| System (period) | Series (epoch) | Stage (age) | Lower boundary, Ma | |
|---|---|---|---|---|
| Cretaceous | Lower | Berriasian | ~145 | |
| Jurassic | Upper (Late) | Tithonian | 149.2±0.7 | |
| Kimmeridgian |  154.8±0.8
| |||
| Oxfordian | 161.5±1.0 | |||
| Middle | Callovian | 165.3±1.1 | ||
| Bathonian | 168.2±1.2
| |||
| Bajocian | 170.9±0.8
| |||
| Aalenian | 174.7±0.8
| |||
| Lower (Early) | Toarcian | 184.2±0.3
| ||
| Pliensbachian | 192.9±0.3
| |||
| Sinemurian | 199.5±0.3
| |||
| Hettangian | 201.4±0.2
| |||
| Triassic | Upper | Rhaetian | older | |
| Subdivisions and "golden spikes" according to IUGS as of September 2023[1] | ||||
The The Middle Jurassic is the second epoch of the Jurassic period, corresponding to the Middle Jurassic series of the Jurassic system in stratigraphy. It lasted from 174.7 ± 0.8 Ma (million years ago) to 161.5 ± 1.0 Ma (IUGS 2023). It is preceded by the Toarcian age of the Early Jurassic and is followed by the Oxfordian age of the Late Jurassic. It is characterized by the emergence of new groups of crocodylomorphs and ornithischian dinosaurs. The last common ancestor of mammals probably lived in this epoch.
Definition[]
The GSSP of the Middle Jurassic series as well as the Aalenian stage was ratified in 2000. The lower boundary is fixed in the base of Bed FZ 107 in Fuentelsaz, Spain, and the first appearance of the ammonites Leioceras opalinum and Leioceras lineatum is related to it.[2]
Subdivisions[]
The Middle Jurassic divided into four stages: Aalenian, Bajocian, Bathonian and Callovian. In geochronology, all of them correspond to the same ages.
Paleogeograhy[]
During this era, the supercontinent Pangea finally broke up. To the north was Laurasia, and to the south was Gondwana, separated by the Tethys Ocean. The territories of modern Europe and western Asia were almost completely under water. There was no Atlantic Ocean because Africa did not separate from South America.
Middle Jurassic life[]
This was a time when cephalopods flourished. Squids, belemnites and ammonites spread throughout all seas. Important changes occurred in the composition of plankton, resulting in the appearance of Leedsichthys, the giant filter-feeding fish. Numerous cartilaginous fish lived along the coasts and in the open ocean. The evolution of marine reptiles took place. In addition to ichthyosaurs and sauropterygians, these were thalattosuchians, relatives of modern crocodiles, but living in the seas.
The diversity of dinosaurs increased. Primitive groups like non-sauropos sauropodomorphs became extinct during this epoch, and specialized forms began to appear, especially among ornithischians. Stegosaurids defended themselves from predators with plates and sharp spines. Among theropods, species have arisen that have adapted to both continental and coastal conditions. Towards the end of the Middle Jurassic, forms close to the bird origin appeared among coelurosaurs. The number of long-tailed pterosaurs, which were the main flying predators of their time, increased. It is assumed that the last common ancestor of all living mammals appeared at this time. This means that the direct ancestors of monotremes, marsupials and placentals appeared around this epoch.
References[]
- ↑ "Latest version of international chronostratigraphic chart". International Commission on Stratigraphy. https://stratigraphy.org/chart#latest-version.
- ↑ "GSSP Table - Mesozoic Era". https://timescalefoundation.org/gssp/index.php?parentid=35.