| 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 Hettangian is the lowermost stage of the Jurassic system, corresponding to the Hettangian age of the Jurassic period. It started after the Rhaetian age of the Late Triassic at 201.4±0.2 Ma (millions ago) and lasted until the Sinemurian age of the Early Jurassic at 199.5±0.3 Ma. The beginning of the Hettangian age and the whole Jurassic period related to the Triassic-Jurassic extinction event.
Definition[]
The GSSP of the Hettangian and the whole Jurassic system was ratified in 2010. The lower boundary is fixed in the Kuhjoch section in Austria and related to the first appearance of the ammonite Psiloceras spelae tirolicum.[2] Since there were almost no conodonts in the Hettangian age, ammonites are used as markers of the lower boundaries of the vast majority of the remaining Mesozoic stages.
Climate[]
The whole Earth was bathed in constant tropical heat.
Hettangian life[]
The ocean was inhabited by numerous cephalopods that survived the T-J mass extinction. The best known fossils are the internal cones of belemnites and the external shells of ammonites. Belemnites looked like squids and probably led a similar lifestyle to them. Ammonites were probably not very fast due to their heavy shells. Their soft organs are scarcely represented in the fossil record, but different groups of ammonites vary in the width, ribbing, and thickness of the whorls of their shells. Megateuthis, the largest known belemtine, as well as the ammonites Dactylioceras, Pseudolioceras and Catacoeloceras are known from the Hettangian rocks of Greenland.
Noticeable changes have occurred in the vertebrate fauna. Conodonts, whose fossils are abundant in Paleozoic and Triassic sediments, became almost extinct. Placodonts and large ichthyosaurs of the Shastasauridae family became completely extinct. However, their smaller relatives survived, and Ichthyosaurus and Protoichthyosaurus lived in this age. Such reptiles completely lost contact with land and gave birth to live young in the water. Along with them lived other marine reptiles. Stratesaurus, Avalonnectes, Eurycleidus, Rhomaleosaurus and Macroplata are rhomaleosaurids. Such animals had elongated necks and relatively large heads, which brings them closer to both Plesiosauroidea and Pliosauriodea that had already appeared at this time. Plesiosaurus is a typical plesiosauroid while Thalassiodracon is an early pliosaurid, although it also had the elongated neck. Both were found in the Hettangian strata of the Lias Group in England.
Terrestrial biota of the Hettangian is lessly known. It’s likely the earliest part of the Jurassic period was similar to the late Triassic landscape, though with more common forests. Water holes would likely be surrounded by conifers, horsetails and other plants. Forests would likely consist of ferns and conifer trees. One of the best known dinosaurs of the time is Scutellosaurus. It lived alongside with tritylodontid mammaliamorphs and indeternimate turtles in what is now Arizona, USA. In the Hettangian strata of the Lufeng Formation, China, several saurischians, lepidosaurs and therapsids were found. Lufengosaurus, Yunnanosaurus and Gyposaurus are sauropodomorphs, while Sinosaurus and Panguraptor are carnivirous theropods that hunted them. Smaller Clevosaurus (also known as Dianosaurus), a relative of modern tuatara, lived in the same ecosystem as well as Bienotherium, a robust herbivorous tritylodontid.
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.