Ichthyosaurus De la Beche & Conybeare, 1821

Massare, Judy A. & Lomax, Dean R., 2018, A taxonomic reassessment of Ichthyosaurus communis and I. intermedius and a revised diagnosis for the genus, Journal of Systematic Palaeontology 16 (3), pp. 263-277 : 272-273

Genus Ichthyosaurus De la Beche & Conybeare, 1821

1818 Ichthyosaurus Konig : 54 [nomen nudum].

1819a Proteosaurus Home : 209.

1821 Ichthyosaurus De la Beche & Conybeare : 563.

1840 Polyostinus Hawkins : 12.

1840 Paramecostinus Hawkins : 15.

1904 Eurypterygius Jaekel : 32.

1979 Protoichthyosaurus Appleby : 492 [in part].

Emended diagnosis. Pineal enclosed within frontals at their posterior edge; large, wide nasals; maxilla excluded from margin of the external naris by subnarial process of premaxilla and anterior process of lacrimal; squamosal present; basioccipital with extensive extracondylar area and well-developed peg; angular with minimal exposure in lateral view; scapula with long shaft, slightly expanded distal end and expanded proximal end, but acromion process absent; coracoid with prominent anterior and posterior notches; humerus with nearly equal width distally and proximally, with slight constriction in the shaft; forefin with distal carpals 3 and 4 contacting the intermedium; metacarpal 5 in the distal carpal row, in contact with the ulnare; digit V as prominent or more prominent than digit IV; digital bifurcation anterior to the primary axis in forefin; forefin with at least five primary digits (although the fin must be preserved to at least p4 row to verify that a bifurcation is present); hind fin with at least four primary digits and an anterior digital bifurcation; rectangular, closely spaced phalanges on most of proximal to middle portion of forefin; pelvic girdle tripartite, without fusion between pubis and ischium; femur longer than wide, with distal end wider than proximal end; preflexural vertebrae <80 (modified from McGowan & Motani 2003; Ji et al. 2016).

Occurrence. The genus is very common from Dorset and Somerset, England, but additional unequivocal occurrences of the genus include: Leicestershire ( Martin et al. 1986), Nottinghamshire ( Lomax & Gibson 2015), Warwickshire ( Smith & Radley 2007), Gloucestershire (e.g. BU 5289, formerly BIRUG 4175), Worcestershire ( WOSMG:1, WOSMG:2), Yorkshire ( Maisch 1997; Massare et al. 2015), Devon (NHMUK PV R5787), England; Glamorgan, Wales ( NMW 2009.35G.1); County Antrim, Northern Ireland (e.g. BELUM K1643, BELUM K28052); Belgium ( Godefroit 1996); Switzerland ( Maisch et al. 2008); Portugal ( Zbyszewski & Moitinho de Almeida 1952; Bardet et al. 2008); and Alberta, Canada ( McGowan 1978). The genus was also reported from Scotland, but contrary to Brusatte et al. (2015), these specimens (teeth and an impression of a lower jaw) are too fragmentary to be assigned to the genus. Similarly, a specimen from British Columbia, Canada ( Dennison et al. 1990) shows no diagnostic characters of the genus, and the relative rostrum length would suggest that it is not Ichthyosaurus .

Ichthyosaurus has previously been reported from the uppermost Triassic (Rhaetian) of Somerset ( McGowan & Motani 2003), but recent studies suggested that specimens from Somerset are probably from the lowermost Jurassic (Hettangian; Benson et al. 2012, 2015; Lomax & Massare 2017). However, DRL recently examined specimens of Ichthyosaurus currently held in a private collection that were collected in situ from the base of the Liassic Group (probably Rhaetian) of northern Somerset ( Carpenter 2001). Thus it is plausible that some specimens of Ichthyosaurus in historical collections are from the Rhaetian. The genus has definitively been reported from the lowermost Jurassic (Hettangian), Pre- planorbis beds of Somerset and Leicestershire ( McGowan 1974; Martin et al. 1986; also see discussion in Lomax & Massare 2017). The youngest occurrence of the genus is from the lower Pliensbachian ( Lomax 2010; Lomax & Massare 2015).