Jurassic

• 1. Introduction

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  The Jurassic is the only boundary between geological periods that remains formally undefined. Geologists can pin the start of the Jurassic to a single ammonite, Psiloceras spelae tirolicum, appearing in rock at the Kuhjoch Pass in Austria. The end is a different matter. The line between the Jurassic and the Cretaceous is the only system boundary in the entire geological record without a defined reference point. This is a period that lasted about 58.3 million years, running from roughly 201.4 million years ago to about 143.1 million years ago. It is the second and middle period of the Mesozoic Era. Yet its closing chapter dissolves into uncertainty. Why does a span of tens of millions of years begin with a precise marker and end in a blur? The answer involves vanishing markers in the rock, a tiny urn-shaped protist proposed as a tiebreaker, and a world remaking itself as a single supercontinent split apart. What grew near the poles when there were no ice caps? Which creatures took to the open ocean, and which learned to glide? The Jurassic holds the answers.

• 2. Naming The Period

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  Alexander von Humboldt toured the Jura Mountains in 1795 and got the age wrong. The German naturalist recognized the carbonate deposits there as distinct from the Triassic-aged Muschelkalk of southern Germany. He concluded they were older, which was a mistake. In 1799 he named them Jura-Kalkstein, meaning Jura limestone. The mountains themselves are a forested range adjacent to the Alps, mainly following the France-Switzerland border. In 1829 the French naturalist Alexandre Brongniart published a book whose title began Tableau des terrains qui composent l'écorce du globe. In it he used the phrase terrains jurassiques, correlating Humboldt's Jura-Kalkstein with similarly aged oolitic limestones in Britain. That act coined the term Jurassic. The German geologist Leopold von Buch built on this in 1839, dividing the period into three parts named from oldest to youngest: the Black Jurassic, the Brown Jurassic, and the White Jurassic. The French palaeontologist Alcide d'Orbigny then divided the Jurassic into ten stages in papers between 1842 and 1852, based on ammonite and other fossil assemblages in England and France. Seven of those stages are still used today, though none kept its original definition. Most of the modern stages were formalized at the Colloque du Jurassique à Luxembourg in 1962.

• 3. The Undefined Boundary

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  The Jurassic-Cretaceous boundary defies every tool geologists normally use. Placing a Global Boundary Stratotype Section and Point there has been difficult because most biostratigraphic markers are strongly regional. There are also no chemostratigraphic events, such as isotope excursions, that could define or correlate the line. Calpionellids offer the most promising candidate so far. These are an enigmatic group of planktonic protists with urn-shaped calcitic tests, briefly abundant during the latest Jurassic to earliest Cretaceous. In particular, the first appearance of Calpionella alpina, which coincides with the base of the Alpina subzone, has been proposed to define the base of the Cretaceous. A working definition has often used the first appearance of the ammonite Strambergella jacobi, once placed in the genus Berriasella. Its use has been questioned, because its first appearance does not correlate with that of C. alpina. Jurassic stratigraphy as a whole leans on ammonites as index fossils. The first appearance of specific ammonite taxa marks the beginning of stages and the smaller ammonite zones within them. Global stratigraphy is built on standard European ammonite zones, with other regions calibrated to the European successions. The Tithonian, the final stage, breaks the naming pattern. Albert Oppel introduced it in 1865 and drew its name not from a place but from Greek mythology. Tithonus was the son of Laomedon of Troy and fell in love with Eos, the goddess of dawn. Oppel chose the name because the Tithonian sits hand in hand with the dawn of the Cretaceous.

• 4. A World Coming Apart

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  At the beginning of the Jurassic, all of the world's major landmasses were joined in the supercontinent Pangaea. During the Early Jurassic it began to break apart into a northern supercontinent, Laurasia, and a southern one, Gondwana. The rifting between North America and Africa was the first to initiate, tied to the emplacement of the Central Atlantic Magmatic Province. The North Atlantic Ocean stayed relatively narrow through the period, while the South Atlantic did not open until the Cretaceous. Madagascar and Antarctica began to rift away from Africa during the late Early Jurassic, linked to the eruption of the Karoo-Ferrar large igneous provinces. This opened the western Indian Ocean and started the fragmentation of Gondwana. Around 190 million years ago, during the Early Jurassic, the Pacific Plate originated at the triple junction of the Farallon, Phoenix, and Izanagi plates. A kink in one of the plate boundaries turned a previously stable junction into an unstable arrangement surrounded by transform faults. The Pacific Plate formed at the centre. Sea levels rose and fell across the period in a long cyclical pattern, with 64 fluctuations, 15 of them over 75 metres. The most noted cyclicity is fourth order, with a periodicity of about 410,000 years. Sea level may have peaked as high as 140 metres above present levels at the Kimmeridgian-Tithonian boundary.

