Roger B J Benson

Leslie Noè, Hilary F Ketchum, Marcela Gómez-Pérez

Academic paper

Palaeontology

54

3

547-571

An almost complete, three-dimensionally preserved plesiosaurian from the Hildoceras bifrons Zone of the Alum Shale Member (Whitby Limestone Formation; Lower Toarcian) of Yorkshire, UK, is described in detail. This represents a new species of Hauffiosaurus, H. tomistomimus, distinguished from H. zanoni (Harpoceras serpentinum Zone, Lower Toarcian, Germany) by the proportionally shorter neck and strongly concave preaxial margin of the tibia. It differs from H. longirostris (previously �Macroplata� longirostris; Har. serpentinum Zone, Yorkshire) by the absence of prominent midline ridges on the dorsal surface of the premaxillae and ventral surface of the mandibular symphysis, and the absence of midline pterygoid contact ventral to the basioccipital. Several synapomorphies support a monophyletic Hauffiosaurus: broad longitudinal troughs occupy the dorsolateral surface of the maxilla and the posterior half of the lateral surface of the dentary; basicranial fontanelle bounded laterally by posterolaterally elongate projections of an undetermined ossification; and the neural arch contacts the rib facet in all postaxial cervical vertebrae. However, the systematic position of Hauffiosaurus, as a pliosauroid or basal plesiosauroid, remains uncertain. There is little evidence for geographic differentiation of Lower Toarcian plesiosaurian faunas in the United Kingdom and Germany as minor differences between abundant taxa may arise from temporal offset of fossils from these regions, and marked taxonomic differences are confined to rare taxa whose absence in one or other area may be attributable to incomplete sampling. Lack of consensus on the relationships of Lower Jurassic plesiosaurians

Plesiosauria, Hauffiosaurus tomistomimus, Toarcian, Whitby Mudstone Formation, Alum Shale Member, Posidonia Shale.

English