Phanerozoic/Triassic period

A Triassic basal crocodylomorphs, sphenosuchian, from Germany.

251 to 200 million years ago

Artistic interpretation of Triassic Earth.

The Triassic period is the oldest of the three periods comprising the Mesozoic (Middle Life) era on the Geologic Time Scale. Named by the German geologist Friedrich August Von Alberti (1795-1878),the Triassic takes its name from the German Trias Supergroup, comprised of "threefold" lithostratigraphic units deposited during and now associated with the Triassic period. These sequences of rock strata are evident in western and central Europe and consist of both marine and continental facies. From space, Triassic Earth would look much different than what we know today, with its land masses contained in one supercontinent, Pangea, centered along the equator extending from the north to south poles.

Beneath the Triassic Earth surface, incredible tectonic and volcanic processes were happening in preparation for the breakup of Pangea. Tensional forces began to build and the supercontinent started to slowly unzip, a process that would span the entire 51 million years that demarcate the Triassic. Eventually, Pangea would break apart to form into the continents we know today. The Triassic began at the end of the Great Dying, a mass extinction event occurring at the end of the Permian period. This mass extinction of most early life on Earth opened the way for a remarkable change of all her inhabitants, and so begins the Mesozoic era of "middle life." Plant life was also remarkably different from the types of diversity we see today. The Triassic closes with the "end-Triassic extinction," another mass extinction event that profoundly changed life on Earth once again.

Continents and Tectonics

Earth appeared remarkably different entering the Triassic period 251 million years ago. All the continental land masses were joined together as one supercontinent we call Pangea, which in Greek means “Entire Earth.” Pangea was centered along the equator extending from the north to south poles and remained relatively unchanged throughout the 51 million year Triassic period. But beneath the vast surface of Triassic-Earth, incredible tectonic processes were actively preparing to break Pangea apart into the continents we are familiar with today.

The Triassic Earth experienced crustal rifting along normal fault lines as the crust initiates the process of separating and fragmenting Pangea into smaller continental units, eventually giving way to basaltic lava flows through the resulting fissures . Extensional tectonics will eventually unzip Pangea and eventually form what is now Europe and Africa, separating Mexico from South America. The Tethys Sea, shaped like a huge tongue protruding into Pangea from the east, eventually becomes the Mediterranean Sea. Seafloor spreading at divergent plate boundaries commenced the formation of today’s Atlantic mid-oceanic ridge.

Triassic Columnar Jointing of the Palisades Basaltic Sills.

Subduction along what is now the western margin of North America produces vast mountain ranges. Accretionary tectonics continues to merge displaced terranes of old island arcs and microcontinents onto the west coast of North America, increasing the land mass of Pangea. Triassic continental land masses were positioned high above sea level, exposing the mountain ranges to relentless erosion. The sedimentary record resulting from these processes on the west coast of North America is represented by the Moenkopi, Shinarump and Navajo formations .

Along the East coast of what is now the United States, fault-bounded rift basins are developing as the Allegheny orogenies are being eroded. The basins become filled with these eroded sediments to become the Newark Supergroup of the Late Triassic. The implication of this formation is that Pangea is unzipping and separating. Sedimentary formations are intruded by igneous dikes and sills indicating significant igneous activity extending from present day Massachusetts down to North Carolina. The Palisades Sill igneous intrusion (see figure) illustrates the immense igneous processes along this area during the Triassic. Such extreme rifting was accompanied by massive basaltic lava flows from the open fissures and exposed magma as the continents broke apart, likely contributing to the Late Triassic mass extinction through increased global warming.

Animal Life

Fossilized Asteroidea starfish from the Triassic sandstone of Burgundy, France.

Trilophosaurus was a lizard-like diapsid reptile from the Late Triassic

Transition of Life from Triassic to Jurassic

• Invertebrates - The Triassic period emerged from the Great Dying of the Permian, an extinction event that decimated life on Earth, providing opportunity for the surviving life forms to adapt and for new forms to evolve. Bivalves, ammonoids and brachiopods survived the Permian-Trassic extinction and dominated invertebrate life of the Triassic, becoming extremely complex and diverse throughout the Triassic and into the Jurassic. New life forms also begin to appear in the fossil record including the squid like coloeids as well as echinoderms, resembling modern day starfish and sea urchins. Small coral and sponge reefs appear in the shallow warm seas along the equator in the Tethys Sea but there is no evidence that they existed in mass as we see today. Also missing from the fossil record are phytoplankton which we know are so important to sea life, but this missing record is likely from a lack of fossilized preservation of their soft-bodies. Arthropods were likely abundant during the Triassic because they are found in both the Permian period preceding the Triassic as well as the later periods, the Jurassic and Cretaceous, of the Mesozoic. The lobster and crab crustaceans also make their first appearance during the Triassic.

