Artistry by bark-tunnelling insects.
Look at the designs they have made!
I peeled back the bark from one section. The wood was still damp, and the contents of the tunnels stayed with the bark. See how the raised design on the bark matches the engraving on the wood beneath.
I don't know what type of creature does this. The branch was green wood, still flexible, not driftwood, so it would have been carved on the tree; these are not marine organisms.
And it was too long for the car; maybe we should be carrying a saw on our outings, too.