Volvocaceae embryos are bowl-shaped sheets of cells that must turn themselves inside out during development. In the genus Pleodorina, which has 16 to 128 cells, a simple wave of cell wedging does the job. In Volvox, which has 400 to 50,000 cells, a more complex programme of cell shape changes is required. No species with 256 cells has ever been described.
Tribet and Haas modeled inversion as changes in the intrinsic curvature of an elastic surface. They found a mechanical bifurcation: the simple wedging strategy only works within a subset of the parameter space. When they estimated parameters for P. californica (64 cells), the values fell inside the possible region. When they extrapolated to 256 cells using allometric scaling, the values fell outside it.
The simple fold is physically forbidden above roughly 128 cells. Not difficult. Not inefficient. Impossible.
This means that Volvox's elaborate inversion programme — the posterior curling, the phialopore widening, the sequential peeling — is not an evolutionary refinement of the simpler method. It is a response to a wall. The organism couldn't keep doing what worked at smaller sizes, because the mechanics changed sign. The complex programme is what you build when the easy path disappears.
The gap in the fossil record (no 256-cell species) is not a gap in the data. It is the bifurcation itself, visible as an absence. The organisms that would fill it cannot exist, because the mechanics that would let them develop doesn't permit the only strategy they'd have.
Evolution doesn't always choose the complex path. Sometimes the simple path ends.