Maastrichtian

soil on the Maastrichtian Chalk in Kozubów Landscape Park, Poland Definition The Maastrichtian was introduced into scientific literature by Belgian geologist André Hubert Dumont in 1849, after studying rock strata of the Chalk Group close to the Dutch city of Maastricht. These strata are now classified as the Maastricht Formation – both the formation and stage derive their names from the city. The Maastricht Formation is known for its fossils from this age, most notably those of the giant sea reptile Mosasaurus, which in turn derives its name from the nearby river Maas (mosa being Latin for the river Maas). The base of the Maastrichtian Stage is at the first appearance of ammonite species Pachydiscus neubergicus. At the original type locality near Maastricht, the stratigraphic record was later found to be incomplete. A reference profile for the base was then appointed in a section along the Ardour river called Grande Carrière, close to the village of Tercis-les-Bains in southwestern France. The top of the Maastrichtian Stage is defined to be at the iridium anomaly at the Cretaceous–Paleogene boundary (K–Pg boundary), which is also characterised by the extinction of many groups of life. Subdivision The Maastrichtian is commonly subdivided into two substages (Upper and Lower) and three ammonite biozones. The biozones are (from young to old): • zone of Anapachydiscus terminus • zone of Anapachydiscus fresvillensis • zone of Pachydiscus neubergicus till Pachydiscus epiplectus The Maastrichtian is roughly coeval with the Lancian North American Land Mammal Age. ==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 , a warm monthly mean temperature of , and a cold monthly mean temperature of . ==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 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. Medium-sized theropods included smaller species of these families, such as Nanotyrannus, non-tyrannosaurid trannosauroids like Dryptosaurus and non-abelisauid abelisauroids like Masiakasaurus, and dromaeosaurids 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. Birds Several archaic clades of birds, such as Enantiornithes, Ichthyornithes, and Hesperornithes, persisted to the latest Maastrichtian but became extinct during the Cretaceous-Paleogene extinction event. 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. ==Notes==