Cassiduloid echinoids are sand-dollar relatives with an expansive fossil record and handful of living species. Cassiduloids have conserved body plans that have never been quantified. We digitized 2D geometric landmarks along the body margin and petals to quantify the shape diversity. Our analysis of disparity through time indicates a loss of hyper-round lineages following the K-Pg extinction. Margin roundness is hypothesized to be related to burrowing ability, with elongated bodies being more adapted to burrowing.