The Last Cretaceous World: Ecosystems Before the K–Pg Boundary

The last part of the Cretaceous contained many kinds of landscapes, not one dinosaur world frozen in a single scene. In western North America, the record includes coastal plains, river channels, wetlands and floodplains. Other regions preserve different climates and communities. The end of the period is often discussed through the Hell Creek Formation and related units, but those deposits represent particular places and intervals rather than the whole planet.

A landscape built from shifting rivers

Hell Creek rocks in Montana, the Dakotas and nearby areas record rivers that moved across a broad coastal plain. Sandstone channel deposits, finer floodplain sediments, soils and plant remains preserve different parts of the environment. A fossil collected from a channel lag may have been transported, while a skeleton in fine sediment may preserve a more local burial. Geological context helps palaeontologists tell these stories apart.

Plants formed the base of these ecosystems. Flowering plants had diversified, while conifers, ferns and other groups remained important. Pollen and leaf fossils complement the bones of dinosaurs and other vertebrates. They can help reconstruct vegetation and climate, although the plants preserved in one layer are not necessarily an exact inventory of what grew across the entire landscape.

Animals varied from one region to another

Hadrosaurs, ceratopsians, ankylosaurs and tyrannosaurids are among the best-known large dinosaurs in the western interior of North America. Smaller dinosaurs, mammals, turtles, crocodilians, fish and invertebrates also formed part of the food web. Their relative abundance changed between formations and between the northern and southern parts of the region. Nearby fossil assemblages should not be collapsed into one timeless community.

Rock layers preserve time unevenly. A formation can cover millions of years, and fossil sites within it may represent different seasons, river systems or ecological conditions. A change in the fossils from one layer to another could reflect biological turnover, migration, climate, preservation, or a shift in which habitats were sampled. Multiple kinds of evidence are needed before choosing among these explanations.

Why the final record is still debated

Researchers have asked whether dinosaur diversity declined in North America before the K–Pg boundary or whether the pattern is partly an effect of incomplete sampling. The answer depends in part on how close fossils are to the boundary, how formations are dated, and whether the sites preserve the habitats where particular species lived. A new statistical analysis of the latest Cretaceous record used occupancy models to account for both the probability that a dinosaur occupied a site and the probability that it was detected there.

Such models do not remove uncertainty, but they show why raw species counts are not enough. A fossil may be missed even when its species occupied the region. Likewise, a gap in exposed rock can erase a long interval. Studies of the Late Cretaceous climate before Chicxulub examine another part of this picture, but climate evidence and local fossil communities operate at different scales.

A changing world at the boundary

The K–Pg boundary marks an abrupt global extinction event about 66 million years ago. At the same time, the ecosystems known from the final Cretaceous were already diverse and regionally structured. To understand what ended, scientists compare the latest Cretaceous fossils with the earliest Paleogene record and ask which organisms crossed the boundary. The details differ among plants, freshwater animals and marine communities.

Every reconstruction is a synthesis of a patchy archive. Sedimentology gives the setting, fossils reveal organisms, and dating establishes sequence. The final Cretaceous was a living, changing world, but no single formation can stand in for all continents or all habitats on the eve of the extinction.