Hauffiosaurus is known from the holotype and only specimen uncatalogued Hauff Museum, an almost complete and articulated skeleton, found from the Posidonien-Schiefer, dating to early Toarcian stage of the Early Jurassic. The holotype specimen preserved in a single block of the original matrix, exposed in ventral view. The body outline visible around the specimen is an artifact of preparation, not preservation; no remains of soft tissue were preserved. The skeleton was discovered during the early 19th Century, in beds of the famous Posidonien-Schiefer lagerstätte at Holzmaden, Baden-Württemberg, in southeastern Germany. However, it was not recognized as a valid taxon, and no thorough description of the fossil was made until 2001. Hauffiosaurus zanoni is a plesiosaur of medium size, measuring. The skull measures about along the midline. The holotype is an adult individual, but incomplete fusion of the and pelvic girdle indicate it is not an old adult. The specimen is displayed at the Urwelt-Museum Hauff, Holzmaden. The exact phylogenetic position of Hauffiosaurus within the Plesiosauria has yet to be resolved, though Vincent states that it may "reasonably be placed within the Pliosauroidea". A second species, H. tomistomimus, was named by Roger B. J. Benson, Hilary F. Ketchum, Leslie F. Noè and Marcela Gómez-Pérez in 2011. It is known from the holotype and only specimen, MMUM LL 8004, an almost complete, three-dimensionally preservedand and articulated skeleton, found from the Hildoceras bifrons Zone of the Alum Shale Member, Whitby Mudstone Formation, dating to early Toarcian stage. This skeleton was discovered in Yorkshire, UK. The holotype H. tomistomimus is about long. Benson et al. 2011 reassigned Macroplata longirostris to Hauffiosaurus. The holotype and only specimen of H. longirostris, MCZ 1033, found from the Harpoceras serpentinum Zone of the Jet Rock Member, Whitby Mudstone Formation, early Toarcian of Yorkshire, England. Few Early Jurassic plesiosaurians have the sort of long, thin snout present in Hauffiosaurus, a shape usually considered to indicate a diet of fish. The teeth are slender and elongated and possess fine longitudinal ridges. This sort of tooth, coupled with the long rostrum seen in this genus is effective in piercing soft prey.
Phylogeny
Smith & Dyke, 2008 were the first who found Hauffiosaurus to be basal pliosauroid. Benson, Ketchum, Noè and Gómez-Pérez who assigned two additional species to this genus, confirmed the affinity of this taxon to Pliosauroidea using cladistic analysis which was based on Ketchum & Benson's analysis. Cladogram after Benson et al., 2011: With the description of Marmornectes in 2011, Ketchum & Benson suggested for the first time that many basal plesiosaurs and pliosauroids are members of Pliosauridae and Rhomaleosauridae. Both Rhomaleosauridae and Pliosauridae were found to be monophyletic, and the relations between Hauffiosaurus's species remained the same. The cladogram below follows Ketchum & Benson, 2011.
