Orientation selectivity is a prominent feature of neurons in the mammalian primary visual cortex, but there is a dichotomy in the spatial arrangement of these neurons across species. Primates, carnivores and marsupials have orientation-selective neurons organized into structured columns forming two-dimensional maps around pinwheel centers. In contrast, rodents display a seemingly random "salt-and-pepper" organization. The evolutionary versus the environmental basis of this distinction remains unclear. Using intrinsic imaging and massed single-cell recordings, we investigated the organization of orientation-selective neurons in a very small (~18g) marsupial, the fat-tailed dunnart (Sminthopsis Crassicaudata). We reveal quasi-repetitive columnar layouts with pinwheel densities close to π, similar to other non-rodent species. Dunnarts have the smallest reported column spacing (0.4-0.5mm) in any mammal. As two marsupials (wallaby and dunnart) with radically different body sizes, environments and behaviours have similar cortical structures, genetic lineage appears to link them more than other factors. Even though the dunnart has a body size and visual cortical area smaller than the mouse, it has pinwheel-like orientation maps. Perhaps rodents cannot express the genetic code required for pinwheel map formation?