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RE: (arthritic) Sauropods vs. Gravity - all



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Paul Cambridge
>
> Let me get this straight, now. It apparently doesn't matter so
> much the size of a planet as what its surface gravity is?

Correct.  The surface gravity is a combination of the mass of the planet and
its radius.  Keep the same mass, shrink the planet down to the size of a
pea, and you'd have white dwarf (maybe even neutron star) material.  Shrink
it down to infintesimal size, and you have a black hole.  Expand Earth to
the size of Jupiter and you'd have cosmic fluff with extremely weak surface
gravity.

> No,
> wait. I'm all confused now. I guess what I am trying to
> understand is why aren't there now sauropod-size animals around
> now? Because there hasn't been given enough time due to the ice
> age? Or because of climate? I would hate to think that the warmth
> of the Mesozoic had a lot to do with size. I get the willies
> thinking about theories describing dinos as overgrown lizards.

Well, nature doesn't care about our willies.  However, Janis & Carranno,
back in:

Janis, Christine M. and Matthew Carrano. 1992. Scaling of reproductive
turnover in archosaurs and mammals: why are large terrestrial mammals so
rare?. Annales Zoologici Fennici 28(3-4): 201-216.

pointed out one possible reason.  This paper turns the question on its head,
from "why did dinosaurs get so big?" to "why do terrestrial mammals not get
dinosaur sized?" (after all, why assume dinosaurs are the oddballs?).  Their
suggestion has to do with the fact that in placental mammals gestation
period scales with body size, so that sauropod-sized mammals would have 3-4
year (or more) gestation periods, and even then producing only 1 young.  In
contrast, even the biggest sauropods could produce a dozen or more eggs
every year.  Therefore, large bodied dinosaurs could sustain large enough
populations for selection to act upon and produce new species, whereas large
bodied mammals would be trapped by their size in a situation where they were
VERY vulnerable as individuals and as populations.

Other factors towards dinosaur giant size might be:
  * Larger continental areas inhabited by particular species (island
biogeography)
  * Higher plant productivity due to more favorable conditions (although
these conditions fluctuate throughout the Mesozoic)
  * Difference in physiological rates due to different atmospheric
composition (ditto)

And others.  None of these require a "lizard-style" physiology for
dinosaurs.

[snip]

> And if it is,
> why do we think that after millions of years of evolutionary
> achievement, that they are still bastards of basic gravity?

Because it doesn't matter how cool an organism looks; we are all gravity's
bastards.

> Something about all of that doesn't sit well with me. And that is
> why I felt like maybe something in the planet's gravity changed.
> I know I know, it's corny.

Stop and think about what you are saying for a moment.  In order to keep our
favorite creatures "cool" in our minds, we are willing to change fundamental
aspects of the nature of the physical universe.  I *HOPE* you see why that
isn't a very scientific approach!

> Elephants today lumber, yes. So what is the point of evolution

[snip]

Nothing; or simple survival.

> to
> create something so much bigger than the elephant in the first
> place? What is the use of megafauna

There is no "use" for species.

> when it appears to be a
> hinderence to itself? Why did they get so big? It seems a waste
> of time.

The insects might say the same thing about all those stupid vertebrates...

> The planet wasn't so much bigger back then was it? So
> why did we need such a huge forager?

Got nothing to do with "need".

> So many questions, so little time...


                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796