Claim: The family is characterised by several traits, most species possess a groove or furrow ("sulcus") on the dorsal surface between the keels (paranota) on each segment, while in males, the opening on the underside of the body where the gonopods (male reproductive appendages) attach has a central constriction, forming the shape of an hourglass or dumb-bell.
Evidence: Paradoxosomatidae, the only family in the suborder Paradoxosomatidea (also known as Strongylosomatidea), is a family of flat-backed millipedes in the order Polydesmida. Most species possess a groove or furrow ("sulcus") on the dorsal surface between the keels (paranota) on each segment, and the keels of the second body segment are situated lower on the body than those of the first segment (collum) and third segment. In males, the opening on the underside of the body where the gonopods (male reproductive appendages) attach has a central constriction, forming the shape of an hourglass or dumb-bell.