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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.
“In 1834, the German salt-mining expert Freidrich August von Alberti proposed the name ‘Trias’ … for a succession of lithostratigraphic units long recognized in southern Germany, which (from oldest to youngest) are the Buntsandstein (colored sandstone), Muschelkalk (clam limestone), and Keuper…. The threefold rock succession established by Alberti corresponds roughly to the standard division of the Triassic into Lower, Middle, and Upper Triassic series, or, in units of geological time, Early, Middle, and Late Triassic epochs. … During the late nineteenth century, geologists, mostly working in the European Alps, established what would become the standard marine stage-level division for the Triassic (from oldest to youngest): Scythian, Anisian, Ladinian, Carnian, Norian, and Rhaetian…. The Scythian was subsequently further divided into the Induan and Olenekian stages” (Sues & Fraser 2010, p. 2).
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).
Land and Sea“During the entire period, there existed a single, vast landmass that encompassed all present-day continents … [named] Pangaea (Greek for ‘all earth’) (Wegener 1915). The northern portion of this landmass is termed Laurasia, whereas the southern part of the supercontinent is named Gondwana. Both portions were partially separated by the vast embayment of Tethys in the east” (Sues & Fraser 2010, p. 6). (Fig. 1)
Over the course of the Triassic, “portions of northern Gondwana separated and moved northward, closing Palaeotethys and eventually forming much of what is now eastern and south-eastern Asia during the Late Triassic and Jurassic. ¶ An immense ocean, Panthalassa (Greek for ‘all sea’), surrounded Pangaea. Global sea levels generally rose after a drop at the end of the Permian to a maximum during the Norian and then dropped again close to the end of the Triassic…” (Sues & Fraser 2010, p. 8).
Climate“Triassic climates, on both a regional and global scale, were markedly different from those of today.... [T]he polar regions ... were significantly warmer, and there is no evidence for polar ice caps. ... Triassic Earth a predominantly warm world (Sellwood & Valdes 2006). ... [M]ost regions of the supercontinent would have experienced a monsoon climate with a long dry season and a shorter wet season with abundant rainfall. … [T]he northward drift of Pangaea during the Triassic led to inceasingly drier conditions in many regions” (Sues & Fraser 2010, p. 8).
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 mid latitude regions were slightly cooler and much wetter” (Brusatte 2018, p. 21).
IntroductionThe 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.
By the end of the Triassic Period, dinosaurs, pterosaurs, lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock. “The Late Triassic (Carnian–Rhaetian, ca. 237–201 million years ago) was a formative phase in the evolution of ‘modern’ terrestrial ecosystems dominated by familiar clades such as mammals, lizards, turtles, and archosaurs” (Brusatte et al. 2015, p. 1).
PlantsAlthough the Triassic flora “included various holdovers from the Paleozoic, in addition, there were many new families as well as one new order, the Bennettitales. In fact, … the Triassic vegetation can be considered a mixture of ancient and modern. Indeed, some Triassic species are considered to be members of living genera, whereas others bear no resemblance whatsoever to modern-day taxa. There was a clear distinction during the Triassic between the land floras of the northern and southern hemispheres. Certain ferns grew in both hemispheres, but in general, there were relatively few taxa common to both the north and the south. … Although angiosperms … did not exist during the early Mesozoic, there were many other Triassic and Jurassic foliage taxa that would likely have formed dense ground cover. Herbaceous seed ferns and extensive stands of horsetails and ferns were probably common. There is certainly every reason to think that open, verdant hillsides, wooded and forested slopes, and hot and steamy forests and swamps were very much part of the Triassic landscape” (Fraser & Henderson (2006), p. 53, 54-55).
Lycopods were widely distributed, but these were herbaceous taxa similar to the modern day Lycopodium rather than the large, tree-like forms (Lepidodendron etc.) of earlier periods. Equisetales (horsetails) were also smaller than their Paleozoic ancestors, although many were still respectable trees. Having declined through the Permian, Filicales (true ferns) underwent a resurgence in diversity during the Triassic, including the appearance of some modern families such as the Osmundaceae, represented by the commonly found fossil, Cladophlebis. Cycads were more widespread than today, achieving a global distribution. Another very commonly found fossil, Taenopteris, belongs to this group. Bennettitales and Gingkoales were important floral taxa. Conifers, including araucarians (a sister group to the podocarps, such as the New Zealand kauri), included both smaller shrubs and large woody trees, such as the massive tree trunks of Araucarioxylon arizonicum, the state fossil of Arizona. (Summarised from Fraser & Henderson (2006), p. 55-60.)
