Early last week, baseball legend Jose Canseco explained gravity to the Internet. He tweeted that "ancient gravity was much weaker" during the time of the dinosaurs, which allowed for their massive and nimble size. Needless to say, Canseco was ridiculed for his comments, but very few people actually took the time to explain why he was wrong. Even Bill Nye failed in this regard. Thankfully, some writers have come to the rescue.
First, a review of Canseco's tweets:
Which prompted this response from Bill Nye:
His nickname in Major League Baseball was "The Chemist," because he was so knowledgeable in the chemistry of performance enhancing drugs and making musculature go big. Reading his recent tweets about the remarkable size of the ancient dinosaurs ... it doesn't sound (read) as though he's especially fluent in physics. This fills me with either joy or dismay depending on what social media messages he provides us with next. Either he's in on the joke and is just throwing us all a curve ball with plenty of break, or we as a society have failed him completely with regard to the fundamentals of planetary science.
The ad hominem attack on Canseco was unbecoming of a scientist, as was his laziness at addressing the issue head on.
But writing in National Geographic, dinosaur expert Brian Switek entered into the fray by explaining how dinosaur evolution — not ancient gravity — allowed super-sized sauropods to evolve:
As part of their respiratory system, sauropods had a complex network of air sacs that gave them two advantages. Not only did the air sacs allow the dinosaurs to breathe more efficiently – more like birds than mammals – but the soft tissues invaded bone to make the skeletons of these dinosaurs lighter without sacrificing strength. Indeed, even at around 100 feet long, Supersaurus has been estimated to weigh in between 35 and 40 tons. That's quite hefty in absolute terms, but consider that the largest African elephant on record weighed about 12 tons, and the extinction rhino Paraceratherium – about 26 feet long and 16 feet tall at the shoulder – weighed about 18 tons. You'd like a dinosaur about four times as long as Paraceratherium would be much heavier – 72 tons or more – but Supersaurus and similar dinosaurs were relatively light. Air sacs allowed sauropods to escape some of the physical constraints that have limited the evolution of mammal body size over the past 66 million years...
...By externalizing birth and development, sauropods and other dinosaurs were able to sidestep the costs and risks that constrain mammal size. For dinosaurs, mechanical and other biological constraints might have prevented them from becoming even larger – the amount of time it would take for nerve impulses to travel to a 100-foot-long dinosaur's brain for example. The fact that all the genera that are contenders for the "largest dinosaur of all time" title – including Argentinosaurus, Supersaurus, and Diplodocus – top out around 100 to 110 feet in length might indicate that these dinosaurs were reaching the anatomical ceiling of how large it was possible for them to get.
But let's be clear about sauropod size. Biological quirks such as air sacs and laying lots of little eggs allowed sauropods to grow to large size, but these features did not drive dinosaur inflation. There were titanic dinosaurs as well as tiny ones. Dinosaurs did not experience the same barriers as mammals, and therefore evolved a greater range of body sizes. The evolutionary driving forces behind the evolution of truly huge body size are not clear, and likely differed from one group of dinosaurs to the next. Paleontologists have determined the features that made it possible for a creature as spectacular as Supersaurus to exist, but the reason why the dinosaur's lineage ended up pushing biological boundaries of body size are still unknown.
And over at Galileo's Pendulum, Matthew Francis debunks the ancient gravity myth:
...while the 65 million years since the last dinosaur is a long time by human standards, Earth has been around 4.5 billion years. The cooling-down period ended long before the colonization of land by animals, which itself happened long before the first dinosaur. The second problem is that, to double Earth's gravity between Supersaurus and today, Earth would have had 1.4 times the diameter in the Jurassic. While that doesn't sound like much, it translates to twice the surface area and nearly three times the volume of modern Earth. That's a much bigger planet!...
...The truth is that Earth's tectonic plates, on which the continents rest, are always in motion, rearranging themselves very slowly over tens of millions of years. Yet the Moon reliably orbits, which wouldn't be true if plate tectonics made a huge difference to Earth's gravity. In fact, there's another sign Earth's gravity hasn't changed much in the last 100 million years: the Moon is actually moving away from Earth, albeit very slowly. If Earth's gravity had doubled since the time of the sauropods, we would expect the opposite effect.
Be sure to read both Switek's and Francis's articles, as they contain plenty more detail and links to relevant studies.
Oh, and in case you were wondering, here's what Canseco had to say about the Chelyabinsk meteor:
Now you know.
Top image: Killdevil.