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The Triassic spans from 251 to 199.6 million years ago. In Germany and northwestern Europe three distinct layers can be found together, red beds capped by chalk, overlaid by black shale. In 1834 a German geologist Friedrich August von Alberti (1795-1878) named these layers the Trias. The Triassic received its name from Alberti’s Trias (Kazlev, 2002, Triassic Page).
Life was sparse in both the seas and on land after the Permian extinction. Adaptive radiations from the few survivors would help to create new flora and fauna representing a new era, the Mesozoic.
Primary Producers & Reefs
Dinoflagellates (phylum Dinoflagellata) make their first appearance in the mid-Triassic. Dinoflagellates are typically, unicellular protists with two flagella. Some dinofagellates have a protective coat made of cellulose and silica, which might remind one of medieval armor. Coccolithophores (phylum Haptophyta) make their first appearance in the late Triassic (DeVargas, Aubry, Probert, & Young, p. 267). Coccolithophores are unicellular protists, with two flagella, that produce coccoliths (calcium carbonate shield structures) as an outer covering. In addition to being an important primary producer today, coccolithophores are responsible for precipitating over half of all the calcium carbonate in the oceans, making them major players in the global carbon cycle (DeVargas, Aubry, Probert, & Young, p. 252). Photosynthetic species of dinoflagellates and coccolithophores make up an important component of today’s plankton. Green algae and cyanobacteria remained the dominant primary producers in the early Triassic; by later Triassic, cyanobacteria took on only a minor role for the first time.
Earth's oceans have experienced two major shifts in the composition of primary producers. Initially, cyanobacteria along with other photosynthetic bacteria were the primary producers during the Proterozoic eon. The first shift occurred during the early Paleozoic era when eukaryotic green algae joined cyanobacteria in being major primary producers. The second shift would occur during the Mesozoic era when dinoflagellates and coccolithofores would be joined by diatoms in the Jurassic. Diatoms, dinoflagellates, and coccolithophores would assume their dominant role as the base of many modern marine ecosystems by Cretaceous times. (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 155).
Reef systems are absent from early Triassic deposits. In the mid-Triassic reestablished reefs were in many ways very similar to that found in the Permian. The main reef-building organisms consisted of sponges, calcareous algae, and the problematic, tiny, branching, tube-like organism Tubiphytes (Kiessling, 2001, p. 45). Corals in the order Scleractinia, which help to build modern reef systems, make their first appearance in the Triassic. By the end of the Triassic Scleractinians were building reefs in southern Europe, Southeast Asia, Alaska, and California (Rich, 1996, p. 136). As Scleractinian corals became more important, a shift to modern-like reefs was occurring. Scleractinian corals had finally acquired rapid growth and reef building potential seen in modern forms. It is at this time that some Scleractinian corals obtained a symbiotic relationship with zooxanthellae. Zooxanthellae are dinoflagellates that live intracellularly as endosymbionts in some marine organisms, such as scleractinian corals. Zooxanthellae can contribute up to 90 percent of the corals energy requirements through photosynthesis. Carbon isotope signatures left by zooxanthellae can be found in Late Triassic corals (Stanley, 2001, p. 26).
Marine Invertebrates
The diversity of species during the early Triassic was reduced. Although ammonoids barley escaped extinction the survivors became abundant and diversified. Ammonites are the most common invertebrate fossils in early Triassic deposits followed by bivalves, and brachiopods. These three groups would prove to be very successful in the Mesozoic. Belemniods (subclass Coleodia) which first appeared in the Carboniferous become very abundant by the late Triassic. Foraminiferans, gastropods, bryozoans, crinoids, sea urchins, and sponges are rare (Stanley, 1988, pp. 109-110). New orders of echinoderms evolve during the Triassic. Although corals are hit hard by the Permian crises they make a rebound and become the dominant structural components of many reef systems, a position their descendents hold today.
Fish
Cartilaginous fish evolve into modern forms during the Mesozoic. Hybodus was a common shark that would remind one of the modern Blue shark. The male Hybodus has modified pelvic fins called claspers. Claspers allow the sperm to be directly deposited into the female. All the descendents of Hybodus have retained this advantage.
