Males of Cosacosa gen. nov. can be distinguished from those of all other Australian lycosine genera for which modern diagnosis exist as follows: From Costacosa Framenau & Leung, 2013, by the lack of serrations apically on the tegular apophysis, from Dingosa Roewer, 1955, by the profile of the carapace, which lacks the elevated cephalic area, from Hoggicosa Roewer, 1960 and Knoelle Framenau, 2006 by the absence of many macrosetae apically on the cymbium, from Mainosa Framenau, 2006 by the obvious differences in burrowing behaviour and colouration, from Portacosa Framenau, 2017 by the lack of a sharp pointed ventral spur on the tegular apophysis, from Tapetosa Framenau, Main, Harvey & Waldock, 2009 by the habitus morphology, which isn't dorso-ventrally flattened, from Tasmanicosa Roewer, 1959 by the lack of a light radial pattern on the carapace, from Tuberculosa Framenau & Yoo, 2006, by the absence of tubercle-like setae on the ventral coxae, from Venator Hogg, 1900 by the lack of a retrolateral incision in the tegular apophysis and by the lack of a black shield on the ventral abdomen, and from Venatrix Roewer, 1960, by the lack of a sclerotised claw on the apical cymbium and the lack of a tubercle on the outer edge of the fangs.

Females differ from those of all other Australian lycosines except for Tasmanicosa hughjackmani Framenau & Baehr, 2016, and species of Hoggicosa by the shape of the epigyne. Both males and females can be easily distinguished from those species by the diminutive size, the lateral carapace profile which is flat and not sloping and the complete lack of a dark ventral pattern.