Ichthyosaurus communis 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 : 273-274

Ichthyosaurus communis De la Beche & Conybeare, 1821

1821 Ichthyosaurus communis De la Beche & Conybeare : 594.

1822 Ichthyosaurus intermedius Conybeare : 108.

non 1834 Ichthyosaurus chiropolyostinus Hawkins : 25; pls 7–12 [nomen dubium].

non 1834 Ichthyosaurus chiroparamekostinus Hawkins : 32; pls 18–22 [nomen dubium].

non 1840 Ichthyosaurus latimanus Owen : 123 [nomen dubium].

non 1884 Ichthyosaurus fortimanus Owen : 176 [nomen dubium].

non 1911 Ichthyosaurus communis hyperdactyla Jaekel : fig. 154 [nomen dubium].

1922 Eurypterygius communis ; Huene: 5 [in part].

1922 Eurypterygius intermedius ; Huene: 9 [in part].

1979 Protoichthyosaurus prostaxalis Appleby : 942 [in part].

Neotype. NHMUK PV R1162 ( McGowan 1974), a practically complete skeleton comprising the skull, an articulated vertebral column extending into the tail fluke, ribs and gastralia, some pectoral elements, forefin, hind fin, and pelvic girdle.

Emended diagnosis. Ichthyosaurus communis is diagnosed relative to other species of Ichthyosaurus by the following unique characters: symmetric, triangular maxilla with long processes, extending anteriorly beyond external naris and posteriorly well under orbit; large, broad, triradiate lacrimal making up at least half of the anterior orbit margin.

The species is further diagnosed relative to other species of Ichthyosaurus by the following unique combination of characters: snout ratio>0.60 but probably less than 0.65 (as in I. anningae and I. larkini ; in the lower range of I. conybeari ); premaxilla supranarial and subnarial processes about equal length, extending about half way across the dorsal and ventral margins of the external naris, with nasal about half of dorsal margin (as in all species except I. breviceps and possibly I. anningae ); pointed anterior process of jugal extends only slightly beyond the orbit, if at all (as in I. breviceps , I. conybeari ); jugal dorsal ramus with a right angle bend (as in all species except I. somersetensis ) makes up about half of posterior margin of orbit; and relatively short, wide, crescentic postorbital makes up remainder of posterior margin (as in I. breviceps ); lacrimal dorsal process participates in anterior orbit margin, with small prefrontal, contributing less than half of anterior orbit margin (as in I. breviceps , I. conybeari and I. anningae ); humerus much longer than wide (as in all species except I. conybeari and I. anningae ); irregular depression on articular surface of humerus (as in I. larkini and I. somersetensis ); humerus dorsal process large, central, extends less than half way down the shaft (similar to I. larkini ); four elements in third (distal tarsal) row of hind fin, with two digits contacting the astragalus (as in I. larkini and I. anningae ); hind fin with five primary digits (as in I. conybeari , I. anningae , unknown in I. larkini ).

Material. Only two specimens referred to the species by McGowan (1974), both from Lyme Regis, can be unequivocally referred to I. communis : NHMUK PV R1073, NHMUK PV OR36256. The following substantially complete skeletons can also be referred to the species: BU 5289, CAMSM J35187, FMNH P 25027, MCZ 1079, MCZ 1493, OUMNH J.10341/P (cast of specimen figured by Buckland 1836, pl. 8, fig. 1), PETMG R174 and ROM 12805. SMNS 13111, a skull discussed above, is also referred to the species.

Occurrence. The geographical and stratigraphical ranges given here are based only on the neotype and referred specimens listed above. The neotype was collected from Lyme Regis, Dorset. Other referred specimens have been collected from the Lyme Regis-Charmouth area in Dorset, possibly from the Whitby coast of Yorkshire ( Maisch 1997, as discussed above), and Gloucestershire (BU 5289). The species is probably also from Street and the surrounding area in Somerset (see Discussion). The species has been reported from Europe ( Zbyszewski & Moitinho de Almeida 1952; Godefroit 1996; Maisch et al. 2008; Bardet et al. 2008) and the Isle of Skye, Scotland ( Brusatte et al. 2015; as discussed above) but the remains are too incomplete for species assignment.

Specimens from the west Dorset coast are most likely from the Blue Lias and Charmouth Mudstone formations ( Page 2010). Historically, the ‘Lyme Regis’ location was often recorded for ichthyosaurs from several localities along the west Dorset coast. The stratigraphical range usually given for Lyme Regis strata is upper Hettangian– lower Sinemurian ( McGowan 1974), although upper Sinemurian–lower Pliensbachian strata are also exposed at nearby localities, such as Charmouth and Seatown ( Benton & Spencer 1995; Lomax 2010; Page 2010). It is likely that many historical specimens of Ichthyosaurus were collected from various localities along the west Dorset coast. However, I. communis has not been unequivocally reported from the upper Sinemurian or Pliensbachian (contrary to Bennett et al. 2011; Massare & Lomax 2016a). The uppermost Triassic (Rhaetian) White Lias Formation is also exposed along the coast, south west of Lyme Regis ( Gallois 2007); however, we have not seen any specimens of I. communis from the ‘White Lias’. Somerset specimens, however, are most likely from the lower Hettangian ( McGowan 1974; Benson et al. 2012, 2015). Thus the species range is at least lower Hettangian–lower Sinemurian.

Remarks. For most of the nineteenth century, all ichthyosaur specimens were identified as Ichthyosaurus , so old museum identifications and identifications from historical literature should be considered carefully. However, even more recent identifications need to be checked against the criteria presented here. Previous diagnoses of the species allowed for so much variation that all large (>1.5 m) specimens of Ichthyosaurus were assigned to I. communis . The species has been treated as a catch-all for specimens that could not be assigned to I. breviceps or I. conybeari , two small species (<1.6 m) within the genus. But not all larger specimens are I. communis . In particular, the Somerset specimens NHMUK PV OR2013 ‡, NHMUK PV OR2013, NHMUK PV R44, CAMSM J59575, and probably CAMSM J35183, previously referred to the species by McGowan (1974), are not I. communis ( Lomax & Massare 2017) . In this analysis, we selected a few practically complete specimens that could be unequivocally assigned to the species based on the revised diagnosis. These were included in the geographical range summary above. Notably, I. communis is substantially more common from Dorset than from Somerset, as had been noted in the early literature ( Owen 1840, 1881). In fact, fairly complete skeletons of I. communis from Somerset seem to be rare.