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.

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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).

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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.

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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).

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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.

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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).

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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.

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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.

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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.