Several commenters checked out the 3-D video of the world’s oldest fossil brain I posted yesterday and were struck by just how tiny the 300-million-year-old fish’s brain was in comparison to its braincase. Their verdict: shrinkage. In the paleontological sense of the word, not the Seinfeldian one. After death, brains that do not simply disappear sometimes get smaller. In this particular fish, Sibyrhynchus denisoni the brain must have gotten a lot smaller. Check out this image, in which the braincase is in red, and the brain is in yellow. (The scale bar is 5 millimeters.)

To move along the discussion of shrinkage, I thought it might be helpful to post the following passage from the paper