Systematic classifications dealing with both extant and extinct taxa vary more widely. Some separate the feliforms (extant and extinct) as: Aeluroidea (superfamily) and Feliformia (suborder). Others include all feliforms (extant, extinct and "possible ancestors") into the Feliformia suborder. Some studies suggest this inclusion of "possible ancestors" into Feliformia (or even Carnivora) may be spurious. The extinct (†) families as reflected in the taxa chart are the least problematic in terms of their relationship with extant feliforms (with the most problematic being Nimravidae).