Differential Growth
A closed loop that gains length faster than its enclosure allows, so it has to fold. The same rule shapes lettuce edges, coral rims and the cortex.
About this pattern
Differential line growth · morphogenesis models
Every node is pulled toward the midpoint of its two neighbours and pushed away from any node within reach. Each pass also inserts new nodes at random points along the curve, whether or not the edges there need them. That is the growth. Perimeter climbs while the area it can occupy does not, and the only resolution is to crenellate.
Because the finished curve passes through everywhere it can reach, its earlier stages cannot survive as separate rings inside it: growth bands draw those stages anyway, and they come out as contours pressed against the final line, the way a coral records its own history.