Old Earth Ministries Online Earth History Curriculum

Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)

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Chapter 10 - The Jurassic Period

Lesson 49: Jurassic Overview


     The Jurassic is a geologic period and system that extends from about 199.6± 0.6 Ma (million years ago) to 145.5± 4 Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the "Age of Reptiles". The start of the period is marked by the major Triassic–Jurassic extinction event. However the end of the Jurassic Period did not witness any major extinction event. The start and end of the period are defined by carefully selected locations; the uncertainty in dating arises from trying to date these horizons.

Chapter 10 - The Jurassic Period


 Lesson 49 - Jurassic Overview

 Lesson 50 - Morrison Formation

 Lesson 51 - Plesiosaurs

 Lesson 52 - Species In-Depth - Archaeopteryx



Jurassic Fast Facts


Started:  199.6 Ma

Ended:  145.5 Ma

Duration:  54.1 Million Years

Preceded By: Triassic Period

Followed By: Cretaceous Period


Mean atmospheric O2 content over period duration ca. 26 Vol %[1]
(130 % of modern level)
Mean atmospheric CO2 content over period duration ca. 1950 ppm[2]
(7 times pre-industrial level)
Mean surface temperature over period duration ca. 16.5 °C[3]
(3 °C above modern level)

Triassic-Jurassic Extinction Event

     The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 199.6 million years ago, and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. A whole class (conodonts), twenty percent of all marine families and all large crurotarsans (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event vacated ecological niches, allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangaea started to break apart. This marked the divide between the Triassic dinosaurs and the Jurassic dinosaurs.
     Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
     Several explanations for this event have been suggested, but all have unanswered challenges:


Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.


Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary.


Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide which would cause either intense global warming (from the former) or cooling (from the latter).

     The isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO2 composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic–Jurassic boundary suggesting that there was a rise in atmospheric CO2 and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO2 outgassing and catastrophic dissociation of gas hydrate. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian Period.



     During the early Jurassic period, the supercontinent Pangaea broke up into the northernLate Jurassic Paleogeography supercontinent Laurasia and the southern supercontinent Gondwana; the Gulf of Mexico opened in the new rift between North America and what is now Mexico's Yucatan Peninsula. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart. The Tethys Sea closed, and the Neotethys basin appeared. Climates were warm, with no evidence of glaciation. As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed.
     The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of the continent was submerged under shallow tropical seas; famous locales include the
Jurassic Coast World Heritage Site and the renowned late Jurassic lagerstätten of Holzmaden and Solnhofen. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation.
      The Jurassic was a time of
calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons (Stanley and Hardie, 1998, 1999).
     In North America, the first of several massive
batholiths were emplaced in the northern Cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are also found in Russia, India, South America, Japan, Australasia and the United Kingdom.

Aquatic and Marine Fauna


     During the Jurassic period, the primary vertebrates living in the seas were fish and marine reptiles. The latter include ichthyosaurs who were at the peak of their diversity, plesiosaurs, pliosaurs, marine crocodiles, of the families Teleosauridae and Metriorhynchidae.
In the
invertebrate world, several new groups appeared, including rudists (a reef-forming variety of bivalves) and belemnites. The Jurassic also had diverse encrusting and boring (sclerobiont) communities (see Taylor & Wilson, 2003), and it saw a significant rise in the bioerosion of carbonate shells and hardgrounds. Especially common is the ichnogenus (trace fossil) Gastrochaenolites.
      During the Jurassic period about four or five of the twelve
clades of planktonic organisms that exist in the fossil record either experienced a massive evolutionary radiation or appeared for the first time.


Terrestrial Fauna


      On land, large archosaurian reptiles remained dominant. The Jurassic was the golden age of the large herbivorous dinosaurs known as the sauropodsCamarasaurus, Apatosaurus, Diplodocus, Brachiosaurus, and many others—that roamed the land late in the period; their mainstays were either the prairies of ferns, palm-like cycads and bennettitales, or the higher coniferous growth, according to their adaptations. They were preyed upon by large theropods as for example Ceratosaurus, Megalosaurus, Torvosaurus and Allosaurus. All these belong to the 'lizard hipped' or saurischian branch of the dinosaurs. During the Late Jurassic, the first birds evolved from small coelurosaurian dinosaurs. Ornithischian dinosaurs were less predominant than saurischian dinosaurs, although some like stegosaurs and small ornithopods played important roles as small and medium-to-large (but not sauropod-sized) herbivores. In the air, pterosaurs were common; they ruled the skies, filling many ecological roles now taken by birds.
      The rest of the
Lissamphibia evolved in this period, introducing the first salamanders and caecilians.




     The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape. Gymnosperms were relatively diverse during the Jurassic period. The conifers in particular dominated the flora, as during the Triassic; they were the most diverse group and constituted the majority of large trees. Extant conifer families that flourished during the Jurassic included the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae and Taxodiaceae. The extinct Mesozoic conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales. Cycads were also common, as were ginkgos and Dicksoniaceous tree ferns in the forest.  Smaller ferns were probably the dominant undergrowth. Caytoniaceous seed ferns were another group of important plants during this time and are thought to have been shrub to small-tree sized. Ginkgo plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere, podocarps were especially successful, while Ginkgos and Czekanowskiales were rare.
     In the oceans modern
coralline algae appeared for the first time.

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