JURASSIC: CHARACTERISTICS, SUBDIVISIONS, FLORA, FAUNA - GEOGRAFÍA - 2024

The Jurassic period is the second of the three that make up the Mesozoic Era. Likewise, it ranks second in terms of duration. Its name comes from the Jura mountain range, which belongs to the Alps on the European continent.

This period is perhaps one of the best known, since because it is the time of the great dinosaurs, it arouses more interest in people. Even a very famous movie is named after him.

Landscape representation in the Jurassic period. Source: Gerhard Boeggemann, via Wikimedia Commons

The Jurassic has been one of the most interesting geological periods to study, taking into account that in it the planet underwent great changes, at a geological, climatic level and in terms of biodiversity.

characteristics

Duration

The Jurassic period lasted 56 million years, beginning about 201 million years ago and ending 145 million years ago.

Wide variety of life forms

During the Jurassic period life diversified widely, both at the plant and animal level. Plants created jungles and forests, in which a large number of animals proliferated.

Among the animals, the dinosaurs were the ones that dominated the landscape, both in terrestrial and aquatic environments.

Great tectonic activity

At the geological level, in the Jurassic period there was an intense activity of the tectonic plates. This resulted in the fragmentation of the supercontinent Pangea beginning to give rise to the continents that are known today.

Divisions

The Jurassic period was divided into three epochs: early, middle, and late. Likewise, these were divided into a total of 11 ages: four in the early Jurassic, four in the middle Jurassic and three in the late Jurassic.

geology

At the beginning of this process, there was only one large land mass on the planet, the supercontinent Pangea, and one immense ocean, the Phantalassa. The most important and momentous geological event that occurred during this period was the breakup of the supercontinent Pangea, a process that began at the beginning of the period.

Rupture of the Pangea

Pangea

During the Jurassic period the activity of the tectonic plates was very intense. Thanks to this, the process of rupture of the supercontinent Pangea took place, which began in this period and culminated in the next.

The fragmentation of the Pangea began with what in the area of ​​geology is known as “rifting”, a geological process that consists of the formation of certain cracks in the lithosphere as a result of the rise of magmatic material towards the crust.

During the Jurassic, a rifting process occurred in which the so-called Hercynic suture was reopened or reactivated. This was nothing more than the site where the Hercynian orogeny occurred, when Euramerica and Gondwana collided in the late Devonian period.

As the gap gradually opened, the ocean water took that place, deepening the separation between what today are the African and European continents.

This was how the Pangea was divided into two huge pieces of land: Laurasia, located to the north, and Gondwana to the south.

Changes in the oceans

At the beginning of the Jurassic period there was a single large ocean that surrounded that great land mass that was the Pangea. That ocean was known by the name of Panthalassa.

As the Pangea fragmented to form Laurasia and Gondwana, that space filled with water, forming what specialists have called the Tethys ocean.

At the level of the middle Jurassic, the Atlantic Ocean began to form and there were the first signs of the Caribbean Sea.

As time progressed, the modifications continued, so that the Pangea being totally fragmented, the Tethys Ocean functioned as a communication channel between the Atlantic Ocean, the Indian Ocean and the Pacific Ocean.

At the end of the Jurassic period there were two continents: Laurasia and Gondwana, which underwent new divisions in later periods, to originate the continents that are known today.

Weather

The Jurassic period was characterized by presenting climatic conditions in which humidity and warm temperatures prevailed.

During this period, plants covered almost the entirety of the existing continents, which caused an increase in humidity, due to perspiration.

At the beginning of the Jurassic, the rains were quite abundant, which favored the growth and proliferation of plants. As time progressed, the climate stabilized, staying humid and with high temperatures.

These climatic characteristics were of great importance in the diversification and permanence of life forms during the period.

Lifetime

The Jurassic period was of great importance for the development of life. There was great biodiversity, both in terms of flora and fauna.

It has been one of the geological periods in which a greater diversification and variety of the species that inhabited the planet was observed.

This was largely due to the fact that the planet's geographical conditions were ideal for life to flourish properly. The Jurassic was the time of the dominance of the great dinosaurs, many of which have been the most representative and known to most people.

