Why Do Things Keep Evolving Into Crabs?

From PBS Eons.

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For some reason, animals keep evolving into things that look like crabs, independently, over and over again. What is it about the crab’s form that makes it so evolutionarily successful that non-crabs are apparently jealous of it? 

Thanks to Franz Anthony for all of the ancient crab (and "crab") illustrations featured throughout this episode! http://franzanth.com/ And special thanks to Dr. Jo Wolfe, Dr. Javier Luque, and Dr. Katie Collins for assisting Franz with research for those illustrations.

And thank you to Dr. Rodney Feldmann for providing a photo of the Paralomis debodeorum fossil!

Produced in collaboration with PBS Digital Studios: http://youtube.com/pbsdigitalstudios

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References: https://docs.google.com/document/d/1q9McRu8tVceDumDFTWyrkVr3D15b3-DHp2ZYyd2sCuc/edit?usp=sharing