Palaeoniscids (primitive ray-finned fishes) diversified in the Triassic. Palaeoniscids possessed thick, heavy, non-overlapping, articulated scales. Two paired sacs connected to the gut could be used to control buoyancy. Palaeoniscids had a single dorsal fin, an asymmetrical caudal fin, and primitive jaws. The term Palaeoniscid has traditionally been used to group basal actinopterygians (primitive ray-finned fish) that span from the Carboniferous to the Triassic. Palaeoniscid is a waste basket term and no longer used because it groups several independent evolutionary lines (Benton, 2005, p. 170).
During the Triassic a new group of ray-finned fish evolved, known as the neopterygians. Neopterygians showed some of the features exhibited by modern ray-finned fish. Lepidotes possessed a new jaw structure. The upper jawbones were freed from the cheekbones. Lepidotes could form its mouth into a tube shape allowing it to “suck” in prey from a distance, as do many modern bony fish (Dixon, 1988, p 37). By the end of the Triassic the neopterygians gave rise to the modern ray-finned fish (Order Teleostei). Leptolepis was one of the first teleosts. Leptolepis skeleton was made entirely of bone and its scales were thin, rounded and lacked an enamel coat.
Amphibians
Many surviving amphibians became adapted to aquatic environments. Paleontologists think members of the temnospondyl labrinthodonts gave rise to today’s frogs and toads. A primitive, intermediate, frog-like organism appears in the early Triassic. Triadobatrachus possessed a frog-like skull with tympanic membranes for hearing. Unlike modern frogs Triadobatrachus had more vertebrae and a tail. The first true frog would not appear until the Jurassic (Dixon, 1988, p. 57). The reptile-like mammals went extinct during the Early Triassic, but one of their evolutionary lines, the amniotes, would live on.
Reptiles
Anapsid reptiles in the family Procolophonidae were insectivores during the Permian and early Triassic, but evolved into herbivorous forms during the middle Triassic. Another group related to the anapsids, the chelonians (turtles, tortoises and terrapins), makes it first appearance during the late Triassic. Proganochelys was a primitive chelonian with a tortoise shape. Proganochelys had a shell structured like a modern tortoise. Proganochelys could not retract its limbs or head into the shell; the shell had more plates than modern forms. Proganochelys also had some teeth on its palate (Dixon, 1988, p. 68).
Diapsid type reptiles and their close relatives evolved to fill many aquatic and terrestrial niches as well as taking to the air. Several groups of marine reptiles make their first appearance in the Triassic. Placodonts, nothosaurs, ichthyosaurs, and plesiosaurs are thought to have evolved from reptiles with diapsid type skulls, although their skull type is sometimes called euryapsid.
Placodonts were the least specialized reptilian swimmers; some evolved turtle-like shells. The evolution of these turtle-like shells is a good example of convergent evolution as placodonts and turtles evolved from different reptilian lines. Many placodonts specialized in eating shellfish. Nothosaurs had streamlined bodies with long necks and tails. Most nothosaurs had webbed feet, while others had flippers. Nothosaurs ate fish. Ichthyosaurs were highly specialized for marine life. Ichthyosaurs filled a niche similar to modern day dolphins. Ichthyosaurs possessed streamline bodies not unlike tuna. Ichthyosaurs were air-breathing reptiles that propelled themselves quickly through the water with a powerful tail. These reptiles were so specialized for aquatic life that they gave birth to live young in their marine habitat. Ichthyosaurs were well adapted for pursuing and eating fish (Dixon, 1988, pp 72-81). The ichthyosaur Shonisaurus popularis is the state fossil for Nevada. This 50 foot long ichthyosaur preyed on chephalopods while cruising in the Triassic seas. The remains of 37 specimens were uncovered from what is now Berlin-Ichthyosaur State Park in Nevada. Plesiosaurs make their first appearance in the Triassic. Plesiosaurs undergo an adaptive radiation early in the Jurassic and become the dominant marine reptiles for the rest of the Mesozoic.
