Opallionectes is a large, around 5 m long, derived cryptoclidid plesiosauroid from the Early Cretaceous of South Australia. It is known from a partial opalised skeleton, which is mounted for display in the South Australian Museum. The holotype specimen lacks a skull.
It is diagnosed by the following unique combination of characters: small needle-like teeth with a distinct ovoid cross-section and no ornamentation; cervical centra with length less than height; platycoelus articular surfaces are markedly ovate, lack a ventral groove (not binocular shape), with prominent mammillae surrounding the notochrdal pit; lateral surfaces of cervical centra lack longitudinal ridge but bear prominent facets for cervical ribs; longitudinal pectoral bar apparently formed by scapula and coracoid only; coracoids unite medially for their entire length; coracoids with prominent posterolateral cornua; humerus massive with slender shaft and marked distal expansion; supernumeracy ossifications in the epipodial row along the anterior margin of the carpus.
Kear (2006) remarked that Opallionectes shared several diagnostic characters with Cryptoclididae, and suggested it could represent the first record (at the time) of this family in the Lower Cretaceous. O’Keefe and Street (2009) suggested Opallionectes may be an aristonectid based on cervical vertebral characters, but at that time aristonectids were allied with cryptoclidids, whereas now they are allied with elasmosaurids. So, the phylogenetic position of Opallionectes was unclear – it was thought to be either a derived cryptoclidid (probably a colymbosaurine) or an aristonectine elasmosaurid. Kear (2016) described Opallionectes as “perhaps the most enigmatic Australian plesiosaurian named in recent years” (p. 21), and reiterated the similarities between it and aristonectids and, overwhelmingly, between it and cryptoclidids. Kear et al. (2018) regarded Opallionectes as “either an aberrant aristonectine elasmosaurid or, more likely, a late-surviving cryptoclidid” (p. 463-464), but still stopped short of a formal assignment. Eventually, Poropat et al. (2023) formally assigned Opallionectes to the Cryptoclididae.
The age range of the strata is Early Aptian to Early Albian.