March 18th, 2010 - Where Are All The Dinosaurs At?

 Well, I went from waking myself up from two nights ago gasping and stifling a shreik dreaming that I had just ridden my bike over the edge of a cliff, to last night's Paleo-philosophical meanderings at four in the morning.
Not sure what set off the bike incident, and it realy wasn't fair because I woke up just as I disappeared off of the cliff into thin air with the land falling away below, and I never got the chance to even try and land it. I mean, I might have made it, I pulled off some servere things when I was mountain bike racing that I never thought I would survive, but I have also failed epicly.
It was just kind of a surprise, like I was flying down a mountain road and going around a bend with a reverse camber when the bike quickly crept further and further towards the apogee of the turn until... whoop, off into space with that feeling of Wile E. Coyote pedaling in place until he realizes it, looks down, gulps, and drops like a rock.
However, the next night before bed my mom and I had watched a program about the evolution of man, with the myths and stories that accompanied mankinds' proposed evolutionary changes. That may have had something to do with me waking up thinking about the extinction of the dinosaurs at four in the morning. Dunno.

My early morning question went something like this: When the dinosaurs inhabited the earth, why was everything so big...? Why is it now all so small in comparison...?

Someone the other day was saying they heard a theory that the dinosaurs went extinct because they choked on their food and their arms were too short to give each other the Heimlich manuever.

My thought was more along the lines of, what if something changed in earth's environment so that it put the largest beings at a disadvantage... say the earth's gravity increased, or the earth's atmospheric pressure increased (now it's around 14.7 psi, unless you're at the top of Mount Everest or the bottom of the Mariana Trench)... a drastic environmental change such as this would affect the large animals drastically while being less of a burden for the small-massed animals.
For the dinosaurs this would be like putting on a few thousand pounds overnight or waddling around in a wet wool coat.
Has anyone looked (or is there a way to even find out) these sorts of things by looking at geologic record, or the fossil record, or any other kind of record...?

Maybe the dinosaurs were just 'squeezed' out of existence. Like the increased pressure just slowed them down to a crawl and they couldn't hunt, or eat, or wallow their way out of the LaBrea Tar Pits.

I don't know much about these questions, but one interesting thing I have learned that is that species such as the beaver exsisted as both 'giant' versions: say about eight feet long, 480 pounds, and with six inch long buck teeth; there is a skeleton in the St. Paul Science Museum, which was found near St. Paul, MN and also one in Chicago's Bell Museum,  and also alongside their equivalent species which were about the size of the animals that enhabit the planet today.
The smaller animals in the species eventually won out. Why? It was put to me that the smaller versions were more manueverable, more adaptable, and could exist without need for a larger habitat, larger trees, etc.

It seems strange to me that both sizes of the same species would overlap in part of the same period of the fossil record though.
In my hypothetical scenario of increased physical pressure, a smaller animal would have a distinct advantage. It would not be as affected by gravity, or the atmospheric downpressure.
A large animal would be sluggish and suffer the crushing forces more and more over time...

Along these lines, have scientists ever tried to figure out if the earths gravity HAS changed over the duration of it's exsistance? Even beyond postulating about animal adaptations, this would be an interesting thing to check out in it's own right. If a goodly-sized comet hit the earth 65 million years ago, did the resulting divet reduce the mass of the earth enough to change it's gravity enough to measure?

Did the resultant dust and ash cloud change the earth's atmospheric pressure enough to be measurable?
Could it have changed it for the rest of it's existance from then on...?
Just a few questions that keep me up at four in the morning that I'm throwing out there, as I don't know the answers.
I wouldn't be surprised if they are embarassingly preposterous. Well, embarrasingly preposterous for anyone else to ask, but for me, merely preposterous.
I still think they are worth asking. Even if they are preposterous, I won't be emabarrased, I ask stupid stuff all the time.
So, all of you Paleo-philosphophical anthropological meterological geologists, help me out here so I can sleep, please.

I'm crushing your head. Crush, crush.