In addition to the marine reptiles mentioned above, several orders of early diapsid type reptiles make their appearance in the Triassic; both aquatic and terrestrial forms evolved within these groups. The tuatara (Sphenodon punctatus) of New Zealand is a sole survivor of one of these orders (Sphenodonta). One of the earliest known lizards living in the Triassic, Kuehneosaurus, possessed a pair of membranous wings. Lizards would undergo a great adaptive radiation in the Jurassic. The rhynchosaurs (a subgroup of archosauromorphs) were most abundant reptiles in the Triassic. These barrel-shaped herbivores were well adapted to eating seed ferns (Dixon, 1988, pp. 88-89). The earliest Archosuars, the thecodontians diversify into both terrestrial and aquatic forms. Although they go extinct by the end of the Triassic they lead to the evolutionary lines for dinosaurs, pterosaurs, and crocodiles. The earliest crocodiles (Crocodylia) make their first appearance in the mid-Triassic. The first crocodiles were small terrestrial carnivores.
The first flying vertebrates, the pterosaurs, make their appearance in the Triassic. Rhamphorhynchs were the first group of pterosaurs to evolve. Eudimorphodon appears in the late Triassic and exhibits many of the characteristics of the group. Eudimorphodon’s wings, like all pterosaurs, were made of membranous skin attached along the length of the elongated forth finger and connecting back along the body to the level of the thigh. Wing-like skin also attached the wrist bones and neck. A vertical, diamond-shaped flap of skin adorned the tip of the tail and was probably used as a rudder in flight. Eudimorphodon had two types of specialized teeth for capturing and consuming fish.
Dinosaurs can be divided into two orders based upon their hip and jaw structures. Saurischia includes dinosaurs with a lizard-like hip structure. Ornithischia includes dinosaurs with hip structures reminiscent of birds. Representatives from both groups appear in the Triassic period.
Saurischian dinosaurs include the bipedal, carnivorous theropods and the quadruped, herbivorous sauropods. Small eating theropods called coelurosaurs were quick, agile and may have hunted in packs. Procompsognathus, Saltopus, and Coelophysis were some of the first coelurosaurs. A primitive carosaur (large theropod) Teratosaurus makes its appearance in the late Triassic. Primitive sauropods, called prosauropods, make their first appearance in the Triassic. Thecodontosaurus, Efraasia, Massospondylus, Plateosaurus, and Mussaurus were among the first prosauropods.
All ornithischians were herbivorous. Ornithischians can be divided into five major groups: ornithopods (bipedal hadrosaurs and duck-bills), ceratopians (quadruped, horned and frilled dinosaurs), stegasaurs (quadruped, plated dinosaurs), pachycephalosaurs (bipedal, dome-headed dinosaurs), and ankylosaurs (quadruped, armored dinosaurs). The first ornithischian to make its appearance in the late Triassic was Pisanosaurus (a ornithopod). The remaining Ornithischians would not appear until the Jurassic and Cretaceous. Different assemblages of dinosaurs appear at different times during the Mesozoic. Roughly 7% of dinosaurs known come from the Triassic and as we can see from the above, almost all are saurischians (Dixon, 1988, p.93).
Protomammals
Different groups of synapsids found success at different times during the Triassic. Therapids (mammal-like reptiles) were numerous and diversified during the Triassic. The dicynodonts were herbivorous therapsids that were very successful during the early Triassic, but become extinct by mid-Triassic. Lystrosaurus is one of the most important fossils in this group. The geographic distribution of Lystrosaurus provides biological evidence that India and the southern continents were connected as Gondwanaland during the late Permian and Triassic times (Dixon, 1988, p. 192). The cynodonts were not only successful during the Triassic and Jurassic; they are the ancestors to present day mammals. While most cynodonts were carnivores, such as, Thrinaxodon and Cynognathus some were herbivorous like Massetognathus. The geographic distribution of Cynognathus helps to establish that present-day Africa and South America were joined during the Triassic period.