-Flora

In the Jurassic period, the vegetation was abundant and very rich. The prevailing climate during that geological period allowed the development of a large number of forests and jungles, which dominated the landscape, also strengthening the diversification of animals.

During this period a great variety of plants flourished, among which the Bennettitales, Cycadales and conifers stand out. Likewise, in this period small plants such as ferns and sphenopsids were also abundant.

Bennettitales

This was the most abundant group of plants that was observed during the Jurassic period, according to the collected fossil records. It belonged to the group of plants with seeds and they became extinct in the period following the Jurassic, the Cretaceous.

According to the fossils collected, the cells of the epidermis of these plants had wavy edges, which constitutes a differential characteristic of this genus.

These plants are, from the evolutionary and phylogenetic point of view, related to the Cycadales. Because of this, for a long time they were described in this order. However, thanks to later studies, it was established that the Bennettitales constitute a separate genre.

Representation of a plant belonging to the Benettitales. Source: MUSE

Of this group of plants, two genera prevailed: Cycadeoidea and Williamsonia. Plants belonging to the genus Cycadeoidea were small in size and rounded in appearance. They also had a small, cylindrical stem without ramifications. At the terminal apex they had pinnate-type leaves.

On the other hand, plants belonging to the genus Williamsonia were made up of thin and tall trunks (up to 2 meters) with ramifications. Its leaves were fern-like and produced large flowers. Their reproductive cells (ovules) were stored in a cup-shaped structure, known as a cone. Each plant stored an average of between 30 - 55 eggs.

Cycadales

This is a group of plants whose origin dates back to the Carboniferous period of the Paleozoic Era. This group of plants have thick and low trunks and others that are not so thick (similar to palm trees).

They also had pinnate-type leaves, located in terminal whorls. These could measure between 50 and 150 cm long. Likewise, this type of plants had masculine and feminine influences. The seeds of this type of plant were oval-shaped, covered with a fleshy texture structure.

These plants were dioecious, which means that there were female and male specimens. Female cells (ovules) were produced and stored in megasporophytes, while male cells (pollen) were produced in microsporophytes.

Conifers

Together with the Benettitales and the Cycadales they dominated the landscape during the Triassic and Jurassic periods. There are even genres that remain to this day. They owe their name to the fact that their seeds are found in structures known as cones.

They belong to the group of gymnosperms. Most of the specimens of these plants were monoecious, which means that they presented both female and male reproductive structures in the same individual.

During the Jurassic, this group of plants was represented by the Taxodiaceae, Pinaceae and Ginkgoales.

The Taxodiaceae were characterized by being monoecious plants that could become very tall, with linear and dimorphic leaves that were located in 2 planes. The male reproductive structure had an axial location on the sole, while the female had a terminal location.

The Pináceas, on the other hand, were plants that were characterized by having resin channels, both in leaves and in the stem. Its leaves were simple, needle-like, located in a spiral shape. They were monoecious plants. The male reproductive structure was made up of a large number of stamens, while the female one was made up of woody cones that presented independent scales, which take a period of 2 or 3 years to mature.

Lastly, ginkgoales were dioecious tree plants. Its leaves presented a parallel veining, with the blade divided or lobed. Most of the species in this group became extinct in the course of time. Today only the

Ginkgo biloba species survives, a plant widely used for ornamental and medicinal purposes.

-Fauna

During the Jurassic period, the fauna diversified and expanded to a great extent. It was a time dominated by the great dinosaurs, perhaps the best known through studies of recovered fossils.

Animal life conquered all habitats: land, sea, and air.

Invertebrates

Of this group of animals, those that predominated were mollusks, especially gastropods, bivalves and cephalopods.

Among the latter several subclasses were distinguished: Ammonoids, Nautiloids (they persist to the present day) and Belemnoids (the most abundant mollusks of the period).

Likewise, another group that experienced some diversification were the echinoderms, their most abundant representatives in this period being those of the asteroid class, to which starfish belong. Within the echinoderms, the echinoids (sea urchins) also stood out, which also populated the marine habitats of the Jurassic.

Arthropods also abounded in this period. Among these, belonging to the class of crustaceans, are crabs, such as those of the genus Mesolimulus. Likewise, there were some specimens such as butterflies, grasshoppers and wasps.

