Life's scale reduction since the Dinosaurs

[swh@eatl.co.uk (Stephen Hurrell)]

I've received some interesting comments about my request
for help to quantify the scale reduction of life. Some 
of the responses I found quite surprising! I've lumped them
all together into one file. (I've just received a lot more 
so I'll have to look at them later).

DARREN NAISH said for example: 

> I disagree with this. There has not been any gradual, observable 
> trend in size reduction 

Darren then went on to sight the biggest of the land animals: mega-dinosaurs, 
indricotheres, recent mammoths and recent mammals. Arguing 
all the time that there has NOT been any reduction in the 
scale of life.

If I plot the weights of these animals in their time periods I get
something like (very roughly)

80 MYA      - Mega-dinosaurs     up to about 75 tonnes
40 MYA      - Indricotheres      about 30 tonnes
3 MYA       - recent mammoths    about 10 Tonnes
Present Day -                    Max. weight about 6 Tonnes

I would have thought the trend was obvious (although there is 
probably considerable scope for improvement in the accuracy of 
the dates and weights)

---------------------------------------------------------------

aplaza.taos.nm.us!darwincr:

> For most of life's time on Earth, some forms have been GROWING in size; 
> recall that life started out as microorganisms.  Only in the latter bit, 
> since the rise of macroorganisms, have we seen individually 
> easily-visible forms, and with the exception of really huge vertebrates 
> like sauropods and whales and elephants and whatnot, the general run has 
> remained more or less 

You point out life has been evolving from microorganisms 
into larger sizes. I would point out that we need to compare 
like-with-like. It would be pointless comparing the first 
Dragonflies with Elephants to prove life has increased in scale.
What we need to do is compare the first Dragonflies (with their
3/4 metre (2 foot) plus wingspan) with the size of present day
Dragonflies.

And of course you are quite right that the key question is:

> But why no mammals as large as large sauropods?

Answer that and you're solved the puzzle.

----------------------------------------------------------------

Bonnie: 

My answer's the same as above. We have to compare like-with-like 
to quantify any scale reduction. 

----------------------------------------------------------------

Thomas R. Holtz, Jr.:

> Hominids have been increasing in size for the last four million 
> years or so

What I want to calculate is an average scale for life at various
time periods. Some animals will be increasing in size while others
are reducing at any particular time. 

> The blue whale is still the largest known animal; the extant 
> whale shark is still the largest known fish 

I'm not interested in any life that is supported by water. The 
buoyancy of the water will affect the scale factor.

> Recent reconstructions have reduced the mass of Indricotherium; 
> it is known believed to be not much larger than some Pleistocene 
> mammoths.

The variation in estimated weights for animals is a problem 
which I need to overcome. I intent to do this by using the 
same calculations for all animals. To do this I will need to
use the original bone dimensions of a wide range of animals.
I need the Length and Diameter of the Femur, Tibia and Largest
Metatarsals and the time period when these animals were alive. 
Any references to these bone dimensions would be most helpful.

That's all for now.

Stephen