Ophthalmothule cryostea was named by Roberts et al. (2020) [A moderately sized cryptoclidid plesiosaur (estimated body length of 5.0–5.5 m), possessing the following autapomorphies unique among Cryptoclididae () and unique character combinations: premaxilla bears 6 alveoli (5 in Tricleidus seeleyi and Muraenosaurus leedsii); medial process of premaxilla terminates anterior to the posterior margin of external naris (); maxilla estimated to contain a similar number of alveoli (>16) as in in Cryptoclidus eurymerus (18) and Tricleidus seeleyi (15); frontal twice as anteroposteriorly long as parietal (subequal or shorter in Cryptoclidus eurymerus and M. leedsii); frontal participates in the medial and posterior margins of the external naris (participates posteriorly in M. leedsii); presence of an interfrontal vacuity (absent in M. leedsii); dorsoventrally low but mediolaterally narrow sagittal crest (flat and mediolaterally broad in Kimmerosaurus langhami); quadrate articulates anterolaterally to the pterygoid (posteromedially in Tricleidus seeleyi and M. leedsii); lateral cotyle of quadrate larger than medial cotyle (reversed in S. larseni); basioccipital tubera mediolaterally broad and dorsoventrally flattened (circular in K. langhami and Cryptoclidus eurymerus); basioccipital tubera triangular in ventral view, following the anteromedial process of pterygoid anteriorly (cylindrical with finished bone anteriorly in K. langhami); exoccipital does not contribute to occipital condyle (contributes in K. langhami and Cryptoclidus eurymerus); posteromedian ridge on supraoccipital absent (present in K. langhami and M. leedsii); palatine and vomer excludes maxilla from internal naris (maxilla participates in M. leedsii); vomer excluded from anterior interpterygoid vacuity (participates in M. leedsii and Cryptoclidus eurymerus); anteromedial process of pterygoid extends as far as the parabasisphenoid (absent in Cryptoclidus eurymerus); dentary with a mediolaterally extended alveolar surface and with laterally shifted, labially inclined alveoli (no mediolateral extension and alveoli positioned centrally in Tricleidus seeleyi); deep glenoid facet of the mandible, constituting over half the dorsoventral height of the mandible (relatively shallow in Colymbosaurus spp., Cryptoclidus eurymerus and K. langhami); retroarticular process slightly dorsally inclined (significantly inclined in Spitrasaurus larseni); faint longitudinal ridges on the teeth, distinct on labial side (distinct on lingual side in M. leedsii and Cryptoclidus eurymerus; ridging absent in K. langhami); slightly recurved tooth crowns (significantly recurved in S. larseni and K. langhami); mamillate hypophyseal eminence is present on the ventral surface of the atlas (ventral keel or keel-like morphology in Cryptoclidus eurymerus, M. leedsii and Tricleidus seeleyi); atlantal rib present (absent in Colymbosaurus megadeirus); 50 cervical vertebrae (32 in Cryptoclidus eurymerus; 44 in M. leedsii; 41 in Colymbosaurus megadeirus; 60 in Spitrasaurus wensaasi); cervical centra are slightly amphicoelous (conspicuously concave in Djupedalia engeri and K. langhami); cervical vertebra eight with anteroposteriorly long postzygapophyses, close to the length of centrum (autapomorphic among Plesiosauria); anterior-most cervical neural spines low and posteriorly angled (straight in K. langhami); cervical prezygapophyses unfused anteriorly and fused posteriorly (unfused throughout in Cryptoclidus eurymerus and completely fused in Spitrasaurus spp. and D. engeri); postzygapophyses fused along the midline (unfused in posterior-most cervicals in D. engeri); lateral ridges present on mid-posterior cervicals (absent in Colymbosaurus megadeirus, Cryptoclidus eurymerus, D. engeri, and Tricleidus seeleyi); posterior cervical—anterior dorsal ribs with a distinct, short (North-facing slopes of Wimanfjellet (Mt. Wiman), which is in a Tithonian/Berriasian marine mudstone in the Agardhfjellet Formation of Norway.