Induan


The Induan is the first age of the Early Triassic epoch in the geologic timescale, or the lowest stage of the Lower Triassic series in chronostratigraphy. It spans the time between 251.902 Ma and Ma. The Induan is sometimes divided into the Griesbachian and the Dienerian subages or substages. The Induan is preceded by the Changhsingian and is followed by the Olenekian.
The Induan is roughly coeval with the regional Feixianguanian stage of China.

Stratigraphic definitions

The Induan stage was introduced into scientific literature by Russian stratigraphers in 1956, who divided the Scythian stage that was used by Western stratigraphers into the Induan and Olenekian stages. The Induan stage is named for the Indus region of Pakistan/India. The Russian subdivision of the Lower Triassic then slowly replaced the one used in the West.
The base of the Induan stage is defined as the place in the fossil record where the conodont species Hindeodus parvus first appears, or at the end of the negative δ18O anomaly after the big extinction event at the Permian-Triassic boundary. The global reference profile of the base of the Induan is situated in Meishan, Changxing County, China.
The top of the Induan stage is at the first appearance of ammonite species Meekoceras gracilitatis.
Though the Induan is an unusually short age at this point in the geologic timescale, its million years' extent still contains five ammonite biozones in the boreal domain and four ammonite biozones in the Tethyan domain.
Marine black shale deposits are common especially during the Dienerian substage of the Induan. These point to low oxygenation in the ocean.

Induan life

The Induan age followed the mass extinction event at the end of the Permian period. Both global biodiversity and community-level diversity remained low through much of this stage of the Triassic.
Much of the supercontinent Pangea remained almost lifeless, deserted, hot, and dry. In higher latitudes, the flora during the Griesbachian was gymnosperm dominated but became lycopod dominated in the Dienerian. This change reflects a shift in global climate from cool and dry in the Griesbachian to hot and humid in the Dienerian and points to an extinction event during the Induan, just ca. 500'000 years after the end-Permian mass extinction event. It led to the extinction of the Permian Glossopteris flora.
The lystrosaurids and the proterosuchids were the only groups of land animals to dominate during the Induan stage. Other animals, such as the ammonoids, insects, and the tetrapods remained rare and terrestrial ecosystems did not recover for some 30 million years. Both the seas and much of the freshwater during the Induan were anoxic, predominantly during the Dienerian subage. Microbial reefs were common, possibly due to lack of competition with metazoan reef builders as a result of the extinction.
from Madagascar
Ray-finned fishes largely remained unaffected by the Permian-Triassic extinction event. Many genera show a cosmopolitan distribution during the Induan and Olenekian. This is well exemplified in the Griesbachian aged fish assemblages of the Wordie Creek Formation, the Dienerian aged assemblages of the Sakamena Formation, Candelaria Formation, and Mikin Formation, and the Smithian aged assemblages of the Vikinghøgda Formation, Thaynes Formation, and Helongshan Formation.
Induan Chondrichthyans include hybodonts, neoselachians and a few surviving lineags of eugeneodontid holocephalians, a mainly Palaeozoic group. Cartilaginous fishes were seemingly rare during the Induan.
Crocodile-shaped, marine temnospondyl amphibians were geographically widespread during the Induan and Olenekian ages. Their fossils are found in Greenland, Spitsbergen, Pakistan and Madagascar.
The bivalve Claraia'' was widespread and common in the Panthalassa and Tethys oceans. The geologically oldest oysters are known from the Induan. They grew on the shells of living ammonoids.

Cartilaginous fishes

Ray-finned fishes

Coelacanths

Lungfishes

†Temnospondyls

†Chroniosuchians

Lissamphibia

†Procolophonomorphs

Diapsids

Archosauromorphs

Therapsids