User blog comment:Lord Loss/Monster Appreciation Day: Nargacuga/@comment-4467532-20130207214749/@comment-4009941-20130207223138

No official word on that but official word does say that Nargacuga abd Barioth are related in a Superfamily called "Pre-Wing Leg Wyvern Species"(Direct translation) then a splitwere Nargas are in the family  called "Naruga Species"(No official english name) and Barioths in the family "Berio Species"(Direct translation again) Tigrex is far off in his own family called "Rekkusu"(Rex) so if you wanna talk about if tigrex is close related the answer is no he is not.