Vertebrates

Of the group of vertebrates, those that completely dominated this period were the reptiles, more specifically the dinosaurs. There were also other types of vertebrates that stood out to a lesser extent, such as the first amphibians (frogs).

In this period there were also a few representatives of the group of mammals, of small size.

Vertebrates in aquatic habitats

The waters of the seas in the Jurassic period were teeming with life. There was a great variety of fish, but the kings of the water were the aquatic reptiles. Among these, the most representative were ichthyosaurs and plesiosaurs.

• Ichthyosaur: it was distributed throughout the seas, it was carnivorous and large (it could measure up to 18 meters). They had several fins: one tail and one dorsal. They had an elongated body and a long snout, similar to that of today's dolphins, toothed. According to the fossil records that have been found, these animals were viviparous (The embryo develops inside the mother's body).
• Plesiosaur: they were the largest marine animals (they measured up to 23 meters). They had an extremely long neck, four fin-like limbs, and a fairly broad body.

Vertebrates in aerial habitats

During the Jurassic period small birds appeared, however those who prevailed were the flying reptiles, the Pterosaurs.

The Pterosaurs had varied sizes, from very small to huge like a bus. They had a body covered with hair and extensive wings formed by a membrane that was hooked to one of the fingers of the hand.

On the upper surface of their head they had showy ridges. They were oviparous and according to specialists, they had very excellent eyesight. In terms of eating habits, they were carnivores, they could feed on fish (their favorite food) or some insects.

Vertebrates in terrestrial habitats

Terrestrial habitats were mainly dominated by large dinosaurs.

Among the herbivorous dinosaurs, we can mention the apatosaurus, the brachiosaurus, the camarasaurus and the gigantspinosaurus, among others.

• Apatosaurus: it was large, it could weigh up to 30 tons, it had a small head and a fairly thick neck. It could measure up to 21 meters.
• Brachiosaurus: it was a quadruped animal, characterized by its large size and long neck. It was one of the largest dinosaurs on record. They could weigh up to 80 tons and were about 13 meters high and 23 meters long.
• Camarasaurus: it was quite long, it could measure up to 18 meters in length. It presented in the vertebrae of the spine some species of air chambers that are believed to reduce its body weight.
• Gigantspinosaurus: this was a dinosaur that was fully armored with bony plates, in addition to pointed spines on its tail and very large spines at the level of the shoulders. It was not that big, compared to others (they were up to 5 meters in length).

Among the carnivorous dinosaurs we can mention: the allosaurus, the compsognathus and the cryolofosaurus, among many others.

• Allosaurus: it was a large animal, in its extremities it had large claws, as well as large teeth. They could measure up to 12 meters in length and weigh a maximum of 2 tons. As a distinctive element, it had a bony ridge above the eyes.
• Compsognathus: It was an extremely small carnivorous dinosaur. If it was a meter in length. It had claws on its limbs and an approximate weight of 3 kg.

Representation of a Compsognathus specimen. Source: Own work, via Wikimedia Commons

• Cryolophosaurus: it was not large. It reached 6 meters in length and 3 meters in height. Its distinctive feature was a crest on the top of the head. On the front limbs it has strong claws capable of destroying its prey.

Divisions

The Jurassic period was found divided into three epochs or series:

Lower Jurassic (early)

It was the first stage of the Jurassic, immediately after the Triassic period. It had an average duration of 24 million years. It was made up of four ages:

• Hettangian: 201 million years - 199 million years.
• Sinemurian: 199 million years - 190 million years
• Pliensbachiense: 190 million years - 182 million years
• Toarcian: 182 million years - 174 million years.

Middle jurassic

It was the intermediate stage of the Jurassic period, with an average duration of 14 million years. It was divided into four ages:

• Aalenian: 182 million years - 174 million years.
• Bajocian: 174 million years - 170 million years.
• Bathonian: 170 million years - 168 million years.
• Callovian: 168 million years - 166 million years.

Upper Jurassic (late)

It was the last stage of the Jurassic period, preceding the Cretaceous period. It lasted approximately 16 million years. It was divided into three ages:

• Oxfordian: 166 million years - 157 million years.
• Kimmeridgian: 157 million years - 152 million years.
• Oxfordian: 161.2 to 155.7 million years ago, approximately.

References

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