“The phragmocone-bearing coleoid cephalopods Sepia, Sepiella, Metasepia and Hemisepius (sepiids) are the most diverse of all extant chambered cephalopods and show the highest disparity. As such, they have a great potential to serve as model organisms to better understand the paleobiology not only of extinct coleoids, but of extinct nautiloids and ammonoids as well. Here, we present new measurements from the phragmocones of Sepia and Sepiella and relate these to known maximum depths. While these latter data remain few for some species, in aggregate they provide progress allowing this work. In so doing, we expand upon the great legacy of Sigurd von Boletzky. We show that deep water-inhabiting sepiids have phragmocones with a higher number of septa per length, a smaller area covered by the last chamber in relation to the volume of all previous phragmocone chambers, are smaller in size (length and volume), and bear dorsal shields that have strengthening central rib(s) compared to most species from shallower water; the very small endemic species of South Africa may be an exception. We show that the various subgenera defined by Khromov (1987) are not monophyletic clades but morphological groupings explicitly related to depth, partially evolved convergently. We conclude with analyses of Australian sepiid assemblages and show that these are depth related in ways analogous to ammonite and/or nautiloid assemblages of the past that are later commingled through post-mortal drift.” Open Access!