Elasmosaurus

Elasmosaurus ( from Greek  elasmos 'thin plate' (referring to thin plates in its pelvic girdle) + σαυρος sauros 'lizard') is a genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous period (Campanianstage), 80.5 million years ago.

Description
Elasmosaurus was about 14.1 m (46 ft) in length and weighed 2,000 kg (2.2 short tons), making it among the largest plesiosaurs. It differs from all other plesiosaurs by having six teeth per premaxilla (the bones at the tip of the snout) and 72 neck (cervical) vertebrae. The skull was relatively flat, with a number of long pointed teeth. The lower jaws were joined at the tip to a point between the fourth and fifth teeth. The neck vertebrae immediately following the skull were long and low, and had longitudinal lateral crests. Like most elasmosaurids,Elasmosaurus had around three pectoral vertebrae. The tail included at least 18 vertebrae.

The pectoral girdle featured a long bar, not present in juveniles. The scapula had margins of approximately equal length for the joint with the coracoid and the articular surface for the upper arm. The anterior edge of the pelvic girdle was made up of three almost straight edges directed to the front and sides of the animal. The ischia, a pair of bones that formed the posterior part of the pelvis, were joined along their medial surfaces. The limbs of Elasmosaurus, like those of other plesiosaurs, were modified into approximately equally sized rigid paddles.

Certain aspects of the anatomy of Elasmosaurus were fairly derived among elasmosaurids, and plesiosaurs in general. As noted, Elasmosaurus can be distinguished by its six premaxillary teeth and 71 cervical vertebrae. Primitively, plesiosaurs and most elasmosaurids had five teeth per premaxilla. Some elasmosaurids had more: Terminonatator had nine and Aristonectes had 10 to 13. In addition, most plesiosaurs had fewer than 60 cervical vertebrae. Aside from Elasmosaurus, plesiosaurs that exceeded 60 cervicals include Styxosaurus, Hydralmosaurus, Thalassomedon, and Albertonectes. Elasmosaurus and Albertonectes are the only known plesiosaurs with more than 70 cervicals. However, Elasmosaurus had roughly the same neck length as The giraffe because the latter has proportionally longer vertebrae. The presence of the pectoral bar is also considered an advanced feature. The long, low axis centrum differs from the condition seen in most other plesiosaurs, which have centra that are either shorter in length than height, or about equidimensional. Styxosaurus and Hydralmosaurus also have the condition present in Elasmosaurus. Another unusual feature of Elasmosaurus is the relatively equal lengths of the margins of the scapula, as mentioned above. Most plesiosaurs had longer margins for articulation with the coracoid than for articulation with the upper arm.