Maastrichtian

Palaeogeography

The breakup of Pangaea was nearly complete in the Maastrichtian, with Australia beginning to break away from Antarctica and Madagascar breaking away from India. However, Arabia had not yet rifted away from Africa. North America was separated from Europe by rift basins, but seafloor spreading had not yet commenced between the two continents.

The Pacific Plate was rapidly growing in size as the surrounding oceanic plates were consumed by subduction, and the Pacific-Izanagi Ridge was rapidly approaching Asia.

Eruption of the Deccan Traps large igneous province began during the Maastrichtian, at around 67 million years ago. This is thought to be a consequence of India drifting over the Réunion hotspot.

Climate

During the Maastrichtian, the global climate began to shift from the warm and humid climate of the Mesozoic to the colder and more arid climate of the Cenozoic. Variation of climate with latitude also became greater. This was likely caused by a major reorganization of oceanic circulation that took place at the boundary between the early and late Maastrichtian. This reorganization was triggered by the breach of tectonic barriers in the South Atlantic, permitting deep ocean water to begin circulating from the nascent North Atlantic to the south. This initiated thermohaline circulation similar to that of the modern oceans. At the same time, the Laramide orogeny drained the Western Interior Seaway of North America, further contributing to global cooling. Nonetheless, the latest Maastrichtian featured a sharp, pronounced warming, which was caused by the activity of the Deccan Traps.

South-central Alaska had a mean annual temperature of 7.42±1.2 °C, a warm monthly mean temperature of 17.08±1.6 °C, and a cold monthly mean temperature of −2.31±1.9 °C.

Paleontology

Dinosaurs remained the dominant large terrestrial animals throughout the Maastrichtian, though mammals with internal organs similar to modern mammals were also present. Both ammonites and pterosaurs were in decline during the Maastrichtian well prior to being wiped out in the extinction event.

Dinosaurs

Ornithiscians

Hadrosaurs and ceratopsians comprised the most abundant clades of ornithiscian fauna in the Maastrichtian, with thescelosaurids, ankylosaurs and pachycephalosaurs also present.

Sauropods

As a part of the so-called sauropod hiatus, sauropods were globally less common than in the Early Cretaceous or the Jurassic Period, and are absent in some localities particularly in the northern hemisphere. By the Maastritchian, large ornithiscians such as the hadrosaurs had spread to most of the Earth's landmass (excluding only India, Madagascar, Australia and Antarctica), displacing or competing with sauropods in many ecosystems.

Nevertheless, sauropods continued to thrive throughout much of the Earth. They were especially widespread in South America, Africa, Madagascar and India. Maastrichtian sauropods are also known from parts of Europe, Asia, and North America, although many localities had limited or no Sauropod presence on those continents.

Though no Maastrichtian sauropods are known from Australia and Antarctica, this may be due to lack of evidence as few Maastrichtian dinosaur fossils are known from those places in general.

Theropods

Apex predator niches were commonly occupied by abelisaurids such as Carnotaurus and Majungasaurus in the southern hemisphere, and tyrannosaurids in the northern hemisphere. The largest tyrannosaurs began to appear in North America and Asia in the late Campanian and throughout the Maastrichtian, including the famous dinosaur Tyrannosaurus.

Medium-sized theropods included smaller ceratosaurs and tyrannosaurs, such as Masiakasaurus and Dryptosaurus, respectively, as well as dromaeosaurids such as Austroraptor and Dakotaraptor.

Other large theropods included herbivores like therizinosaurids and the giant ornithomimosaur Deinocheirus.

Smaller theropods included alvarezosaurids, ornithomimosaurs, troodontids, small dromaeosaurs, and oviraptosaurians such as Avimimus.

Avialans (including crown birds)

Several stem-group clades of avialans (the group which includes modern birds and their close extinct relatives), such as Enantiornithes, Ichthyornithes, and Hesperornithes, persisted to the latest Maastrichtian but became extinct during the Cretaceous-Paleogene extinction event. Modern birds belonging to Neornithes unambiguously appeared in the fossil record during the Maastrichtian, as indicated by taxa such as Asteriornis, which is suggested to be a close relative of modern fowl, as well as Vegavis of uncertain affinities.

Pterosaurs

Traditionally, pterosaur faunas of the Maastrichtian were assumed to be dominated by azhdarchids, with other pterosaur groups having become extinct earlier on. However, more recent findings suggest a fairly composite pterosaur diversity: at least six ("Nyctosaurus" lamegoi, a Mexican humerus, a Jordanian humerus, and several taxa from Morocco) nyctosaurs date to this period, as do a few pteranodontids, and Navajodactylus, tentatively assigned to Azhdarchidae, lacks any synapomorphies of the group. This seems to underscore a higher diversity of terminal Cretaceous pterosaurs than previously thought.

Flora

The radiation of angiosperms (flowering plants) was well under way in the Maastrichtian. From 50% to 80% of all genera of land plants were angiosperms, though gymnosperms and ferns still covered larger areas of the land surface.