The genus Atychodracon was erected by Smith (2015) to accommodate ‘Rhomaleosaurus’ megacephalus, because it is generically separarate from Rhomaleosaurus sensu stricto (Smith and Dyke, 2008). A. megacephalus is closely related to Eurycleidus and some authors have regarded A. megacephalus a distinct species of Eurycleidus.
The holotype specimen of A. megacephalus was housed in the Bristol City Museum and Art Gallery but was destroyed by enemy action during the World War II. However, surviving plaster casts of the holotype skull and forelimb provide enough data for a modern diagnosis of the taxon.
A neotype specimen of A. megacephalus was erected by Cruikshank (1994b). This fossil, nicknamed the ‘Barrow Kipper’ after the location it was discovered (Taylor and Martin, 1990), is on display at the New Walk Museum in Leicester, UK. This specimen has become a referred specimen (Smith, 2015). The skull of the Barrow Kipper was described in detail by Cruikshank (1994b) and is also on display at the New Walk Museum in Leicester, UK, in a case adjacent to the displayed skeleton. The skeleton has a replica skull cast from the original.
Cruickshank et al. (1991) proposed a specialised olfaction (or ‘underwater smelling’) system for plesiosaurs based on observations of the skull of A. megacephalus. The internal nares (or bony nostrils) on the palate are positioned anteriorly on the skull, and positioned anterior to the external nares on the dorsal skull roof. The internal nares are also associated with grooves that have been interpreted as an adaptation for channelling water into the nostrils. Under this hypothesis, the flow of water over the external nares was helped by the animal swimming, which maintained hydrodynamic pressure. The flow of water through the nasal ducts could have been ‘tasted’ by olfactory epithelia. If such a system existed it might have been a common adaptation among plesiosaurians, since all plesiosaurs have anteriorly positioned internal nares (Brown and Cruickshank, 1994).
Street-on-the-Fosse, a village about 14 km ENE of Street, Somerset, UK
BRSMG Cb 2335 (destroyed)
Smith (2015) provided a list of referred material: LEICT G221.1851 (the neotype specimen) from Barrow-upon-Soar, Leicestershire, UK (Cruickshank, 1994a, 1994b); NMING F10194, a partial skeleton including the skull (but no mandible) from Street (Smith, 2007; Benson et al., 2012); NMING F8749, a partial skeleton including a damaged skull and suffering from pyrite decay, also from Barrow-upon-Soar (Smith, 2007). A complete skeleton from Wilmcote, Warwickshire, UK, sometimes referred to ‘P.’ megacephalus (WARMS G10875, Wright, 1860; Cruickshank, 1994a) represents a new species (Smith, 2007).