A New Ophthalmosaurid (Ichthyosauria) from Svalbard, Norway, and Evolution of the Ichthyopterygian Pelvic Girdle

Abstract

In spite of a fossil record spanning over 150 million years, pelvic girdle evolution in Ichthyopterygia is poorly known. Here, we examine pelvic girdle size relationships using quantitative methods and new ophthalmosaurid material from the Slottsmøya Member Lagerstätte of Svalbard, Norway. One of these new specimens, which preserves the most complete ichthyosaur pelvic girdle from the Cretaceous, is described herein as a new taxon, Keilhauia nui gen. et sp. nov. It represents the most complete Berriasian ichthyosaur known and the youngest yet described from the Slottsmøya Member. It is diagnosed on the basis of two autapomorphies from the pelvic girdle, including an ilium that is anteroposteriorly expanded at its dorsal end and an ischiopubis that is shorter or subequal in length to the femur, as well as a unique character combination. The Slottsmøya Member Lagerstätte ichthyosaurs are significant in that they represent a diverse assemblage of ophthalmosaurids that existed immediately preceding and across the Jurassic–Cretaceous boundary. They also exhibit considerable variation in pelvic girdle morphology, and expand the known range in size variation of pelvic girdle elements in the clade.

Peer Review #2 of "A new specimen of Palvennia hoybergeti: implications for cranial and pectoral girdle anatomy in ophthalmosaurid ichthyosaurs (v0.1)"

“…Its placement along the anterior margin of the ventral side is similar to Ophthalmosaurus icenicus (Moon & Kirton, 2016) There are three distal articular facets for a preaxial accessory element, and the radius and ulna. This is similar to all other SML ophthalmosaurids (including the partial humerus of the Palvennia hoybergeti holotype) with the exception of Cryopterygius kristiansenae, which has two facets on one humerus and three facets on the other in addition to several other ophthalmosaurids e. g. Caypullisaurus bonapartei, Acamptonectes densus and Undorosaurus (Fernández, 1997;Efimov, 1999a;Druckenmiller et al, 2012;Fischer et al, 2012;Arkhangelsky & Zverkov, 2014;Roberts et al, 2014;Delsett et al, 2017). In contrast, Aegirosaurus leptospondylus, Maiaspondylus lindoei, Grendelius and Brachypterygius extremus have a third facet for the intermedium (McGowan, 1976;Bardet & Fernández, 2000;Maxwell & Caldwell, 2006;Zverkov, Arkhangelsky & Stenshin, 2015), whereas Platypterygius americanus has a third facet for a postaxial accessory element (Maxwell & Kear, 2010).…”

“…Relative to the size of the anteromedial process, the anterior notch is mediolaterally narrow compared to Ophthalmosaurus icenicus and Acamptonectes densus (Fischer et al, 2012;Moon & Kirton, 2016). The anterior notch is anteroposteriorly shorter than Keilhauia nui (Delsett et al, 2017) but longer and narrower than in PMO 222.667 and Arthropterygius chrisorum (Maxwell, 2010). The anterior process is straighter in dorsal view than the processes found in Sveltonectes insolitus (Fischer et al, 2011) and Janusaurus lundi (Roberts et al, 2014).…”

“…The dorsal process extends to the proximodistal midpoint, as in Undorosaurus gorodischensis (Efimov, 1999a) and Ophthalmosaurus icenicus (Moon & Kirton, 2016). This is proximodistally shorter than in Arthropterygius chrisorum (Maxwell, 2010) and Platypterygius hercynicus (Kolb & Sander, 2009), but longer than in Keilhauia nui (Delsett et al, 2017). In anterior view the deltopectoral crest is dorsoventrally short and does not reach the midpoint of the element on the ventral side, similar to Keilhauia nui (Delsett et al, 2017) and…”

“…obs. AJR), somewhat more than Keilhauia nui (Delsett et al, 2017) and Baptanodon natans (Gilmore, 1905), but less than in Sveltonectes insolitus (Fischer et al, 2011),Platypterygius americanus (Maxwell & Kear, 2010) and Grendelius alekseevi (Zverkov, Arkhangelsky & Stenshin, 2015). The posterior shaft is relatively short compared to the anterior portion compared to the more slender shaft in Grendelius alekseevi (Zverkov, Arkhangelsky & Stenshin, 2015) Coracoid (Figs.…”

“…In PMO 222.669, the facet for the radius is only slightly larger than the ulnar facet. In Ophthalmosaurus icenicus specimens, the three facets vary in relative size and orientation (Moon & Kirton, 2016 Manuscript to be reviewed Gengasaurus nicosiai (Paparella et al, 2016) and Arthropterygius chrisorum (Maxwell, 2010) the ulnar facet is not posteriorly deflected, unlike in Keilhauia nui (Delsett et al, 2017).…”