 However, a 2018 analysis retains Thalassocninae within Nothrotheriidae.Given that the two families in question may be sisters, and that the position of Thalassocninae within either would likely be fairly basal, correct family placement may be difficult.The five species seem to form one direct lineage (chronospecies), however it is possible "T. antiquus" is not the ancestor of "T. natans".