Maastrichtian

The Maastrichtian (

Definition

The Maastrichtian was introduced into scientific literature by

The Maastrichtian is commonly subdivided into two substages (Upper and Lower) and three ammonite biozones. The biozones are (from young to old):^[11]

• zone of Anapachydiscus terminus
• zone of Anapachydiscus fresvillensis
• zone of Pachydiscus neubergicus till Pachydiscus epiplectus

The Maastrichtian is roughly coeval with the

Palaeogeography and paleoclimate

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 sea floor spreading had not yet commenced between the two continents.^[12]

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.^[13]

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.^[14]

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.^[14] 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.^[15]

Paleontology

Dinosaurs remained the dominant large terrestrial animals throughout the Maasastrichtian, though mammals with internal organs similar to modern mammals were also present. Both

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.^[17]

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 Jordan 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.^[18][19] This seems to underscore a higher diversity of terminal Cretaceous pterosaurs than previously thought.^[20][21][22]

Flora

The radiation of

References

External links

Wikimedia Commons has media related to Maastrichtian.

• GeoWhen Database - Maastrichtian
• ghK Classification - Maastrichtian
• Late Cretaceous timescale, at the website of the subcommission for stratigraphic information of the ICS
• Stratigraphic chart of the Late Cretaceous, at the website of Norges Network of offshore records of geology and stratigraphy
• Maastrichtian Microfossils: 60+ images of Foraminifera