Warm-blooded reptiles?

Modern reptiles are cold blooded, but what about extinct reptiles?
Based on their teeth, large extinct swimming reptiles, like ichthyosaurs, plesiosaurs, and mosasaurs are thought to have been predators in the Mesozoic oceans. Their streamlined body profile also suggests an active lifestyle. An active lifestyle requires high metabolic rates, which are usually correlated with at least some ability to regulate body temperature.
Scientists analyzed oxygen isotopes from the teeth of these extinct marine reptiles and compared the values with those from cold-blooded fossil fish. The results showed ichthyosaurs and plesiosaurs differed significantly from fish, and suggest that these two reptiles, both of whom were active, "pursuit predators", probably controlled their body temperature. The data for mosasaurs were equivocal and suggest that mosasaurs led a different lifestyle perhaps as an opportunistic ambush predator.
Illustration of an ichthyosaur is from here.