We reach the conclusion that posterior prevalence, a collinear property considered important for Hox complex function, is so far unique, in a global form, to vertebrates. Why is this? We suspect this is because posterior prevalence is explicitly connected to the vertebrate form of Hox temporal collinearity, which is central to axial patterning. Developmental Dynamics 241:1799–1807, 2012. © 2012 Wiley Periodicals,Inc.