Sauropterygia, a successful clade of marine reptiles abundant in aquatic ecosystems
of the Mesozoic, inhabited nearshore to pelagic habitats over >180 million years of
evolutionary history [1]. Aquatic vertebrates experience strong buoyancy forces that
allow movement in a three-dimensional environment, resulting in structural convergences
such as flippers and fish-like bauplans [2, 3], as well as convergences in the sensory
systems. We used computed tomographic scans of 19 sauropterygian species to determine
how the transition to pelagic lifestyles influenced the evolution of the endosseous
labyrinth, which houses the vestibular sensory organ of balance and orientation [4].
Semicircular canal geometries underwent distinct changes during the transition from
nearshore Triassic sauropterygians to the later, pelagic plesiosaurs. Triassic sauropterygians
have dorsoventrally compact, anteroposteriorly elongate labyrinths, resembling those
of crocodylians. In contrast, plesiosaurs have compact, bulbous labyrinths, sharing
some features with those of sea turtles. Differences in relative labyrinth size among
sauropterygians correspond to locomotory differences: bottom-walking [5, 6] placodonts
have proportionally larger labyrinths than actively swimming taxa (i.e., all other
sauropterygians). Furthermore, independent evolutionary origins of short-necked, large-headed
"pliosauromorph" body proportions among plesiosaurs coincide with reductions of labyrinth
size, paralleling the evolutionary history of cetaceans [7]. Sauropterygian labyrinth
evolution is therefore correlated closely with both locomotory style and body proportions,
and these changes are consistent with isolated observations made previously in other
marine tetrapods. Our study presents the first virtual reconstructions of plesiosaur
endosseous labyrinths and the first large-scale, quantitative study detailing the
effects of increasingly aquatic lifestyles on labyrinth morphology among marine reptiles.