This ocean was extended between the continents of Gondwana and Laurasia. At this time, where Monte Bolca is today, M. rhombea, and its relative, M. oblonga, lived in a tropical lagoon.
The passage describes an ancient ocean that existed between the continents of Gondwana and Laurasia.  In this specific area, now Monte Bolca, there was a tropical lagoon where M. rhombea and its relative, M. oblonga, lived.