“Triassic tetrapods have a rich fossil record…. Most of the principal groups of extant amphibians and amniotes or their closest relatives made their first appearance in the fossil record during the Triassic…. In addition, the first dinosaurs and pterosaurs originated during this period” (Sues & Fraser 2010, p. 11).
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 Metoposauridae (Fig. 2), a clade of temnospondyl amphibians that were globally distributed in low paleolatitudes during the Late Triassic, were some of the most common and characteristic non-marine vertebrates, particularly during the early Carnian–middle Norian. They achieved mid- to large size (1.25–3 m body length), and filled crocodile-like predatory niches in lacustrine and fluvial environments (after Brusatte et al. 2015).
The Archosauromorpha, which comprises true 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 remained so for much of the Mesozoic. One of the oldest known crown members of this group is the Olenekian (Early Triassic) species Ctenosauriscus koeneni known from Germany. Vytshegdosuchus zbeshartensis, from Russia, is also Early Triassic but less certainly a member of the group. Another species of similar or slightly younger age is Xilousuchus sapingensis from China (Nesbitt et al. 2011; also see Butler et al. 2011, p. 24, for further discussion of the age). The archosaurs gave rise to the dinosaurs in the Late Triassic and continues to be an important component of modern ecosystems. In general, however, little is known about the timing and tempo of their early evolution. (After Parrish 1997 and Butler et al. 2011.)
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.
The origins of dinosaurs are poorly understood, and many details remain unclear, but the event is likely to have occurred in the Early Triassic.
One of the earliest contenders for a “proto-dinosaur” is the ichnofossil Prorotodactylus, a trackway known from Poland and dated to approximately 250 Ma (Early Triassic). The individual prints are “about the size of a cat’s paw, arranged in narrow trackways, with the five-fingered handprints positioned in front of the slightly larger footprints, which have three long central toes flanked by a toe nubbin on each side. … [T]he narrow distance between left and right tracks [indicates] that they belong to a specialized group of reptiles called archosaurs that [had] an upright posture…. Almost as soon as the archosaurs originated, they branched into two major lineages … the pseudosuchians, which led to today’s crocodiles, and the avemetatarsalians, which developed into dinosaurs. … [The Prorotodactylus prints have characteristics] that link them to signature features of the dinosaur foot: the digitigrade arrangement of the bones, in which only the toes make contact with the ground while walking, and the very narrow foot with three main toes” (Brusatte 2018, p. 21).
“Moderately large size and bipedal posture among dinosauromorphs are first known in the latest Olenekian–Early Anisian [Early to Middle Triassic], and tracks that may belong to true dinosaurs are present in the Ladinian of Europe ... and South America.... The first dinosaur body fossils are known from near the Carnian–Norian boundary [Late Triassic] ... but only in the Norian did dinosaurs diversify into the range of shapes and sizes (morphological disparity) characteristic of their post-Triassic history.... Finally, after the Triassic–Jurassic transition, dinosaurs experienced a burst of diversification … and became the dominant mid-to-large size terrestrial vertebrates in ecosystems worldwide” (Brusatte et al. 2011, p. 1112).
True dinosaurs are distinguished from dinosauromorphs by “a long scar on the upper arm that anchored bigger muscles, some tablike flanges on the neck vertebrae that supported stronger ligaments, and an open, windowlike joint where the thighbone meets the pelvis that stabilized upright posture” (Brusatte 2018, p. 22).
These innovations are believed to have evolved between about 240 and 230 Ma. The oldest unequivocal dinosaur fossils lie at the younger end of this range: several species including, most famously, Herrerasaurus, Eoraptor, and Pisanosaurus, are known from the middle Carnian age (~230 Ma) Ischigualasto Formation of Argentina (Sereno & Novas 1992, Sereno et al. 1993). Pisanosaurus is an ornithischian and Herrerasaurus is a theropod. Eoraptor was also originally described as a theropod although it now seems more likely to be a sauropodomorph (Martinez et al. 2011). It appears that all three main branches of the dinosaur clade were present from very soon after the group first evolved.
In an article for Scientific American, Stephen Brusatte (2018) describes what he describes as dinosaurs’ “humble” origins, and somewhat protracted emergence through the Triassic.
