Thoughts on FMNH PR 2081 mass

Hey, everyone. These are my thoughts on the mass of FMNH PR 2081, also known as “Sue,” the most complete Tyrannosaurus rex skeleton found.

We all know that T.rex is big. It’s enormous. But for more than a hundred years, paleontologist based its mass off of scale models and water displacement, not taking into account where actual muscles were situated and how large or dense they were. In 2009, John Hutchinson and his team used a technology known as LiDAR (Light Detection and Ranging) to examine the skeletons of BHI 3033 and MOR 555 in more detail than had ever been done before, and they found out some pretty interesting things.

The go-to estimate, for a long time, for T.rex’s upper mass limits was six short tons. Twelve thousand pounds: pretty impressive. But with Giganotosaurus, Carcharodontosaurus and Spinosaurus all exceeding that size with even the most conservative limits (except for Gregory S. Paul, who in 1989 put Spinosaurus at 4 tons. He’s changed his mind since then), it doesn’t seem that big.

BHI 3033, one of the specimens used in the study, is the largest gracile specimen of Tyrannosaurus that has so far been discovered, at 11.3 meters long. “Stan” is significantly more lightweight than Sue–it is a gracile morph, after all–and Hutchinson’s measurements still greatly exceeded the primitive six ton estimate.

I like working in short tons, since I’m an American and I’m weird. Six short tons is equal to 5,454 kilograms, and Stan is–look at that number again, please. A gracile morph of Tyrannosaurus coming in at 2,200 kilograms more than the old maximum? What do you mean, water displacement with plastic models misrepresented theropod mass?

So what does this mean for Sue, who is obviously quite a bit larger?

Well, let’s find out.

Hutchinson’s team did another study, again using LiDAR, in 2011, examining ontogeny in T.rex, and boy, was it earth-shattering. You may have seen the headlines–something along the lines of “T.rex grew bigger and faster than previously thought”–but even the interpretations by the press fell short of how big the specimens were actually measured to be.

First off, the minimum mass of Sue is just stunning. It rounds off to 10.45 short tons, which is bigger than all but the most radical estimates for Giganotosaurus or Carcharodontosaurus, and the other specimens aren’t shrimps either. Out of the four adults, working only off of the minimum measurements, it creates an average mass of 7,151 kilograms (7.8 short tons). If you look at the midranges, it’s even more shocking, with an average of 10.9 tons. That’s bigger than a large Spinosaurus. As far as the maximal estimates go…well. Somehow, I don’t think it likely that any tyrannosaur reached the mass of a small sauropod, but that’s what averages are for, right?

Thanks for reading!


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