Jurassic

Jurassic
201.4 ± 0.2 – ~145.0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Map of Earth as it appeared 170 million years ago during the Middle Jurassic, Bajocian Age[citation needed]
Chronology
−200 — – −195 — – −190 — – −185 — – −180 — – −175 — – −170 — – −165 — – −160 — – −155 — – −150 — – −145 — – Mesozoic Triassic Jurassic Cretaceous Early Middle Late Hettangian Sinemurian Pliensbachian Toarcian Aalenian Bajocian Bathonian Callovian Oxfordian Kimmeridgian Tithonian       ← Triassic–Jurassic extinction event Subdivision of the Jurassic according to the ICS, as of 2023.[1] Vertical axis scale: millions of years ago.
Etymology
Name formality	Formal
Usage information
Regional usage	Global (ICS)
Time scale(s) used	ICS Time Scale
Definition
Chronological unit	Period
Stratigraphic unit	System
Time span formality	Formal
Lower boundary definition	First appearance of the ammonite Psiloceras spelae tirolicum.
Lower boundary GSSP	Kuhjoch section, Karwendel mountains, Northern Calcareous Alps, Austria 47°29′02″N 11°31′50″E / 47.4839°N 11.5306°E / 47.4839; 11.5306
Lower GSSP ratified	2010
Upper boundary definition	Not formally defined
Upper boundary definition candidates	Magnetic—base of Chron M18r Base of Calpionellid zone B FAD of ammonite Berriasella jacobi
Upper boundary GSSP candidate section(s)	None

The Jurassic (/dʒʊˈræsɪk/ juurr-ASS-ik^[2]) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period 201.4 million years ago (Mya) to the beginning of the Cretaceous Period, approximately 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era as well as the eighth period of the Phanerozoic Eon and is named after the Jura Mountains, where limestone strata from the period were first identified.

The start of the Jurassic was marked by the major Triassic–Jurassic extinction event, associated with the eruption of the Central Atlantic Magmatic Province (CAMP). The beginning of the Toarcian Stage started around 183 million years ago and is marked by the Toarcian Oceanic Anoxic Event, a global episode of oceanic anoxia, ocean acidification, and elevated global temperatures associated with extinctions, likely caused by the eruption of the Karoo-Ferrar large igneous provinces. The end of the Jurassic, however, has no clear, definitive boundary with the Cretaceous and is the only boundary between geological periods to remain formally undefined.

By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north and Gondwana to the south. The climate of the Jurassic was warmer than the present, and there were no ice caps. Forests grew close to the poles, with large arid expanses in the lower latitudes.

On land, the fauna transitioned from the Triassic fauna, dominated jointly by dinosauromorph and pseudosuchian archosaurs, to one dominated by dinosaurs alone. The first stem-group birds appeared during the Jurassic, evolving from a branch of theropod dinosaurs. Other major events include the appearance of the earliest crabs and modern frogs, salamanders and lizards. Mammaliaformes, one of the few cynodont lineages to survive the end of the Triassic, continued to diversify throughout the period, with the Jurassic seeing the emergence of the first crown group mammals. Crocodylomorphs made the transition from a terrestrial to an aquatic life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. Modern sharks and rays first appeared and diversified during the period, while the first known crown-group teleost fish appeared near the end of the period. The flora was dominated by ferns and gymnosperms, including conifers, of which many modern groups made their first appearance during the period, as well as other groups like the extinct Bennettitales.