A 2018 study found Sandownidae to represent a sister group to Thalassochelydia, under the clade Angolachelonia. Similarly, a 2021 study found Protostegidae and Sandownidae to possibly be closely related to or even members of Thalassochelydia, although the exact affinities of the clade are still an open question. If Protostegidae is included, the clade might exist just outside Chelonoidea, which contains the extant sea turtles.
