Re: Life's scale reduction since the Dinosaurs

[JCMcL <darwincr@laplaza.taos.nm.us>]

On Wed, 24 Jan 1996, Stephen Hurrell wrote:

> 
> Considering the recent discussions about how BIG is BIG 
> in relationship to the Dinosaurs, my request for help to 
> quantify the scale reduction in life over time my well 
> interest a few of you. 
> 
> We all know that the Dinosaurs were big. What is not so generally 
> realized is that life has been reducing in size over periods of 
> hundreds of millions of years. 
> 
> There can be no doubt that this size reduction has taken place. 
> After the extinction of the Dinosaurs a range of Supergiant Mammals 
> approaching the size of the smaller Dinosaurs evolved. These Supergiant 
> Mammals were superseded by Giant Mammals which were larger 
> versions of the Mammals alive today. These Giant Mammals have in 
> their turn died out to be replaced by a smaller scale of life.
> 
> I am currently trying to quantify this scale reduction in life and I 
> would welcome help with this task. If you are interested, contact me 
> and I will send some additional information.


Okay, with all the strength of opinion out there in this list, someone 
ought to have some pretty good ideas on why [some] dinosaurs got so big.  
However, Mr. Hurrell goes on to say:


> (Some of you may well be aware that this scale reduction has occurred
> throughout life's time on Earth. I am of course interested in scale 
> reduction over time for Dinosaurs, Insects, Plants, etc.).

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 the same in size on a trans-specific scale.  

As witnesses to the sorry end of the Cenozoic, we can certainly see 
what's happening to large mammals like elephants and whales; the global 
biomass is being diverted to human purpose as fast as humans can do it.  

But why no mammals as large as large sauropods?

How long after the first broad adaptive radiation of archosaurs did really 
large sauropods (bigger than elephants) arise?

Presuming that the first really broad adaptive radiation of mammals was 
in the Paleocene, how long (uninterrupted by human activity) would the 
Cenozoic have been, in comparison, before really behemoth mammals 
arose?   We can never know, of course.  But it's fun to speculate.