Today, formulated keyhole-shaped tumuli are considered to have been first built in the beginning of the second half of the third century, a little earlier than this the Tsubaiotsukayama-kofun Tumulus was built.
しかし、近年では、この椿井大塚山古墳より若干遡り、3世紀中葉過ぎに定型化した前方後円墳が現れたと考えられている。