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AbstractThis page describes the Triassic Period, including stratigraphy, paleogeography, and famous lagerstätten, followed by a sketched outline of some of the major evolutionary events.
Keywords: Triassic Period, Triassic biota, fossil record, evolution, extinction
During the early part of the Triassic Period, much of the world was arid, becoming cooler and wetter towards the end. Popular interest in the Mesozoic is largely focused upon the Jurassic and Cretaceous and essentially preoccupied with crown group dinosaurs. The interesting evolution, however, mostly occurred deep in the Triassic. By the end of this period, dinosaurs, pterosaurs, lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock.
Lower (Permian–Triassic) Boundary
The mass extinction event which marks the Permian-Triassic boundary is discussed elsewhere. Stratigraphically, the boundary is defined at the first occurrence of the conodont Hindeodus (al. Isarcicella) parvus within the evolutionary lineage Hindeodus typicalise – H. Latidentatus praeparvuse – H. parvuse – H. postparvus as the primary correlation marker for the base of the Mesozoic and Triassic (Gradstein et al. 2012, p. 683).
Upper (Triassic–Jurassic) Boundary
“The end-Triassic mass extinction terminated many groups of marine life, including the conodonts, whose distinctive phosphatic jaw elements constitute a primary zonation for much of the Paleozoic and Triassic, and the majority of ammonoids. Indeed, in the few regions with continuous deposition there is an interval devoid of either typical latest-Triassic taxa (e.g., conodonts or Choristoceras ammonoids) or earliest-Jurassic forms (e.g., Psiloceras ammonites). A sea-level fall produced extended gaps in many shallow-marine sections; therefore, the boundary between upper Triassic and the overlying lower Jurassic was commonly a sequence boundary and hiatus” (Gradstein et al. 2012, p. 733, and references therein).
The GSSP for the base of the Jurassic is set at 5.80 m above the base of the Tiefengraben Member of the Kendelbach Formation, corresponding to the local lowest occurrence of the ammonite Psiloceras spelae subsp. tirolicum, in the Kuhjoch section, Northern Calcareous Alps, Austria. Other useful markers include the FAD of Cerebropollenites thiergartii (a pollen grain), Praegubkinella turgescens (a foraminifer), and Cytherelloidea buisensis (an ostracod) (see Gradstein et al. 2012).
The Triassic Period spans the interval from 252.17 ±0.06 to 201.3 ±0.2 Ma (Cohen et al. 2015).
Major Tectonic Events
Land and Sea
During the Triassic, the Earth “was much warmer, and because Pangea was centred on the equator, half the land was always scorching in the summer while the other half was cooler in the winter. These marked temperature differences fueled violent ‘mega monsoons’ that divided Pangea into environmental provinces characterized by varying degrees of precipitation and wind. The equatorial region was unbearably hot and muggy, flanked by subtropical deserts on both sides. The midlatitude regions were slightly cooler and much wetter” (Brusatte 2018, p. 21).
The large terrestrial animals living at the end of the Permian – “an exotic menagerie of large amphibians, knobbly-skinned reptiles and flesh-eating forerunners of mammals” (Brusatte 2018) – were decimated by the end-Permian mass extinction. The generally small creatures which survived into the Triassic underwent rapid radiation from which arose modern lineages of amphibians, reptiles and mammals, as well as the dinosaurs.
The earliest unambiguous dinoflagellate fossils are Triassic in age. Like most microscopic fossils, the dinoflagellates have a rather low profile with the general public, but they are important for Mesozoic and Cenozoic biostratigraphy.
Triassic vertebrates were dominated by two different amniote clades: the Synapsida and the Archosauromorpha. The Synapsida, which includes mammals and their close relatives, were the most abundant and diverse of terrestrial vertebrates. Non-mammalian synapsids were extremely abundant during the Early and Middle Triassic, although the true mammals are not known until the Late Triassic.
The Archosauromorpha, which comprises crocodilians, dinosaurs, pterosaurs and others, first appeared in the Late Permian and became important components of the Early and Middle Triassic faunas. A group of archosauromorphs known as archosaurs became dominant in the Middle Triassic and gave rise to the dinosaurs in the Late Triassic. (After Parrish 1997.)
Major Biotic Events
By the end of the Triassic Period, dinosaurs, pterosaurs, lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock.
A Triassic origin for the angiosperms?
Modern squamates (lizards, snakes and amphisbaenians) are the world’s most diverse group of tetrapods along with birds and have a long evolutionary history, with the oldest known fossils dating from the Middle Jurassic period – 168 million years ago. Megachirella wachtleri is a lepidosaurian reptile from the Middle Triassic of the Italian Alps. Simoes et al. (2018) analysed X-ray computed tomography data to re-evaluate the diapsid phylogeny and present evidence that M. wachtleri is the oldest known stem squamate. Megachirella is 75 million years older than the previously known oldest squamate fossils, partially filling the fossil gap in the origin of lizards, and indicating a more gradual acquisition of squamatan features in diapsid evolution. Divergence time estimates using relaxed combined morphological and molecular clocks show that lepidosaurs and most other diapsids originated before the Permian/Triassic extinction event, indicating that the Triassic was a period of radiation, not origin, for several diapsid lineages.
Origin of the Mammals
The earliest fossil mammals are early Mesozoic, the exact age being dependent upon which fossils one accepts as meeting the definition of ‘mammal.’ Conventionally, mammals are recognised by their jaw morphology: how the jaw articulates with the skull and incorporation of two small bones into the inner ear. In reptiles – including the mammalian ancestors – the jaw joint is hinged on two small bones; one (the quadrate) linked to the squamosal bone of the skull and the other (the articular) to the lower jaw itself (the dentary). In true mammals, the dentary is hinged directly to the squamosal; the quadrate and articular bones are incorporated into the mammals inner ear, becoming the incus and malleus respectively.
The Late Triassic morganucodontids exhibit an intermediate jaw morphology, neither completely reptilian nor yet fully mammalian: They had a jaw in which the dentary articulated with the squamosal but which still included articular and quadrate bones; these had not yet evolved to form the malleus and incus of the true mammalian inner ear. The morganucodontids are the oldest and most primitive of the triconodontans so are sometimes (e.g. Rich et al. 1996, p. 519) regarded as the first mammals.
New Zealand Occurrences
Brusatte, S. 2018: Dinosaurs: From Humble Beginnings to Global Dominance. Scientific American 23 May: 20-25.
Cohen, K.M.; Finney, S.C.; Gibbard, P.L.; Fan, J.X. 2015: The ICS international chronostratigraphic chart v 2015/01. Episodes 36: 199-204.
Gradstein, F.M.; Ogg, J.G.; Schmitz, M.D.; Ogg, G.M. 2012: The Geologic Time Scale 2012. Elsevier 1-2.
Ogg, J.G.; Ogg, G.; Gradstein, F.M. 2008: The Concise Geologic Time Scale. Cambridge University Press: 1-177.
Parrish, J.M. 1997: Triassic Period. In Currie, P.J.; Padian, K. (ed.) 1997: Encyclopedia of dinosaurs. Academic Press: 747-748.
Rich, P.V.; Rich, T.H.; Fenton, M.A.; Fenton, C.L. 1996: The Fossil Book. Dover.
Simões, T.R.; Caldwell, M.W.; Tałanda, M.; Bernardi, M.; Palci, A.; Vernygora, O.; Bernardini, F.; Mancini, L.; Nydam, R.L. 2018: The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps. Nature 557: 706-709.
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