Despite the presence of excellent fossil assemblages (e.g., of amphibians and reptiles) in the arid, equatorial regions of Triassic Pangea, early dinosaurs do not occur among them; the group appears to have been confined to the temperate regions of the Triassic world, and unable to colonise the deserts during the early stages of their evolution. In more humid climates, however, “the dominant large herbivores of the time – reptiles called rhynchosaurs and mammal cousins called dicynodonts – went into decline, disappearing entirely in some areas for reasons still unknown. Their fall from grace between 225 million and 215 million years ago [Norian Age] gave primitive plant-eating sauropodomorphs such as Saturnalia, a dog-size species with a slightly elongated neck, the opportunity to claim an important niche. Before long these sauropod precursors were the main herbivores in the humid parts of the Northern and Southern Hemispheres. Second, around 215 million years ago [mid Norian] dinosaurs finally broke into the deserts of the Northern Hemisphere…”
However, “wherever they lived … they were surrounded by all kinds of bigger, more common, more diverse animals. In Argentina’s Ischigualasto, for instance, those earliest dinosaurs made up only about 10 to 20 percent of the local ecosystem. The situation was similar in Brazil and, millions of years later, at the Hayden Quarry [New Mexico]. In all cases, the dinosaurs were vastly outnumbered by mammal forefunners, giant amphibians and eccentric reptiles. … All throughout the Triassic, the pseudosuchians were more anatomically diverse than the dinosaurs…” (Brusatte 2018, p. 24-25).
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.
Fig. 2: Metoposaurus algarvensis reconstruction by Marc Boulay, copied from Brusatte et al. 2015 (fig. 12).
ExtinctionsThe final three ages of the Triassic are the Carnian, Norian and Rhaetian, and it seems as if significant extinction events occurred within or at the end of all of them, most significantly at the end of the Rhaetian, the Triassic-Jurassic boundary.
Notably among vertebrates, the “crocodile-line archosaurs were decimated, with only a few species – the ancestors of today’s crocodiles and alligators – able to endure” (Brusatte 2018, p. 25), the removal of which may have contributed to the great success of the dinosaurs in the succeeding Jurassic Period.
New Zealand Occurrences
Brusatte, S. 2018: Dinosaurs: From Humble Beginnings to Global Dominance. Scientific American 23 May: 20-25.
Brusatte, S.L.; Butler, R.J.; Mateus, O.; Steyer, J.S. 2015: A new species of Metoposaurus from the Late Triassic of Portugal and comments on the systematics and biogeography of metoposaurid temnospondyls. Journal of Vertebrate Paleontology 35 (3): 1-23.
Brusatte, S.L.; Niedźwiedzki, G.; Butler, R.J. 2011: Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic. Proceedings of the Royal Society, Series B 278: 1107-1113.
Butler, R.J.; Brusatte, S.L.; Reich, M.; Nesbitt, S.J.; Schoch, R.R.; Hornung, J.J. 2011: The sail-backed reptile Ctenosauriscus from the latest Early Triassic of Germany and the timing and biogeography of the early archosaur radiation. PLoS One 6 (10): e25693.
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.
Fraser, N.; Henderson, D. (illus.) 2006: Dawn of the dinosaurs. Indiana University Press: 1-307.
Gradstein, F.M.; Ogg, J.G.; Schmitz, M.D.; Ogg, G.M. 2012: The Geologic Time Scale 2012. Elsevier 1-2.
Martinez, R.N.; Sereno, P.C.; Alcober, O.A.; Colombi, C.E.; Renne, P.R.; Montañez, I.P.; Currie, B.S. 2011: A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea. Science 331: 206-210.
Nesbitt, S.J.; Liu, J.; Li, C. 2011: The oldest archosaur: A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Earth Env Sci Trans Roy Soc Edinburgh 101: 271-284.
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.
Sellwood, B.W.; Valdes, P.J. 2006: Mesozoic climates: General circulation models and the rock record. Sedimentary Geology 190: 269-287.
Sereno, P.C.; Forster, C.A.; Rogers, R.R.; Monetta, A.M. 1993: Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria. Nature 361: 64-66.
Sereno, P.C.; Novas, F.E. 1992: The complete skull and skeleton of an early dinosaur. Science 258: 1137-1140.
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.
Sues, H.-D.; Fraser, N.C. 2010: Triassic life on land. Columbia University Press: 1-236.
Wegener, A. 1915: Die Entstehung der kontinente und ozeane. Braunschweig: Friedrich Vieweg & Sohn.
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