Moderately sized polycotylid plesiosaur; resembles P. latipinnus in having stout and coarsely striated teeth unlike D. osborni which has slender teeth with fine striations and

T. bentonianum, which has slender teeth with coarse striations; resembles P. latipinnus and D. osborni, but differs from T. bentonianum, in having non cylinder-like, but laterally and ventrally constricted vertebral centra; resembles P. latipinnus, but differs from D. osborni and T. bentonianum, in having some centra compressed anteroposteriorly, although not to the extent seen in

P. latipinnus; resembles T. bentonianum in having lateral foramina on dorsal vertebrae; interclavicle, clavicle, scapula, and coracoid very similar to Dolichorhynchops and Trinacromerum; humerus resembles P. latipinnus in having four facets for articulation of epipodials, but differs in lacking greatly broadened distal end; humerus also differs from P. latipinnus and all other polycotylids in having short, broad, dorsoventrally compressed, less sigmoidal neck; femur differs from P. latipinnus in shorter neck and lack of greatly broadened distal end; ilium nearly identical to P. latipinnus in curvature and tapered sacral end, unlike the straight ilium with expanded sacral end of T. bentonianum; pubis very similar to T. bentonianum in lacking scalloped anterolateral edge seen in T. kirki and P. latipinnus; ischia very similar to Trinacromerum, not tapered posteriorly as in P. latipinnus, not as broad anteriorly as in D. osborni.

Eo, from eos, Greek for dawn, earliest, oldest; rankini, for David O. Rankin of Big Water, Utah, discoverer of this and several other important specimens from the Tropic Shale (see acknowledgments).