Rhomaleosaurids Temporal range:201–161 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Rhomaleosaurus cramptoni (NHMUK PV R.34), Natural History Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	†Sauropterygia
Order:	†Plesiosauria
Family:	†Rhomaleosauridae Kuhn,1961
Genera
†Anningasaura? †Archaeonectrus †Atychodracon †Avalonnectes †Bishanopliosaurus? †Borealonectes †Eurycleidus †Lindwurmia? †Macroplata †Maresaurus †Meyerasaurus †Rhomaleosaurus †Sthenarosaurus †Stratesaurus? †Thaumatodracon †Trematospondylus? †Yuzhoupliosaurus?

Rhomaleosauridae is afamily ofplesiosaurs from theEarliest Jurassic to the latestMiddle Jurassic (Hettangian toCallovian stages) ofEurope,North America,South America and possiblyAsia. Most rhomaleosaurids are known fromEngland, many specifically from lowerBlue Lias deposits that date back to the earliestJurassic, just at the boundary with theTriassic. In fact, to date only two undisputed rhomaleosaurids were named from outsideEurope - the closely relatedBorealonectes russelli andMaresaurus coccai fromCanada andArgentina, respectively. These two species are also the onlyMiddle Jurassic representatives of the family.^[1] Rhomaleosauridae was formally named byKuhn in 1961, originally proposed to includeRhomaleosaurus cramptoni and its relatives, which have short necks and large heads relatively toplesiosauroids likeElasmosaurus andPlesiosaurus, but longer necks and smaller heads relatively to advancedpliosaurids likePliosaurus andKronosaurus.^[2]

Rhomaleosauridae is astem-based taxon defined in 2010 (and in earlier studies in a similar manner) as "all taxa more closely related toMeyerasaurus victor than toLeptocleidus superstes,Pliosaurus brachydeirus orPolycotylus latipinnis".^[2] Moderncladistic analyses are divided in the position of Rhomaleosauridae; some recover it at the base ofPlesiosauria in a positionbasal toNeoplesiosauria that contains both thepliosauroids and theplesiosauroids,^[3] while other analyses recover Rhomaleosauridae aspliosauroids, to the exclusion ofPlesiosauroidea, either as thesister taxon ofPliosauridae or, rarely, as aparaphyletic array of taxa leading to it.^[4][2] Additionally, many putative rhomaleosaurids from the early deposits ofBlue Lias, vary greatly in their position across variousphylogenetic analyses. Some of these are recovered asbasal rhomaleosaurids in certain analyses, or alternatively outside Rhomaleosauridae in more basal positions within Plesiosauria.^[4][2][3][1] The following twocladograms are simplified after two recent analyses, showing only the relationships within Rhomaleosauridae, and some other relevantbasal taxa whose position within the family is highly uncertain.

Following Bensonet al. (2012):^[3]

Following Benson & Druckenmiller (2014), withMacroplata andEurycleidus excluded, andBorealonectes added:^[1]

• Hettangian first appearances
• Rhomaleosaurids
• Sauropterygian families

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