| 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 Sinemurian is a second stage of the Lower Jurassic series, corresponding to the Sinemurian age of the Early Jurassic epoch. It lasted from approximately 199.5 Ma to around 192.9 Ma (million years ago). It is preceded by the Hettangian and is followed by the Pliensbachian.
Plesiosaurus dolichodeirus, the first described plesiosaur species, was discovered in the Sinemurian rocks.
Definition[]
The GSSP of the Sinemurian stage was ratified in 2000. The lower boundary is fixed in the section on East Quantoxhead, England, and related to the first appearance of the ammonites Vermiceras quantoxense and Vermiceras palmeri.[2]
Sinemurian life[]
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This was the heyday of cephalopods. Free-swimming ammonites are notable for their external shells, which in most species are curled and resembles a ram's horn. Ammonite shells are well preserved as fossils, unlike the soft bodies of their close relatives, squids. Not many squid fossils are known. Loligosepia from the Sinemurian of the Dorset coast, England, is notable for the presence of soft organs in a fossil rock. Numerous other representatives of marine fauna were found in the same sediments. Decapods like Coleia and Aeger likely fed on decaying remains. There was a great variety of cartilaginous fish. Numerous chimaeriforms (Myriacanthus, Recurvacanthus), hybodontiforms (Acrodus, Hybodus, Polyacrodus) and synechodontiforms (Palaeospinax) are known mainly from dental fossils because the cartilaginous parts of the skeleton are poorly preserved in the fossil state. The skeleton of sturgeon fish also consists mainly of cartilage; therefore, fish such as Chondrosteus are known mainly from partial remains. Ichthyosaurs preyed on numerous small fish. Large forms as Temnodontosaurus still existed in the Sinemurian. Having completely lost contact with land, these animals acquired a resemblance to fish. However, like all reptiles, they had to come to the surface to breathe air. Long-necked plesiosaurs like Eurycleidus or the famous Plesiosaurus lived with ichthyosaurs in the Sinemurian oceans. Plesiosaurs are considered relatives of turtles, although it is not known exactly who their ancestors were.
On land there was an active evolution of dinosaurs. By this time, large predators like Dilophosaurus already existed. Crocodylomorphs occupied the niche of ambush predators. Among them, specialized forms are already known, such as the Dianosuchus, whose flat jaws probably helped it remain invisible at the bottom of reservoirs. Sinoconodon, a small mammaliamorph, probably hunted insects and small vertebrates.
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.