The phrase "single landmass" refers to a large piece of land that is not divided by bodies of water, such as oceans or seas. It means that all the land is connected and forms a continuous mass. Full definition
Based on the fossil record, it is generally assumed that mammals arose some 200 million years ago, when the continents were still joined together in a single landmass called Pangaea. (scientificamerican.com)
In 50 million to 200 million years» time, all of Earth's current continents will be pushed together into a single landmass around the North Pole. (scientificamerican.com)
Most paleontologists believed that any dinosaurs alive in the late Triassic would simultaneously inhabit all of Pangaea — a reasonable assumption, since the continents were melded into a single landmass stretching nearly from the North Pole to the South Pole, allowing animals to roam freely. (discovermagazine.com)