Rhomaleosaurus is a big basal plesiosaur.
Some specimens reached 7m in length.
In the large reptile tree (LRT, 1413 taxa) Rhomaleosaurus nests basal to plesiosaurs + pliosaurs, all derived from Late Triassic Yunguisaurus and kin. Smith 2007, Druckenmiller 2006 and O’Keefe 2001 nested Rhomaleosaurus with pliosaurs. The skull is larger than in traditional plesiosaurs and yunguisaurs, but not in more primitive and Early Jurassic Hauffiosaurus, which nested with Anningasaura a plesiosaur-mimic, evolving hyerphalangeal flippers in convergence with traditional plesiosaurs.
Rhomaleosaurus cramptoni (originally Plesiosaurus Carte and Bailey 1863, Seeley 1974; Early Jurassic, Toarcian, 180 mya; 7m long; NMING F8785) nests basal to Plesiosaurus, the elasmosaurids and pliosaurids in the LRT.
Smsith 2007 reported, “The Rhomaleosauridae ranges throughout the Lower Jurassic and extends into the Lower part of the Middle Jurassic. The latter part of the Middle Jurassic sees the emergence of the first pliosaurids and it is possible that the rhomaleosaurids were out competed by these very short-necked predators during the Middle Jurassic.”
Carte A, Bailey WH 1863. Description of a new species of Plesiosaurus, from the Lias, near Whitby, Yorkshire. J R Dublin Soc 4:160–170.
Conybeare W 1824. On the Discovery of an almost perfect Skeleton of the Plesiosaurus. Geological Society of London. Retrieved 2010-1-15.
Cruickshank ARI 1994. Cranial anatomy of the Lower Jurassic pliosaur Rhomaleosaurus megacephalus (Stutchbury) (Reptilia: Plesiosauria). Philosophical Transactions of the Royal Society Lond Ser B 343:247–260.
Seeley HG 1874. Note on some of the generic modifications of the plesiosaurian pectoral arch. Quarterly Journal of the Geological Society Londong 30:436–449.
Smith AS 2007. Anatomy and systematics of the Rhomaleosauridae (Sauropterygia, Plesiosauria). Ph.D. thesis, University CollegeDublin.
Smith AS and Dyk GJ 2008. The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissenschaften. 95: 975–980. doi:10.1007/s00114-008-0402-