(i.e. less specialized) than other elasmosaurids with the exception of Libonectes. In 2005 Sachs suggested that Elasmosaurus was closely related to Styxosaurus, and in 2008 Druckenmiller and Russell placed it as part of a polytomy with two groups, one containing Libonectes and Terminonatator, the other containing Callawayasaurus and Hydrotherosaurus. Ketchum and Benson's 2010 analysis included Elasmosaurus in the former group. Benson and Druckenmiller's 2013 analysis (below, left) further removed Terminonatator from this group and placed it as one step more derived. 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. Danielle Serratos, Druckenmiller, and Benson could not resolve the position of Elasmosaurus in 2017, but they noted that Styxosaurinae would be a synonym of Elasmosaurinae if Elasmosaurus did fall within the group. 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. Topology A: Benson et al. (2013) Topology B: Otero (2016), with clade names following O'Gorman (2020) Paleobiology Elasmosaurids were fully adapted to life in the ocean, with streamlined bodies and long paddles that indicate they were active swimmers. 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. Plesiosaurs were even believed to have been able to maintain a constant and high body temperature (homeothermy), allowing for sustained swimming. A 2015 study concluded that locomotion was mostly done by the fore-flippers while the hind-flippers functioned in maneuverability and stability; a 2017 study concluded that the hind-flippers of plesiosaurs produced 60% more thrust and had 40% more efficiency when moving in harmony with the fore-flippers. The paddles of plesiosaurs were so rigid and specialized for swimming that they could not have come on land to lay eggs like sea turtles. Therefore, they probably gave live-birth (viviparity) to their young like some species of sea snakes. Evidence for live-birth in plesiosaurs is provided by the fossil of an adult Polycotylus with a single fetus inside. Neck movement and function Cope, in 1869, compared the build and habits of Elasmosaurus with those of a snake. Although he suggested that