Two extremely-long necked elasmosaurids, AMNH 1495, holotype of Hydralmosaurus serpentinus, and AMNH 5835, previously referred to H. serpentinus, are here reviewed in detail. Unique features of the cervical vertebrae, which are only present on elasmosaurids from the Western Interior Seaway, are recognized based on these specimens and by comparison with penecontemporaneous taxa with biogeographic affinities. Phylogenetic analysis, bivariate graphic analysis of cervical vertebrae proportions, comparisons of different cervical vertebral types, paleobiogeographic distribution and study of the elasmosaurid axial evolution throughout the Cretaceous are here integrated. As a result, at least two separate lineages within the Elasmosauridae are identified by independently acquired extremely-long necks (over 60 cervical vertebrae). First, a still scarcely known lineage is so far represented by the lower Cenomanian Thalassomedon haningtoni, the Turonian Libonectes morgani and close relatives. A second lineage is here defined as a new clade, the Styxosaurinae, which groups the Campanian genera Terminonatator, Styxosaurus (='Hydralmosaurus'), Albertonectes and Elasmosaurus, the two latter forming a derived branch that includes the most extreme amniote necks known to date (more than 70 cervical vertebrae). Phylogenetic analysis supports AMNH 1495 and AMNH 5835 as being closely related to Styxosaurus snowii. Therefore, the species Styxosaurus browni is re-validated, while AMNH 1495 is here referred to Styxosaurus sp. This research also recognizes the 'Cimoliasauridae' (nomen dubium) as a paraphyletic group but informative of a plesiomorphic cervical vertebral morphology of elasmosaurids which was persistent throughout the whole Cretaceous and from whom aristonectines, styxosaurines and Thalassomedon and close relatives are derived. The genus Hydralmosaurus is recommended for being abandoned.