Two new ophthalmosaurids (Reptilia: Ichthyosauria) from the Agardhfjellet Formation (Upper Jurassic: Volgian/Tithonian), Svalbard, Norway

摘要

Ichthyosaur diversity near the Jurassic-Cretaceous boundary, particularly from high paleolatitudes, is poorly known. Two recently collected specimens of medium- to large-bodied ichthyosaurs from the Slottsm?ya Member of the Agardhfjellet, Svalbard, Norway represent two new taxa of ophthalmosaurids. The holotype specimen of Cryopterygius kristiansenae gen et sp. nov., PMO 214.578, is a nearly complete and largely articulated skeleton. The specimen consists of a nearly complete skull, the entire presacral and preflexural vertebral series, numerous dorsal ribs and gastralia, an articulated pectoral girdle and nearly complete forelimb, and an articulated left pelvic girdle and hindlimb. The new taxon is diagnosed on a unique suite of features, including a robust and moderately elongate rostrum, a reduced supranarial process, an elongate maxilla that bears a high number of teeth, the absence of a lacrimal-external naris contact, and an anteroposteriorly broad postorbital bar possessing an unidentified element (supratemporal?) that lies posterior to the quadratojugal. Cryopterygius has 52 presacral vertebrae, a distinctive forelimb, including a humerus that bears only two facets at its distal end, and an articulated left pelvic girdle and hindlimb, which facilitates the unequivocal orientation of the ophthalmosaurid femur. The holotype specimen of Palvennia hoybergeti, SVB 1451, includes a nearly complete skull and fragmented postcranial remains. It is diagnosed on its relatively short rostrum, greatly enlarged orbit, narrow postorbital bar, very large pineal foramen, basioccipital with broad extracondylar area laterally, and a gracile stapedial shaft. The Slottsm?ya Lagerst?tte is established as one of the most productive horizons for Upper Jurassic ichthyosaurs and considerably expands our knowledge of ophthalmosaurid diversity and distribution in the latest Jurassic.