Fostering curiosity in kids (and their parents) since 2011

“I thought T. Rexes only had three toes. What’s that back thing for?”

A father holds his daughter as they look at the T. Rex skeleton in the Field Museum.

The Five-Year-Old Meets Sue. (Photo: Shala Howell)

Last year, The Five-Year-Old had a chance to cross a major item off her bucket list. She met Sue. For those of you who do not have your own in-house five-year-old dinosaur specialist, Sue is the largest, most complete, and best preserved T. Rex skeleton ever found. She’s named after Sue Hendrickson, the dinosaur hunter who found her on August 12, 1990 in the hills near Faith, South Dakota.

Sue enjoys an appropriate amount of notoriety here at Caterpickles Central, so as you can imagine, The Five-Year-Old could not rest until we had gone to Chicago’s Field Museum to see Sue in the not-flesh. Here she is, preparing to introduce Sue to Daddyo, just as soon as those pesky tourists are done pretending that Sue is going to eat them.

In the meantime, Daddyo has a question.

As a dedicated student of Dinosaur Train, Daddyo knows that T. Rexes are therapods. While marveling at the fact that Sue’s famously three-toed feet were nearly as large as The Five-Year-Old, Daddyo noticed something peculiar: “Hey, I thought T. Rexes only had three toes. What’s that back thing for?”

The mysterious fourth toe. (Image: Shala Howell)

The answer, it turns out, is nothing. Or perhaps more accurately, almost nothing. In my Googling, I’ve found that the word most commonly used to describe that fourth claw is “vestigial” — the term anatomists use to describe parts of the body that are no longer needed to perform their main function and so have shrunk.

T. Rex leg bones. (Image: Philcha, via Wikipedia)

Sue’s dewclaw, for example, most likely didn’t touch the ground when Sue walked so probably wasn’t used to maintain balance while walking like her other toes would have been.

Sue’s most famous vestigial limbs are her arms. They’re remarkably small for a dinosaur of her size. Of course, vestigial doesn’t mean entirely useless. Sue’s arms, so tiny against her 40.5-foot long, 7-ton body, are still pretty darn impressive compared to mine.

As we’ve discussed elsewhere on Caterpickles, wrap those bones in muscles and tendons and you’ve got a vestigial limb that scientists think could have hoisted as much as 400 pounds. Given that Sue’s arms weren’t long enough to reach her mouth, it’s not entirely clear what she would have done with that 400-pound hauling capacity. But I think that we can safely reserve that question for some other day. Or maybe I’ll just outsource it completely.

Looking for more Caterpickles Investigative Reports on Dinosaurs? Try:

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