Sixty-five million years ago marks the Cretaceous-Paleogene (K-Pg) boundary, representing the end of the Mesozoic Era. The world map was in a state of rapid transformation, driven by continental drift. The ancient supercontinent Pangaea was long dissolved, but its fragments had not yet settled into their modern positions. This period captures the final moments of the dinosaur-dominated world, set against a backdrop of continents still moving, separating, and colliding.
The Major Continental Divisions
The global landmasses were broadly structured into two main, partially fragmented groupings, mirroring the earlier split of Pangaea into Laurasia and Gondwana. The northern continents, remnants of Laurasia, were beginning to define the shape of North America and Eurasia. These masses were separated from the southern continents by a wide expanse of ocean, including the early Atlantic and the shrinking Tethys Seaway.
The continents were recognizable, but their positions differed significantly from their present-day locations. Ongoing tectonic activity ensured that the connections and separations between these major land blocks were unstable and constantly evolving. The vast oceanic separation between the northern and southern continents resulted in distinct evolutionary pathways for terrestrial life.
Geography of the Northern Landmasses
North America was undergoing a massive mountain-building event known as the Laramide Orogeny. This deformation, caused by the shallow subduction of the Farallon Plate beneath the continent’s western edge, was actively uplifting the early Rocky Mountains. The uplift was linked to the rapid retreat of the Western Interior Seaway, the shallow sea that had long split the continent into western Laramidia and eastern Appalachia.
At this time, the seaway was in its final stages of disappearance, leaving behind a narrow channel as the continent’s interior rose. Across the newly forming North Atlantic, Eurasia was not yet a unified landmass. Europe existed as a fragmented series of archipelagoes, separated by shallow seas. The major Asian landmasses were still moving toward consolidation, which would close the ancient Tethys Ocean and form the Himalayan chain.
Isolation and Movement in the Southern Continents
The southern continents, once the core of Gondwana, were characterized by isolation and rapid northward movement. South America and Africa were completely separated, with the South Atlantic Ocean having fully opened between them. This wide oceanic barrier had already led to the independent biological evolution of terrestrial life on both continents.
The Indian subcontinent was an isolated island continent, traveling rapidly toward its collision with Asia. India’s journey north was one of the fastest known continental movements, having rifted from Africa and Antarctica much earlier. Further south, Australia and Antarctica remained physically connected, but active rift zones were creating the weakness necessary for their separation. The stage was set for Australia’s northward drift and the opening of the deep ocean between the two landmasses.
High Sea Levels and Global Climate
The period was defined by a consistently high global mean sea level, a characteristic feature of the late Cretaceous. With minimal polar ice caps, the planet was in a greenhouse state, and the lack of ice-sheet volume contributed significantly to the elevated water line. Younger oceanic crust was warmer and more buoyant, taking up less space in the ocean basins and pushing water onto the continents.
This combination of factors caused extensive marine inundation, with shallow epicontinental seas covering large portions of the continental interiors. Global temperatures were substantially warmer than today, with estimates suggesting average temperatures were several degrees Celsius higher. The remnants of the vast Tethys Ocean formed a prominent east-west seaway, separating the northern and southern landmasses and influencing global ocean currents.