Is it likely that mesosaurus swam back and forth across the atlantic ocean

Mesosaurus was significant in providing evidence for the theory of continental drift, because its remains were found in southern Africa and eastern South America, two widely separated regions. As Mesosaurus was a coastal animal, and therefore could not have crossed the Atlantic Ocean, this distribution indicated that the two continents used to be joined together.

No because it is small and it lives in freshwater not salt water

Currently, as of february 2016, an american traveling in france would be by the exchange rate. the weakness of the euro and the strength of the us dollar means that items in france cost less to an american than they otherwise would. of course, if this trend reverses and the us dollar weakens and the euro strengthens, such as was happening in early 2008, an american traveling in france would have an adverse experience because items would cost relatively more.