The taxon was initially erected in 1869 by Edward Drinker Cope to include the type genus Elasmosaurus with the related Cimoliasaurus, although he did not argued in detail why.

Over the following years, many authors recognized this classification on the basis of predominantly postcranial features, becoming one of the three groups in which plesiosaurs were often classified during the 19th century, along with the Pliosauridae and the Plesiosauridae.

With a maximum length ranging from 5 to 12 m (16 to 39 ft) depending on the genera, elasmosaurids have a streamlined body with paddle-like limbs, a short tail, a small head, and an extremely long neck.

Elasmosaurids probably ate small fish and marine invertebrates, seizing them with their long teeth, and may have used gastroliths (stomach stones) to help digest their food.

Like many plesiosaurs, elasmosaurids are easily recognizable by their compact, streamlined bodies, long paddle-like limbs, short tails, proportionately small heads, and very elongated necks.

[2] One of the most easily recognizable characteristics of elasmosaurids is their long neck formed by a fairly considerable number of cervical vertebrae, of which a large majority of genera have between 50 and 70.

[2] Though Edward Drinker Cope had originally recognized Elasmosaurus as a plesiosaur, in an 1869 paper he placed it, with Cimoliasaurus and Crymocetus, in a new order of sauropterygian reptiles.

He found no evidence of a clavicle or an interclavicle in the shoulder girdle of Elasmosaurus; he noted that the absence of the latter bone was also seen in a number of other plesiosaur specimens, which he named as new elasmosaurid genera: Eretmosaurus, Colymbosaurus, and Muraenosaurus.

[16] Richard Lydekker subsequently proposed that Elasmosaurus, Polycotylus, Colymbosaurus, and Muraenosaurus could not be distinguished from Cimoliasaurus based on their shoulder girdles, and advocated their synonymization at the genus level.

He characterized elasmosaurids by their long necks and small heads, as well as by their rigid and well-developed scapulae (but atrophied or absent clavicles and interclavicles) for forelimb-driven locomotion.

He provided a revised diagnosis of the Elasmosauridae; aside from the small head and long neck, he characterized elasmosaurids by their single-headed ribs; scapulae that meet at the midline; clavicles that are not separated by a gap; coracoids that are "broadly separated" in their rear half; short ischia; and the presence of only two bones (the typical condition) in the epipodialia (the "forearms" and "shins" of the flippers).

He also removed several plesiosaurs previously considered to be elasmosaurids from this family due to their shorter necks and continuously meeting coracoids; these included Polycotylus and Trinacromerum (the Polycotylidae), as well as Muraenosaurus, Cryptoclidus, Picrocleidus, Tricleidus, and others (the Cryptoclididae).

The cited variability in the number of heads on the neck ribs arises from his inclusion of Simolestes to the Elasmosauridae, since the characteristics of "both the skull and shoulder girdle compare more favorably with Elasmosaurus than with Pliosaurus or Peloneustes."

[27] Per Ove Persson, however, considered Welles' classification too simplistic, noting in 1963 that it would, in his opinion, erroneously assign Cryptoclidus, Muraenosaurus, Picrocleidus, and Tricleidus to the Elasmosauridae.

He also retained the Cimoliasauridae as separate from the Elasmosauridae, and suggested, based on comparisons of vertebral lengths, that they diverged from the Plesiosauridae in the Late Jurassic or Early Cretaceous.

[25] However, David S. Brown noted in 1981 that the variability of neck length in plesiosaurs made Persson's argument unfeasible, and moved the aforementioned genera back into the Elasmosauridae; he similarly criticized Welles' subdivision of elasmosaurids based on the pelvic bar.

[28] In 2009, F. Robin O'Keefe and Hallie Street synonymized the Cimoliasauridae with the Elasmosauridae, noting that most of the diagnostic traits previously established to distinguish them are also found in elasmosaurids.

He found that Libonectes and Dolichorhynchops shared characteristics such as an opening on the palate for the vomeronasal organ, the plate-like expansions of the pterygoid bones, and the loss of the pineal foramen on the top of the skull, differing from the pliosaurs.

In 2008 Patrick Druckenmiller and Anthony Russell moved the Polycotylidae back into the Pliosauroidea, and placed Leptocleidus as their sister group in the newly named Leptocleidoidea;[33] Adam Smith and Gareth Dyke independently found the same result in the same year.

[36] In Rodrigo Otero's 2016 analysis based on a modification of the same dataset (below, right), Elamosaurus was the closest relative of Albertonectes, forming the Styxosaurinae with Styxosaurus and Terminonatator.

[43] In 2020, O'Gorman formally synonymized Styxosaurinae with Elasmosaurinae based on the inclusion of Elasmosaurus within the group, and also provided a list of diagnostic characteristics for the clade.

[45] Topology A: Benson et al. (2013)[36] Cryptoclididae Leptocleididae Polycotylidae Thalassomedon Libonectes Elasmosaurus Terminonatator Styxosaurus Hydrotherosaurus Callawayasaurus Eromangasaurus Kaiwhekea Aristonectes Topology B: Otero (2016),[39] with clade names following O'Gorman (2020)[3] Cryptoclididae Leptocleididae Polycotylidae Eromangasaurus Callawayasaurus Libonectes Tuarangisaurus Thalassomedon Specimen CM Zfr 115 Hydrotherosaurus Futabasaurus Kaiwhekea Alexandronectes Morturneria Aristonectes Terminonatator Elasmosaurus Albertonectes Styxosaurus

[2] The unusual body structure of elasmosaurids would have limited the speed at which they could swim, and their paddles may have moved in a manner similar to the movement of oars rowing, and due to this, could not twist and were thus held rigidly.

The neck anatomy of elasmosaurids was capable of making a gentle slope to allow them to breathe at the surface but would have required them to engage in energy-expensive swimming at the sub-surface.

Elasmosaurids may also have been active hunters in the pelagic zone, retracting their necks to launch a strike or using side-swipe motions to stun or kill prey with their laterally projected teeth (like sawsharks).

[59] Several different functions have been proposed for gastroliths, including aiding in digestion, mixing food content, mineral supplementation, and storage and buoyancy control.