In the east, the chart shows the remnants of the Crusader strongholds and the final throes of the Byzantine Empire. The clues initially led Clausen to suspect the map had been drawn around 1420.

The Barry Lawrence Ruderman antique map shop is listing the chart, now dubbed the Rex Tholomeus Portolan Chart of 1360, for $7.5 million. With such a price tag, it probably won’t attract a ...

The rare 14th-century portolan chart is worth $7.5 million. Robert Gauthier / Los Angeles Times / Getty Images Last fall, Christie’s auctioned off a “highly unusual” nautical map as part of ...