Why were dinosaurs so big?

Dear Cecil: When I look at dinosaur skeletons in museums the thing that always impresses me is their incredible size. It has occurred to me that perhaps these animals could grow to such enormous size because effective gravity was lighter then. If earth spun faster at the time of the dinosaurs then centrifugal force might have counteracted gravity enough to make a substantial difference in the weight of massive animals. What are the scientific merits of this idea? I read somewhere that the Earth’s day is getting shorter by a fraction of a second every year. Extrapolating backwards, how much faster [etc.] Andrew Murphy, Philadelphia

Cecil replies:

We have two questions here, Andrew — the one you asked, and the one you would have asked if the drugs hadn’t kicked in first. The best that can be said for question #1 is that if you mention it to a stranger on the bus you’re guaranteed to get the seat to yourself. Question #2 is more interesting: why were dinosaurs so big? Some theories:

Dinosaurs had to be big to reach the leaves on the tops of the trees. This is the answer favored by fourth graders. It probably was a factor in the size of some dinosaurs, notably brachiosaurus, a plant-eater described as basically a crane on legs. But dinosaurs were huge whether they ate plants or not. One estimate puts their median weight at two tons.

Dinosaurs had to be big because they were (a) cold-blooded or (b) warm-blooded. Paleontologists have been debating whether dinosaurs were warm-blooded or cold-blooded for 25 years, and the great size of the animals has been enlisted as an argument on either side. Cold-blooded advocates say dinosaurs were big because their sheer thermal mass protected them against the sudden temperature swings to which small cold-blooded creatures were vulnerable. Objection: “bulk homeothermy,” as it’s called, only works in animals weighing more than a ton; some dinosaurs were smaller.

Warm-blooded advocates say dinosaurs had to be big because their uninsulated skin made them easily chilled. Great size gave them a lower ratio of skin surface to volume and thus reduced heat loss. Objection: come on, they couldn’t wear sweaters? There have to be easier ways to stop heat loss than putting on twenty thousand pounds. Equally to the point, how did they survive long enough to get big?

Dinosaurs had to be big because the early mammals had taken all the small slots, ecologically speaking. It’s thought mammals were more agile than the relatively stiff-limbed dinosaurs and thus better equipped to survive. Objection: OK, but why couldn’t mammals have become huge too and beaten out dinosaurs at both ends of the size continuum?

The question may never be resolved because so much about dinosaurs is unknown and, given the limitations of the fossil record, may be … well, I don’t know that it’s completely unknowable. But Cecil can appreciate that it’s a lot easier holding forth on your latest grand theory in the faculty room than excavating in some former swamp with a spoon.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.