During the final millions of years of the Cretaceous, life on Earth was already undergoing a period of instability before the asteroid impact. Rather than representing a uniformly thriving world, Late Mesozoic ecosystems show clear signs of transformation, environmental stress, and progressive changes in biodiversity.
Continental fragmentation, initiated long before, had produced increasingly regionalized ecosystems. Many faunas became endemic, with limited geographic ranges and a growing dependence on local conditions. This specialization increased regional diversity, but it also made many lineages more vulnerable to rapid environmental change.
At the same time, Late Cretaceous climate conditions began to fluctuate more noticeably. Gradual cooling trends, shifts in sea level, and changes in ocean circulation affected both terrestrial and marine ecosystems. The retreat of epicontinental seas reduced extensive coastal habitats that had long supported diverse biological communities.
The fossil record indicates that several dinosaur groups were already experiencing declines in diversity and abundance prior to the end of the period. These trends were neither uniform nor simultaneous across the globe, but they suggest that the biosphere was not in a state of stable equilibrium. Some lineages continued to thrive locally, while others entered prolonged phases of decline.
In the oceans, significant changes were also underway. The progressive disappearance of certain plankton groups and marine reptiles altered trophic networks, reducing the resilience of shallow marine ecosystems. These transformations occurred over geological timescales rather than as abrupt collapses, but they set the stage for a more severe disruption.
The asteroid impact did not strike an intact world. It occurred within a biosphere already shaped by millions of years of environmental, climatic, and ecological change. Understanding this context is essential for interpreting the K–Pg extinction not as an isolated event, but as the culmination of a complex process in which multiple factors were already in play.