The synapsid fossil record is rich with transitional forms illustrating evolutionary trends in skeletal structure, which lead to and define mammals. The synapsid article in the Science Olympiad section of our website explores some of these skeletal transitions (jaw and ear) revealed by the fossil record. Triassic carnivorous cynodonts over time acquired an increasing number of mammalian skeletal features. Determining when these organisms became mammals is difficult because they exhibit a mosaic of mammalian and primitive amniote characteristics
The
traditional paleontological view suggests the dentary-squamosal
jaw joint is the key mammalian character that defines the node for
the clade mammalia, although others use the presence of an incus
and malleus in the middle ear to define a mammal (Prothero, 2004,
p. 399). For further discussion on the debate regarding how the clade
Mammalia should be defined read our article Mammals: Characteristics
and Origins in the Science Olympiad section of our museum.
Mammals
Adelobasileus from the Late Triassic of Texas, at 225 Ma,
represents the earliest mammalian candidate. Ironically, Adelobasileus is not defined as a mammal by its jaw or teeth. Known only from
a partial skull, it possesses a number of mammalian braincase features
(Benton, 2005, pp. 298-300).
Haramiyidans (order Haramiyida) represent the next earliest known
mammals and range from the Late Triassic to the Mid Jurassic. Haramiyidans
first appear in the Late Triassic of Germany (Kemp, 2005, p. 140).
Haramiyavia represents one of the most complete haramiyidan fossils
from the Late Triassic of Greenland. Haramiyidans teeth indicate
they were small herbivores or at least omnivores (Kemp, 2005, p.
140). At present, Haramiyidans are the earliest known herbivorous
mammals.
Morganucodonts (order Morganucodonta) is a well-known group of early
Mesozoic mammals. Morganucodonts were shrew-sized nocturnal insectivores.
Morganucodon is a weasel like morganucodon of the Late Triassic and
Early Jurassic of England and China. Morganucodon possesses a mammalian
dentary/squamosal jaw joint as well as a reduced, but functional,
quadrate/articular jaw joint (Rose, 2006, p. 52). The fact that Morganucodon had a double jaw articulation implies that the malleus and incus
were not yet a part of the middle ear. The differentiated teeth of
Morganucodon are diphyodont, meaning they have two sets of teeth
during life; this is in contrast with early amniotes and reptiles
that have undifferentiated teeth that are constantly replaced throughout
life (polyphydont). However, its teeth did not show precise occlusion
like later mammals. Morganucodon is known only from skulls, but the
skeleton of its close relative Megazostrodon of South Africa is known.
The skeleton of Megazostrodon exhibits a mosaic of mammal and primitive
synapsid characteristics. Megazostrodon, like modern mammals, had
its ribcage restricted to the thorasic vertebrae. Megazostrodon had
a pelvic girdle that was more mammal-like; however, their pectoral
girdle was more like early synapsids. Megazostrodon was designed
as a predator, probably hunting insects and smaller vertebrates among
the leaf litter at night.
Sinoconodon from the Late Triassic of China, at (208 Ma),
may also occupy a basal position in mammalian phylogeny. Sinoconodon is
an intermediate form in that it possessed both mammal-like and early
amniote characteristics. Sinoconodon has braincase features
like
Adelobasileus and possesses a fully developed dentary/squamosal
jaw joint. Sinoconodon also possesses some non-mammalian
features, which include serial tooth replacement (polyphydont) and
sustained
growth
throughout life (Kemp, 2005, p. 142). Triassic mammals were small,
nocturnal, highly active organisms with relatively large brains.
Most mammals would remain this way throughout most of the Mesozoic.
Insects
New orders of winged insects make their first appearance in the Triassic. Among winged insects with incomplete metamorphosis (exopterygote neoptera) walking sticks (order Phasmida) appear. Among winged insects with complete metamorphosis (endopterygote neoptera) primitive flies from the order Diptera make their appearance and primitive, non-social members of the order Hymenoptera (sawflies) (Carpenter & Burnham, 1985, p. 309). The first definitive beetle borings are from the Triassic. Leaf mines are meandering tunnels produced by the feeding larvae of some beetle, fly, and sawflies. The first definitive leaf mines first appear in the leaves of Triassic conifers and pteridosperms. Interestingly, the abundance and diversity of fossil leaf mines coincides with the radiation of flowering plants (Angiosperms) during the Cretaceous (Grimaldi & Engel, 2005, pp. 53-54).
