The median artery was not caught in the act of “evolving,” either in the micro- or macro- sense. It has been caught in the act of “persisting.”
For an animal whose brain is the size of a pinhead, one would think its capabilities would be hopelessly limited. Not so.
Remember when fossil hunters found Marble Canyon, a fossil bed in Canada that surpassed the Burgess Shale in extent and species richness?