  We do have at least one floater today as
big as the dinos, though it does not come on land, the great blue

However, if a new animal came along and developed the light weight yet
strong bone structure of the large dinosaurs, today's gravity would
not be a reason for it to not reach the size of the old dinos.

search - google
Terms - dinosaur gigantism

The following websites were used to compose the answer:

The Nine Planets - Here you will find figures on the mass
of the Earth, meteorite information and much, much more.

Dinosaurnews Webzine - - this is a
magnificent collection of dinosaur resources.  You can browse here for

E-Magazine: Big Ideas About Big Bugs" -

National Geographic -


Discovery Channel - 


If I may clarify anything before rating the answer, please ask.


Clarification of Answer by digsalot-ga on 20 May 2004 04:59 PDT
Sorry - I forgot to include the link ""Where giants roamed: The big
bug theory"


Request for Answer Clarification by doright-ga on 02 Jun 2004 19:17 PDT
I appreciate and was rather amazed at the speed of your response to my
question. Only now have I had the opportunity to examine the
background information you cited. It seems to confirm the lack of any
role by the increase in Earth's gravity as a cause for the decrease in
the maximum size of land animals since the dinosaur era. That the
giant insects and dinosaurs existed at different points in time was
for some reason a complete surprise to me. So was the theory of a
surge in atmospheric oxygen levels 300 million years ago, even though
that theory is relatively new and probably would surprise a lot of
people, people who might have thought that our planet's atmosphere has
been as relatively unchanged as its gravity seems to have been for the
last 65 million years. Most useful to me of the information you cited
was the Arizonia State University site where researcher Jon Harrison
indirectly reveals that scientists at this point can not say what
causes either "... gigantism or gigantism gone extinct." It looks like
it's not gravity.

Clarification of Answer by digsalot-ga on 03 Jun 2004 18:44 PDT
I'm glad the material helped.  While there is no real reason for
gigantism not to develop again as far as gravity is concerned, the
main reason is probably "opportunity."

With the fall of dinos and the rise of mammals, sheer competition for
resources may be the main reason gigantism has not repeated itself
more frequently.  Though it has done so.  Some mammals did reach an
enormous size but early man did them in.

Here is an article about the American Indians and the reasons they may
have hunted many species to extinction, including such animals as the
mammoth, big horned bison, North American camel, etc. - - "The big
game hunters had many ways of hunting large mega-fauna and gradually
modified many tools to do so. One of the popular techniques of hunting
was to encircle a herd of Mastodons or other animals, and drive them
off of a cliff, or into a large thick swamp where they would be shot
with poison darts, projectile points, or large stones. Once killed in
the swamp, the prey was often skinned and taken from literally the
knees up (to about the bog or swamp water level). This technique has
been evidenced by large bone remains and deposits at many
archeological sites. - quote from - From
Minnesota State University.

That the Native Americans respected their environment is largely 20th
century myth-making.  Native Americans were responsible for most of
the die (kill) off of North American Mega-fauna.  The idea of
prehistoric man hunting species to extinction is offensive to some
contemporary Native Americans but the archaeological evidence is
there. - - - "...the catastrophic extinction of dozens of species of
large animals. Across the Americas millions of large animals?known as
megafauna?disappeared. These animals included the mammoth, mastodon,
and the giant ground sloth, as well as the horse, the camel, and many
other herbivores. Some very large and formidable carnivores also died
out, including the American lion, the saber-toothed tiger, and the
giant short-faced bear. These extinctions were thought to coincide
with the arrival of Clovis groups, a chronological coincidence that
led University of Arizona ecologist Paul Martin to propose the
hypothesis of Pleistocene overkill. This hypothesis, first put forward
in 1967, contends that Clovis big-game hunters caused the extinctions.
Martin suggested that overkill was especially likely?even
inevitable?if Clovis groups were the first Americans. For if the
megafauna had never before faced human hunters, they would have been
especially vulnerable prey to this new, dangerous, two-legged
predator." - From MSN Encarta - Migration to the Americas

In fact, one large mammal which has vanished since the arrival of
humans is the Eremotherium which was not only much larger than a
modern elephant but was much larger than the mammoths.  They stood 6 m
(20 ft) tall and weighed several tons.  You will find one illustrated
to scale in the above website.

There is also a good photo here: -
From the National Museum of Natural History

So, in reality gigantism is also a modern development and only the
intervention of man has brought it to an end.  While science may be at
somewhat of a loss at explaining 'why' gigantism takes place, the only
reason it still 'does not' take place is human meddling.

If I may be of further service, please ask.


Clarification of Answer by digsalot-ga on 03 Jun 2004 21:14 PDT
I really do wish we had an "edit" feature.  The link to the
Eremotherium drawn to scale is not the 'one above' but the second one

I should probably also wait till I've had some sleep before tackling
questions and clarification requests.  My error entirely.


Clarification of Answer by digsalot-ga on 20 Jun 2004 16:10 PDT
Thank you.

You made my day when you said: - "The researcher cited sources which
addressed my question at its edges. Reading between the lines in them,
I picked up the information that I had sought - and a lot faster than
had I researched it on my own."

Perhaps this was a homework question, perhaps not.  I went on the
assumption that it was simply by its nature.

I have often wondered if my method of handling homework questions had
any validity or not.

That is, instead of doing a finished paper, which we should not do
anyway, is to provide the information requested but point to its
"outer edges" or a "general resource" in the answer.

All the information is there but the student has to dig for some of it
and learn in the process.  And as you say, there is a need to be able
to "read between the lines" and associate what is suggested one place
with what is being read in another.

Thank you for letting me know it works.

And if you are not a student, thanks for letting me know it worked
anyway (and an apology for making you do some of your own legwork)

Sometimes knowing if a question is homework related is a toss-up.

doright-ga rated this answer:4 out of 5 stars
My question was answered, and faster than I had expected. The
researcher cited sources which addressed my question at its edges.
Reading between the lines in them, I picked up the information that I
had sought - and a lot faster than had I researched it on my own.
Count me a satisfied customer.

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