Plants
The
transition from the Paleophytic flora (dominated by clubmosses,
ferns, seed ferns and
cordaite types) to the Mesophytic flora (dominated
by conifers, cycad-like plants, and ginkgophytes) started in the
Permian and extended into the Triassic period. The change in
flora represents
a change in reproductive strategy. Clubmosses, horsetails, and ferns,
which all reproduce with spores constituted the dominant floras
of the Paleophytic.
The conifers, which reproduce using seeds, would be the dominant
floras of the Mesophytic (Kenrick & Davis, 2004, p. 143-144).
Overall, the climate of the Triassic was warmer and dryer.
This would have given seed plants an advantage over many spore
producing plants. Oxygen levels in the atmosphere were lower.
These conditions may explain why the recovery of plants during
the Triassic was slow and why plant diversity remained low.
Coal deposits during the
Early
Triassic
are almost
unknown. By the Middle Triassic
plant diversity started to increase with many modern families
of ferns and conifers appearing (Cleal & Thomas, 2009,
p. 211). Benettitales, Woodworthia,
and Schilderia make their first
appearance during the Triassic.
Gres a Voltzia
The Gres a Voltzia is a Fossil-Lagerstatte in the northern Vosges Mountains of northeastern France that preserves a Triassic deltaic environment. Sedimentary deposits in this Lagerstatten represent point bars, brackish ponds, and incursions from seawater during storms. The delta was in a semi-arid environment, which experienced wet and dry seasons. Gres a Votzia takes its name from the most abundant conifer found at this site Voltzia heterophylla. Along with confiers, horsetails, lycopods, ferns, cycads and ginkos have been found. Among the aquatic life is found: jellyfish, Lingula (an inarticulate brachiopod), polychaete worms, mollusks, horseshoe crabs, insect larva and eggs, shark egg cases, primitive ray-finned fish, lobe-finned fish, and temnospondyl amphibians. Among the terrestrial animals found are: the first funnel-web spider, scorpions, millipedes, mayflies, dragonflies, cockroaches, beetles, scorpion flies, true flies, bugs, and reptilian trackways (Selden & Nudds, 2004, pp 71-78).
The Chinle Group
The Chinle Group consists of late Triassic deposits exposed in Arizona, Colorado, Nevada, New Mexico, and Utah. The Chinle deposits were formed in a low relief floodplain with meandering rivers and lacustrine environments. Strong monsoonal seasons punctuated by severe draught conditions defined the weather patterns at this time and location. Volcanic activity is also recorded within the formations. Three locations are of particular importance in giving paleontologist a fossil lagerstatte, which provides a window into the late Triassic. The Petrified Forest National Park in Arizona, Placerias Quarry in Arizona, and Ghost Ranch in New Mexico.
Permineralized logs in the Petrified Forest National Park represent large gymnosperm trees, mostly Araucarioxylon arizonicum, transported by water and deposited on flood plains. Araucarioxylon arizonicum is the state fossil for Arizona. Placerias Quarry near St. John, Arizona contains a variety of terrestrial organism, with Placerias, a heavily built herbivorous dicynodont protomammal, being the most common. It is believed that herds of Placerias concentrated around a diminishing water supply during draught conditions. Later, their bodies were covered where they lay with sediment from floods. Ghost Ranch in New Mexico represents a bone bed dominated by specimens of Coelophysis, a small theropod dinosaur. Coelophysis bauri is New Mexico's state fossil. The victims of this bone bed may have died as a result of drought and were subsequently transported and deposited into a bone bed by flooding. The term allochthony is used when organisms are preserved in a location at which death did not occur, as is the case for the large trees that were transported or the Coelophysis carcasses, which were redeposited. The term autochthony is used to describe fossils that are preserved where they lived and died, as is the case for the Placerias herds.