• Vertebrates - As the Triassic begins and the Permian-Triassic extinction comes to an end, 70% of land species present in the Permian have disappeared. Earth is now ready for a new line of vertebrates to emerge in place of the departed and that’s exactly what happened. Life on land attained new levels of modernization and diversity. Reptilian populations increase in number and variety existing both on land and in the sea. The fish-eating plesiosaurs and the dolphin-like giant reptilian ichthyosaurs are just two examples of many sea dwellers. Pterosuars were the first group of reptiles to take to the air in flight. Many diminutive diapsids, lizard-like reptiles, also populated land and sea, and tortoises also made their debut in the Triassic. The therapsids, endothermic mammal-like reptilian synapsids, greatly diversified from their Permian ancestors and although prominent in the beginning of the Triassic, they eventually lose their stronghold to the ectothermic archosaurs, a large group of diapsids notable for the single openings in front of their eyes on each side of their skull. These predominantly carnivorous creatures lived both on land and in semi-aquatic environments. Some bipedal varieties walked upright while other quadruped types walked agilely on all fours. Archosaurs later develop into the first dinosaurs, getting their start in the late Triassic. Archosaurs are the ancient ancestors of our modern day birds and crocodiles. It’s important to recognize that throughout the Triassic, these creatures are constantly adapting, evolving and diversifying into new and fantastic life-forms. The first small shrew-like mammals appeared toward the end of the Triassic. These tiny creatures occupied a tiny niche in the hostile continents of the Triassic that would eventually allow them to populate the Earth through to present day. Throughout the Triassic, mammals remain relatively insignificant until the end the Mesozoic in the Cretaceous.

Plant Life

Petrified wood of a Triassic Sequoia sempervirens.

During the Triassic, forestation of conifers, ginkgos, cycads, and bennettitaleans dominated the lands. The northern hemisphere was consisted primarily of conifers and seedless ferns, and the southern hemisphere eventually was taken over by seed ferns. The arid interior of Pangea sustained the gymnosperms, like the evergreen conifer, which relied on the wind for pollination and exposed its seeds to more easily reproduce. The conifers and cycads remain throughout the entire Triassic period. Coal-rich deposits suggest accumulations of plant matter and amphibian fossils indicate swamplands existed. Abundant ferns point to tropical forests and as Pangea separated later in the Mesozoic, the vegetation likely thinned out. Some smaller lycopod varieties survived from the Permian, but their spores didn't thrive in the dry Triassic climate. The ferns, however, flourished and subjugated the Triassic landscape.

Climate

Pangea 237 million years ago.

The unique climate of the Triassic Period resulted from a combination of the conditions and processes happening at that time. The supercontinent Pangea remained centered on the equator, extending from the north to the south pole. One massive ocean, Panthallasa, encompasses the globe and surrounds Pangea. Volcanism in the sea and across Earth's continental crust begins the slow process of unzipping the supercontinent as fault zones create rift valleys across the lands.

Together, these conditions created a warm and dry climate. Pangea sat relatively high above sea level and because the continent was so massive, the interior became increasingly dry and arid because of its increased distance from the sea. Volcanism released enormous volumes of carbon dioxide into the air, further warming the atmosphere, melting any remaining polar ice and ultimately causing global sea levels to rise. The supercontinents vast size disrupts ocean water circulation which further contributes to global warming conditions. These processes took place over tens of millions of years and Triassic Earth continued on a steady path of global warming throughout its history.

Extinction Events

Timeline of Earth's Mass Extinctions

Two Extinction Events and their impact on life on earth.

• Triassic–Jurassic Extinction 205 million years ago

The Triassic-Jurassic extinction occurred 205 million years ago and this event is believed to be second greatest mass extinction in Earth’s history, second only to the Permian-Triassic extinction. It is estimated to have wiped out 50% of all species, greatly affecting the larger species including many therapsids, all large crurotarsans and most large amphibians, which until then, dominated the terrain. The fossil evidence indicates this extinction occurred in a geologic instant of about 10,000 years. Its cause is unknown, but the usual suspects have been suggested, including extra-terrestrial impact, volcanic traps or massive sustained volcanic eruptions over the course of a million years, eventually resulting in catastrophic atmospheric changes. Unfortunately, enough concrete evidence just doesn’t exist at this time to allow scientists to achieve general consensus.

• Permian-Triassic Extinction 251 million years ago

The beginnings of the Triassic period and the Mesozoic era are marked by the end of an extinction event that destroyed over 90% of marine and 70% of land species. This Permian-Triassic extinction is notably referred to as "the Great Dying" because the desolation to life far exceeds that inferred by any of Earth’s other extinction events. It is believed that a chain of related events brought about the Great Dying. At that time, Earth’s land masses sat high above sea level and were concentrated into one supercontinent, Pangea. This supercontinent would have significantly influenced Earth’s processes and Earth’s atmosphere. The interior of early Pangea would likely have been arid, dry and possibly cold. Earth’s poles were extremely cold and the huge continent would have prevented equatorial circulation, significantly restricting marine fauna habitation, not to mention the reduction of coastline habitat that Pangea now offered to marine life. As the shallow epicontinental seas receded from the outer parts of the continental mass, shallow warm sea environments were lost, along with the diverse life forms they supported. Dynamic volcanic outgassing activity likely disrupted atmospheric conditions plunging Earth on a path of global warming, melting Earth's ice and releasing frozen methane gas into the atmosphere. Finally, isotopic evidence found in Permian-Triassic sediments supports the possibility of an extraterrestrial impact that would have resulted in tsunamis, volcanism and further atmospheric changes. This extinction event was significant to the Triassic period because it left Earth full of rich and vacant environments hungry to be filled with the new and diversified life forms known to have developed during the Triassic.