Most rhomaleosaurids are known from England, many specifically from lower Blue Lias deposits that date back to the earliest Jurassic, just at the boundary with the Triassic.

In fact, to date only two undisputed rhomaleosaurids were named from outside Europe - the closely related Borealonectes russelli and Maresaurus coccai from Canada and Argentina, respectively.

[2] Rhomaleosauridae is a stem-based taxon defined in 2010 (and in earlier studies in a similar manner) as "all taxa more closely related to Meyerasaurus victor than to Leptocleidus superstes, Pliosaurus brachydeirus or Polycotylus latipinnis".

[2] Modern cladistic analyses are divided in the position of Rhomaleosauridae; some recover it at the base of Plesiosauria in a position basal to Neoplesiosauria that contains both the pliosauroids and the plesiosauroids,[3] while other analyses recover Rhomaleosauridae as pliosauroids, to the exclusion of Plesiosauroidea, either as the sister taxon of Pliosauridae or, rarely, as a paraphyletic array of taxa leading to it.

[4][2][3][1] The following two cladograms are simplified after two recent analyses, showing only the relationships within Rhomaleosauridae, and some other relevant basal taxa whose position within the family is highly uncertain.