Triassic Period

Friedrich August von Alberti, a German salt-mining engineer and geologist, coined the term "Trias" in his 1834 publication Beitrag zu einer Monographie des Bunten Sandsteins, Muschelkalks und Keupers, und die Verbindung dieser Gebilde zu einer Formation. He recognized the unity of three distinctive sedimentary rock units in the Germanic Basin of central Europe: the Buntsandstein ("colored sandstone," predominantly continental red beds), the Muschelkalk ("clam limestone," shallow marine carbonates and evaporites), and the Keuper (continental marls and clastics). Alberti's threefold succession roughly corresponds to the modern Lower, Middle, and Upper Triassic series. The name was later anglicized from "Trias" to "Triassic."

Subsequent researchers correlated marine invertebrate fossils from Alpine successions with the Germanic strata. Starting in the late 19th century, geologists working primarily in the Austrian Alps established the global standard marine stages. The modern ICS-recognized stages are, from oldest to youngest: Induan and Olenekian (Early Triassic), Anisian and Ladinian (Middle Triassic), and Carnian, Norian, and Rhaetian (Late Triassic). As of the ICS chart v2024/12, three stages (Induan, Ladinian, Carnian) have ratified GSSPs, while four (Olenekian, Anisian, Norian, Rhaetian) still lack formally ratified boundary stratotypes.

The base of the Triassic (= base of the Induan Stage, = base of the Mesozoic Era) is defined by the GSSP at Meishan Section D, Changxing County, Zhejiang Province, China. The boundary marker is the First Appearance Datum (FAD) of the conodont Hindeodus parvus at the base of Bed 27c. This GSSP was ratified by IUGS in March 2001. The boundary is dated at 251.902 ± 0.024 Ma.

The top of the Triassic (= base of the Jurassic, = base of the Hettangian Stage) is defined by the GSSP at the Kuhjoch section, Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria. The boundary marker is the FAD of the ammonite Psiloceras spelae tirolicum. This GSSP was ratified in April 2010 and inaugurated in 2011. The boundary is dated at approximately 201.4 ± 0.2 Ma.

The Triassic is divided into three epochs (series) and seven ages (stages), based on the ICS International Chronostratigraphic Chart v2024/12:

• Early Triassic (Lower Triassic): Induan (251.902–249.9 Ma) and Olenekian (249.9–246.7 Ma)
• Middle Triassic: Anisian (246.7–241.464 Ma) and Ladinian (241.464–237.0 Ma)
• Late Triassic (Upper Triassic): Carnian (237.0–227.3 Ma), Norian (227.3–205.7 Ma), and Rhaetian (205.7–201.4 Ma)

The Norian is the longest stage of the Triassic, spanning approximately 21.6 million years.

The Triassic began in the wake of the Permian–Triassic extinction event ("the Great Dying"), the most severe mass extinction in Earth's history. According to multiple estimates, roughly 81% of marine species, 70% of terrestrial vertebrate species, 57% of biological families, and 83% of genera were lost. The primary cause is widely attributed to massive volcanism of the Siberian Traps, a large igneous province that erupted approximately 252 Ma in what is now Siberia, releasing enormous volumes of CO₂, SO₂, and other volatiles. The resulting consequences included rapid global warming, ocean acidification, marine anoxia, ozone depletion, and acid rain.

Ecosystem recovery was slow and protracted. Some studies suggest recovery took approximately 5–10 million years, extending well into the Middle Triassic (Anisian) before marine and terrestrial ecosystems achieved pre-extinction levels of ecological complexity. Early Triassic ecosystems were marked by low biodiversity, dominance of opportunistic species (so-called "disaster taxa" such as the bivalve Claraia and the therapsid Lystrosaurus), and several distinct phases of recovery and setback.

During the Triassic, Earth's landmasses were assembled into the supercontinent Pangaea, which straddled the equator. The single global ocean, Panthalassa, surrounded Pangaea, while the Tethys Ocean (or Tethys Sea) formed a large embayment on its eastern margin between what are now southern Europe/North Africa and central-to-eastern Asia.

Pangaea began to rift apart during the Middle to Late Triassic. The initial rifting separated what would become North America and Africa, marking the incipient opening of the Central Atlantic. Seafloor spreading at divergent plate boundaries in the Tethys also drove the separation of Laurasia (north) and Gondwana (south). However, full continental dispersal did not occur until the Jurassic and Cretaceous. The rifting was accompanied by extensive igneous activity, including the formation of the Newark Basin rift basins in eastern North America and the Palisades sill.

The Triassic was one of the hottest periods in Earth's history. There is no evidence of polar ice caps at either pole. The continental interior of Pangaea experienced extreme aridity, with vast desert environments (aeolian sandstones of this age are widespread). Seasonal monsoons, driven by differential heating of land and ocean, dominated coastal regions. The temperature gradient between equator and poles was significantly weaker than at present.

High-latitude regions, particularly at the northern and southern extremes of Pangaea, were relatively wetter and supported coal-forming swamp environments. The presence of coal deposits in these high-latitude regions, as well as large amphibians, indicates more humid conditions compared to the continental interior. Sea levels were generally low relative to the continental elevations, limiting the extent of epicontinental seas.

Climate shifted toward more humid conditions as Pangaea began to break apart in the Late Triassic, increasing coastline length and oceanic influence. The Carnian Pluvial Episode (~234–232 Ma) is recognized as a significant interval of increased rainfall, possibly linked to volcanic activity and major biotic turnover.

