Ichthyosaurs were uniformly low-drag

[Halichoeres]

This study uses data from dolphins to model the drag coefficient of ichthyosaur bodies during steady swimming. Perhaps counterintuitively, the authors find that basically all ichthyosaurs had low drag profiles, even quite early in their evolution:



That doesn't mean that other hydrodynamic improvements didn't occur, of course. This study doesn't really address the effect of changes in thrust (by adopting tuna-like locomotion) and maneuverability (by foreshortening the torso and tail).

paper is open access! https://royalsocietypublishing.org/doi/full/10.1098/rspb.2018.2786