The Chinle deposits preserve a variety of organisms, which represent terrestrial and freshwater environments. Among Saurischian dinosaurs Coelophysis is well represented, while only the teeth of the Ornithischian Revueltosaurus have been found. Among protomammals the dicynodont Placerias is well represented, while only the teeth of a carnivorous cynodont have been recovered. The most common predators are represented by archosaurs of the crocodile lineage. Phytosaurs, Crocodylomorphs, and rauisuchids remains are represented, with Postosuchus being the top predator. Stagonolepis is an interesting archosaur in the order Aetosauria. These crocodile-like organisms with pig-shaped snouts and peg-like teeth were herbivores. Amphibians are represented by the labyrinthodont Metoposaurus. Sharks, lungfish, coelacanths, paleoniscids, redfieldiids, and semionotids represent fish. Freshwater mollusks, insects, and the first appearance of freshwater crayfish can be found in the Chinle group. A variety of fossilized plant material is found in the Chinle including lycopods, hosetails, ferns, seed ferns, cycads, bennettitaleans, ginkos, and conifers (Nudds & Selden, 2008, pp. 138-149).
Using the Chinle material as a model, Walt Wright, describes a possible forest of the time. Gymnosperms such as Araucarioxylon, Dadoxylon, Woodworthia, Schilderia, and Ginko helped to make up the canopy of many forests. Cycadales such as Lyssoxylon and Charmorgia and Bennettitales such as Williamsonia and Bucklandia helped to make up the understory of forests. Cycadophytes include the orders Cycadales (true cycads) and Bennettitales (Cycadeoidales). Cycadophytes had short, squat to columnar trunks with a covering of leaf bases and mostly pinnately divided leaves. The difference between these two orders is in their cone attachment and the structure of their leaf traces (Tidwell, 1988, p. 196). Ferns like Itopsidema and Donwelliacaulis helped to make up the ground cover. Interestingly, lightning scars, damage by fungus, insects, and fire are also preserved within the permineralized wood structure of some specimens (Wright, 2002, pp. 125-131).
Mass Extinction
The Triassic ended with mass extinctions in marine and terrestrial environments. The terrestrial extinctions took place millions of years before the marine crises. The Triassic crisis is actually several extinctions that took place over a 17 million year time span (Prothero, 2004, p. 91). Labyrinthodont amphibians and dicynodonts (a group of mammal-like reptiles) went extinct. Land plants were hit hard, especially the gymnosperms with 23 of their 48 known families going extinct during the last third of the Triassic (Cleal & Thomas, 2009, p. 211). In the marine realm placodonts, nothosaurs, and conodonts went extinct. Ammonoids, brachiopods, gastropods, and bivalves took heavy losses. It is estimated that 20% of marine families went extinct during the Triassic crises. Reef growth was greatly reduced as well as marine limestone and dolomite deposition.
What caused the end-Triassic extinction (ETE) 201 million years ago? There is some evidence for sea level changes and some cooling. An abundance of black shales and geochemical anomalies indicate massive oceanic changes. Some believe the rifting of the North Atlantic may have released large volumes of volcanic gasses contributing to global climate change (Prothero, 2004, p. 91). The Central Atlantic Magmatic Province (CAMP) is a LIP that formed from the rifting of Pangea and spans the Triassic-Jurassic boundary. CAMP is associated with the breakup of the supercontinent Pangea and the formation of the Atlantic ocean basin. At an estimated 11 million square kilometers CAMP covers the largest area of any known LIP. It is also one of the most voluminous at an estimated 2 to 3 million cubic kilometers.
Remnants of CAMP are found on four continents including North America, South America, Europe, and Africa. Using samples from these remnants Blackburn et al. (2013) demonstrated that zircon uranium-lead geochronology provides a temporal link between the ETE and CAMP. The release of magma and associated atmospheric flux occurred in four pulses over 600,000 years. The earliest known eruptions took place at the same time as the extinction events. Further pulses of CAMP occurred as life was recovering from the extinction event. Although a temporal link between early pulses of CAMP and ETE has been established, we still do not understand the details of how these massive eruptions induced a global biological crises (Blackburn, 2013, p. 943). As a group, dinosaurs benefited from this extinction event, as they would undergo a great adaptive radiation during the Jurassic period.
| Back to Triassic |
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