| System (period) | Series (epoch) | Stage (age) | Lower boundary, Ma | |
|---|---|---|---|---|
| Jurassic | Lower | Hettangian | 201.4±0.2 | |
| Triassic | Upper (Late) | Rhaetian | ~208.5 | |
| Norian | ~227 | |||
| Carnian |  ~237
| |||
| Middle | Ladinian | ~242
| ||
| Anisian | 247.2 | |||
| Lower (Early) | Olenekian | 251.2 | ||
| Induan | 251.902±0.024
| |||
| Permian | Lopingian | Changhsingian | older | |
| Subdivisions and "golden spikes" according to IUGS as of September 2023[1] | ||||
The Anisian is a first stage of the Middle Triassic series, corresponding to the Anisian age of the Middle Triassic epoch. It lasted from approximately 247.2 Ma to around 242 Ma (million years ago). It is preceded by the Olenekian age of the Early Triassic and is followed by the Ladinian age of the Middle Triassic. The earliest probainognathians are known from this stage.
It includes four substages (from lower to upper): Aegean, Bithynian, Pelsonian and Illyrian.
Definition[]
As of 2024, there is no GSSP for the Anisian stage and the whole Middle Triassic series. The candidate is a section at Desli Caira, Romania. Other significant sections located in Guizhou Province (China) and South Primorye (Russia). The first appearance of the conodont species Chiosella timorensis or the base of magnetozone MT1n are suggested as the markers of the lower boundary of this stage.
Anisian life[]
Nautilus, the modern nautilid genus, already existed in the Anisian
The Tethys Ocean, which washed the eastern coast of central Pangea, was in full swing with life. At the bottom lived brachiopods (Decurtella, Piarorhynchia), similar to bivalves, but with shells consisting of an upper and lower valve instead of a right and left one. Numerous cephalopods, covered with a hard external shell, hunted in the water column. Grypoceras, Pleuronautilus, and Nautilus are members of the large group Nautiloidea, with Nautilus being its only genus that still lives in the ocean. Ammonites like Bulogites, Beyrichites, Paraceratites, Ptychites, and Norites also had shells, but their closest living relatives are squid and octopuses. Numerous cartilaginous hybodontiform fish (Palaeobates, Acrodus), characterized by spines at the base of their dorsal fins, swam near the seabed. Numerous marine reptiles, whose ancestors were once land-dwelling, lived along the coasts and in the open sea. Ichthyosaurs like Cymbospondylus, Tholodus, Mixosaurus, and Phalarodon are the oldest group of marine Mesozoic reptiles, which appeared at the beginning of the Triassic and by this time had reached gret number. Sauropterygians, other sea reptiles, appeared later, but in the Anisian, different forms also arose among them. Pachypleurosaurids (Neusticosaurus) resembled sea lizards, while nothosaurs (Nothosaurus) were specialized fish eaters, and placodonts like Placodus have acquired strong teeth for chewing shells. Tanystropheus, a strange long-necked reptile, was likely semiaquatic.
The animals that lived on the continents were also diverse. Fresh waters were inhabited by lungfish (Ceratodus) and temnospondyls including medoum-sized Xenotosuchus and such huge members as Mastodonsaurus, Cyclotosaurus, and Paracyclotosaurus. On land, the real dawn of archosauromorph reptiles began. Among them, herbivorous forms appeared, such as the beaked rhynchosaur Langeronyx, and carnivorous, such as Sarmatosuchus and the big-headed Erythrosuchus. Among the parareptiles by this time, only small species like Teratophon remained.
At this time, quite a variety of synapsids still lived. Dicynodonts in the process of evolution gave rise to large forms like Kannemeyeria and Sangusaurus. With their fangs they dug up the roots of plants, and with their beaks they bit through strong stems and cones. Cynognathus, a large cynodont, could have hunted dicynodonts and large reptiles or their young. Probainognathians, the most successfull group of cynodonts, which include later mammals, appeared in this age. The earliest genera, Lumkuia and Aleodon, are known from Anisian sediments of South Africa and Tanzania, respectively. The number of therocephalians gradually declined, although several genera (Theriognathus, Microgomphodon, Antecosuchus) are known from the Anisian strata.
References[]
- ↑ "Latest version of international chronostratigraphic chart". International Commission on Stratigraphy. https://stratigraphy.org/chart#latest-version.