Cecil, one good reason why Mammals couldn’t grow to compete with the big Dinosaurs is that being warm blooded means you have a higher metabolic rate and thus need an awful lot more food. Certain mammals (IIRC one was a giant sloth called Megatheridon) did get pretty impressive but nothing like as big as diplodocus (before anyone says, yes I know baleen whales are bigger but they have the best of both worlds, being able to “browse” rather than hunt for their food and still get an energy dense animal based diet).
I read recently that a lot of dinosaurs weren’t big at all - in the Jurassic era they occupied the rat-type niches that they were outcompeted in by mammals (one advantage of being warm blooded is being able to run fast first thing in the morning) toward the end of the Triassic. I admit to cheating by looking up the last bit here, partially because I had a feeling my memory came from My Big Book of Dinosaurs a long time ago…
Dinosaurs were uninsulated? Didn’t many of them have feathers?
Cosmic debris piles up on the Earth every day (100,000 lbs/day, according to the Discovery Channel). Over hundreds of millions of years, wouldn’t the added mass raise its gravity?
The added mass from micrometeorites and the like falling on the Earth is totally trivial compared to the total mass of the planet.
Earth has a mass of about 5.97 x 10[sup]24[/sup] kg; this recent paper estimates about between 4.2 - 7.4 x 10[sup]6[/sup] kg/yr of flux. Assuming that constant over the last half billion years (the dinosaurs arose, lived, and died out during less than that time), we get about 10[sup]16[/sup] kg of total accretion during that time, assuming zero loss. That’s about one hundred millionth of the mass of the Earth. Stated otherwise, it’s about a millionth of a percent.
Newton’s law of gravity gives the force of gravitation as proportional to the mass of the attracting objects (one of which is the Earth in this case); increasing the mass of the earth by one part in a hundred million will increase the gravitational force by that same, immeasurably small, proportion.
Even if we extend that out to the total age of the Earth, it’s still less than a ten-millionth of the Earth’s mass.
But dinosaurs were warm-blooded. Or at least homeothermic, which operates similarly.
From what I’ve read, scientists currently seem to think the primary reason mammals couldn’t compete with dinosaurs for most (almost all, really) ecological niches is that the dinos already filled those niches in large numbers.