• 5. A Greenhouse Earth

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  There were no ice caps in the Jurassic, and forests grew close to the poles. The climate was generally warmer than today, by around 5 to 10 degrees, with atmospheric carbon dioxide likely about four times higher. Forests near the poles experienced warm summers and cold, sometimes snowy winters. High summer temperatures prevented snow from accumulating into ice sheets, though there may have been mountain glaciers. Dropstones and glendonites in northeastern Siberia point to cold winters during the Early to Middle Jurassic. Large areas of desert and scrubland spread across the lower latitudes between 40 degrees north and south of the equator. The warmth was not uniform. The Early Jurassic Cool Interval ran between 199 and 183 million years ago, and glaciation may have reached the Northern Hemisphere during the early and latest Pliensbachian. Then came a sharp reversal in the early Toarcian, a spike of around 4 to 8 degrees corresponding to the Toarcian Oceanic Anoxic Event. During this Toarcian Warm Interval, ocean surface temperatures likely exceeded 30 degrees, and the interior of Pangea likely surpassed 40 degrees. The Kimmeridgian Warm Interval, between 164 and 150 million years ago, was one of the wettest stretches of the entire period, judged from fossil wood distribution.

• 6. The Toarcian Catastrophe

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  Around 183 million years ago, the oceans turned anoxic. The Toarcian Oceanic Anoxic Event, also called the Jenkyns Event, was an episode of widespread oceanic anoxia during the early Toarcian Age. It is marked by a globally documented high amplitude negative carbon isotope excursion, the deposition of black shales, and the collapse of carbonate-producing marine organisms. It is often attributed to the eruption of the Karoo-Ferrar large igneous provinces and the resulting rise in atmospheric carbon dioxide, possibly with a release of methane clathrates. Brachiopods suffered especially badly, enduring one of the most severe extinctions in their evolutionary history. Ammonites, ostracods, foraminifera, bivalves, and cnidarians were also hit. Marine reptiles, by contrast, were little affected. The event reshaped landscapes far from the sea. The Sichuan Basin became a giant lake, probably three times the size of modern Lake Superior, recorded in the Da'anzhai Member of the Ziliujing Formation. That lake likely sequestered about 460 gigatons of organic carbon and about 1,200 gigatons of inorganic carbon. Ocean acidity drove the carbonate collapse. Seawater pH dropped to its lowest point around the middle of the event. Anoxia then fed on itself, as high acidity and temperature inhibited phosphorus from mineralising into apatite, which freed more phosphorus to cause further eutrophication in a positive feedback loop.

• 7. The Rise Of The Dinosaurs

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  Dinosaurs became the dominant vertebrates on land during the Early Jurassic. In the aftermath of the end-Triassic extinction, which wiped out other reptile groups, they surged in diversity and abundance. Among the strangest is Chilesaurus, a herbivorous dinosaur from the Late Jurassic of South America whose relationships are so uncertain it has been recovered as a member of all three main dinosaur groups in different analyses. Sauropods became the dominant large herbivores, and some reached gigantic sizes, becoming the largest organisms ever to live on land. The quadrupedal Ledumahadi from the earliest Jurassic of South Africa reached an estimated 12 tons, far heavier than other known basal sauropodomorphs. Among theropods, the ceratosaur Limusaurus from the Late Jurassic of China had a herbivorous diet, with adults bearing edentulous beaked jaws. It is the earliest known theropod to convert from an ancestrally carnivorous diet. Feathers and flight emerged here too. The basal neornithischian Kulindadromeus from the Middle Jurassic of Russia shows that at least some ornithischians were covered in protofeathers. Scansoriopterygids, small feathered coelurosaurs from the Middle to Late Jurassic of China, had membraneous, bat-like wings for gliding. The earliest avialans appear in the Middle to Late Jurassic, definitively represented by Archaeopteryx from the Late Jurassic of Germany.

• 8. Oceans, Air, And The First Mammals

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  Some species of Pliosaurus had skulls up to two metres long. These short-necked, large-headed thalassophonean pliosaurs reached body lengths estimated around 10 to 12 metres, making them the apex predators of Late Jurassic oceans. They evolved during the Middle Jurassic from ancestrally small-headed, long-necked forms. Ichthyosaurs reached their peak species diversity in the Early Jurassic, including the huge predator Temnodontosaurus and the swordfish-like Eurhinosaurus. Crocodylomorphs took to the water as well. Within the marine clade Thalattosuchia, the Metriorhynchidae became highly adapted for the open ocean, transforming their limbs into flippers, developing a tail fluke, and growing smooth, scaleless skin. Overhead, pterosaurs dominated the skies. The anurognathids, which first appeared in the Middle Jurassic, had short heads and densely furred bodies and are thought to have been insectivores. The largest bony fish known to have existed swam these seas: Leedsichthys, a filter-feeding pachycormiform with an estimated maximum length of over 15 metres. On land, mammals were quietly diversifying from cynodonts. The docodontan Castorocauda was adapted to aquatic life like a platypus or otter. The aardvark-like Fruitafossor likely specialized on colonial insects, much as living anteaters do. The earliest known eutherian, Juramaia, appears in the early Late Jurassic, closer to the ancestry of placentals than marsupials. It is so much more advanced than expected for its age that some have suggested it may actually originate from the Early Cretaceous.