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The Straight Dope



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 backward [etc]. --Andrew Murphy, Philadelphia

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 number one is that if you mention it to a stranger on the bus you're guaranteed to get the seat to yourself. Question number two is more interesting: Why were dinosaurs so big? Some theories:

Dinosaurs had to be big to reach the leaves on the tops of 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 tree leaves 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 both sides. Cold-blooded advocates say dinosaurs were big because their sheer thermal mass protected them against 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 20,000 pounds.

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, maybe . . . 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.


The debate over whether the millennium should be celebrated on January 1 of the year 2000 or 2001 prompts me to suggest a third (and fourth) alternative. Both sides of the debate overlook the fact that the millennium should be counted from the year Christ was born. Although our present AD/BC dating system is based on the assumption that Christ was born at the beginning of 1 AD, subsequent scholarship indicates the correct date was either 4 BC or 6 BC. If 4 BC is the correct date, then the millennium should be celebrated on January 1, 1997. If 6 BC, then the millennium begins on January 1, 1995, only a few months away. Do you think the best restaurants are already booked? --Gary Greenberg, New York

Why stop at four dates? Add the ten-day correction for the Julian calendar and we get eight dates. Allow for variations in the dating of Christmas and New Year's, orbital eccentricities, and fluctuations in the gravity field and we can celebrate every two weeks for the next 20 years.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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