Nevertheless, even based on surviving descriptions alone, many differences were noted even from its closest known relative at the time, Eusaurosphargis dalsassoi from the Anisian-Ladinian boundary of the southern Alps.

Various hypotheses existed for the affinities of these species, and together with Helveticosaurus, they were originally thought to be placodonts, but later studies suggested relatedness to other sauropterygians and / or ichthyopterygians.

The second species L. qianensis, known from three specimens, and the newly prepared portions of L. polycarpon postcranial skeleton, revealed the same osteoderm "body armor" and broadened rib basket that are unique to saurosphargids.

[2] All four known specimens of Largocephalosaurus came from the Luoping Biota, from Member II of the Guanling Formation, that yielded Sinosaurosphargis, alongside various invertebrates, fish, basal ichthyosaurs, Atopodentatus, and several species of eosauropterygian, including both pachypleurosaurs and nothosaurids.

[2][4][5] The following cladogram is simplified after Li et al. (2014) phylogenetic analysis, showing interrelationships of all known Saurosphargidae species, and the placement of the clade within Sauria.