Flora: Triassic vegetation was dominated by gymnosperms. Modern conifer families, cycads, cycadeoids (bennettitaleans), and ginkgos were prominent. In the Southern Hemisphere, the seed-fern Dicroidium dominated early and middle Triassic floras. By the end of the Triassic, conifer-cycad vegetation had become more cosmopolitan. Ferns, including the families Osmundaceae and Dipteridaceae, were common in wetter habitats. There were no flowering plants (angiosperms) during the Triassic.

Archosaurs and the rise of dinosaurs: The Triassic is sometimes described as the "Dawn of the Dinosaurs," but for most of the period, the dominant large terrestrial animals were non-dinosaurian archosaurs, especially pseudosuchians (crocodile-line archosaurs). These included:

• Rauisuchians: Large, often quadrupedal apex predators, some exceeding 6 meters in length (e.g., Postosuchus, Saurosuchus).
• Phytosaurs: Superficially crocodile-like semi-aquatic predators that filled ecological niches later occupied by crocodylians.
• Aetosaurs: Heavily armored, herbivorous or omnivorous pseudosuchians.

The earliest undisputed dinosaurs appeared during the Carnian stage of the Late Triassic, approximately 231 Ma. The most informative early dinosaur assemblages come from the Ischigualasto Formation of northwestern Argentina, which has yielded representatives of both major dinosaurian clades: Saurischia (including early theropods such as Herrerasaurus and Eoraptor, and early sauropodomorphs) and Ornithischia. High-precision U–Pb dating (Marsicano et al., 2015) has calibrated the transition from dinosauriform-dominated assemblages (Chañares Formation, ~234–236 Ma) to dinosaur-bearing assemblages within a geologically brief interval. Notably, during the Late Triassic, dinosaurs were generally small-bodied (1–3 m), bipedal, cursorial animals and were not yet ecologically dominant.

Other well-known Triassic dinosaurs include Coelophysis (a small theropod from the Late Triassic of North America), and Plateosaurus (an early sauropodomorph from the Late Triassic of Europe, reaching up to 10 m in length and representing one of the first large-bodied dinosaurs).

Mammaliaforms: The first mammaliaforms (mammals and their closest relatives) appeared in the Late Triassic, around 225 Ma. These were small-bodied, likely nocturnal insectivores. Examples include Morganucodon and Adelobasileus.

Pterosaurs: The first pterosaurs appeared during the Late Triassic, around 228 Ma (Norian stage). Early pterosaurs such as Eudimorphodon were the earliest vertebrates capable of powered flight.

Other groups: The Triassic also witnessed the first appearances of true crocodylomorphs, lepidosauromorphs (the lineage leading to lizards and snakes), and possibly the earliest turtles (e.g., Proganochelys, Norian).

Marine ecosystems underwent dramatic restructuring during the Triassic. Following the Permian–Triassic extinction, the recovery of marine invertebrate communities was gradual. Modern-type scleractinian corals first appeared and began constructing small reefs, though large reef ecosystems did not fully recover until the Middle to Late Triassic.

Marine reptiles diversified significantly. Ichthyosaurs first appeared in the Early Triassic and rapidly diversified into a range of body sizes and ecologies, including some of the largest marine predators of the era. Nothosaurs and placodonts were other prominent marine reptile groups during the Middle Triassic.

Among invertebrates, ammonoids (ceratitid ammonites), conodonts, and bivalves were important components of Triassic marine faunas. The Triassic marks the last period in which conodonts were present; they went extinct at the end of the Triassic.

The end-Triassic extinction event (also known as the Triassic–Jurassic extinction) occurred at approximately 201.4 Ma and is one of the "Big Five" mass extinctions in Earth's history. It resulted in the loss of approximately 76% of all species, including about 23–34% of marine genera and ~20% of marine families. On land, all non-dinosaurian, non-crocodylomorph, and non-pterosaurian archosauromorph reptiles were eliminated, including rauisuchians, phytosaurs, and aetosaurs.

The primary cause is widely attributed to the massive volcanic eruptions of the Central Atlantic Magmatic Province (CAMP), one of the largest known large igneous provinces. CAMP volcanism occurred as Pangaea rifted apart, producing enormous volumes of basaltic lava (covering an estimated 11 million km²) and releasing vast quantities of CO₂ and SO₂ into the atmosphere. Recent high-precision geochronological studies (e.g., those published in PNAS in 2024) have demonstrated that pulsed CAMP eruptions correlated with the terrestrial extinction at sub-centennial resolution. The consequences included rapid global warming, ocean acidification, marine anoxia, and disruption of the carbon cycle.

The extinction cleared ecological space and removed the primary competitors of dinosaurs, enabling the surviving dinosaurian lineages to diversify and achieve ecological dominance throughout the subsequent Jurassic and Cretaceous Periods. As noted by Sues (2016), the "Age of Dinosaurs" truly commenced at the beginning of the Jurassic.

The Triassic occupies a pivotal position in Earth history. It represents the interval during which life recovered from the greatest mass extinction ever recorded, and during which the foundations of modern terrestrial and marine ecosystems were established. The period witnessed the origins of dinosaurs, pterosaurs, mammaliaforms, modern conifers, and many marine reptile lineages. Pangaea's existence and subsequent initial breakup shaped global climate, ocean circulation, and biogeographic patterns in ways that would persist for hundreds of millions of years. The Triassic bookended by two mass extinction events—the Permian–Triassic at its start and the end-Triassic at its close—underscores the recurring interplay between large igneous province volcanism, climate change, and biological turnover that characterizes Earth's deep history.