Olenekian


In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma. The Olenekian is sometimes divided into the Smithian and the Spathian subages or substages. The Olenekian follows the Induan and is followed by the Anisian.
The Olenekian saw the deposition of a large part of the Buntsandstein in Europe. The Olenekian is roughly coeval with the regional Yongningzhenian stage used in China.
in Heligoland

Stratigraphic definitions

The Olenekian stage was introduced into scientific literature by Russian stratigraphers in 1956. The stage is named after Olenëk in Siberia. Before the subdivision in Olenekian and Induan became established, both stages formed the Scythian stage, which has since disappeared from the official timescale.
The base of the Olenekian is at the lowest occurrence of the ammonoids Hedenstroemia or Meekoceras gracilitatis, and of the conodont Neospathodus waageni. It is defined as ending near the lowest occurrences of genera Japonites, Paradanubites, and Paracrochordiceras; and of the conodont Chiosella timorensis. A GSSP has in 2009 not yet been established.

Olenekian life

Life was still recovering from the severe end-Permian mass extinction. During the Olenekian, the flora changed from lycopod dominated to gymnosperm and pteridophyte dominated. These vegetation changes are due to global changes in temperature and precipitation. Conifers were the dominant plants during most of the Mesozoic. Among land vertebrates, the archosaurs - a group of diapsid reptiles encompassing crocodiles, pterosaurs, dinosaurs, and ultimately birds - first evolved from archosauriform ancestors during the Olenekian. This group includes ferocious predators like Erythrosuchus.
In the oceans, Microbial reefs were common during the Early Triassic, possibly due to lack of competition with metazoan reef builders as a result of the extinction. However, transient metazoan reefs reoccurred during the Olenekian wherever permitted by environmental conditions. Ammonoids and conodonts diversified, but both suffered losses during the Smithian-Spathian boundary extinction at the end of the Smithian subage.
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.
Marine temnospondyl amphibians, such as Aphaneramma or Wantzosaurus show wide geographic ranges during the Induan and Olenekian ages. Their fossils are found in Greenland, Spitsbergen, Pakistan and Madagascar. The first marine reptiles appeared during the Olenekian. Hupehsuchia, Ichthyopterygia and Sauropterygia are among the first marine reptiles to enter the scene. Sauropterygians and ichthyosaurs ruled the oceans during the Mesozoic Era.
A major extinction event occurred during the Olenekian: the Smithian-Spathian boundary extinction. This event was probably caused by late eruptions of the Siberian Traps and led to extinctions among several groups, especially nektonic and pelagic taxa such as ammonoids and conodonts. One of several exceptionally diverse Early Triassic assemblages, the Paris Biota, was deposited in the wake of the Smithian-Spathian boundary extinction. It features at least 7 phyla and 20 distinct metazoan orders, including leptomitid protomonaxonid sponges, thylacocephalans, crustaceans, nautiloids, ammonoids, coleoids, ophiuroids, crinoids, and vertebrates.
''

Ray-finned fishes

Coelacanths

Lungfishes

†Temnospondyls

†Chroniosuchians

Lissamphibia

†Procolophonomorphs

Archosauromorphs

Lepidosauromorphs

†Sauropterygia

†Ichthyosauromorphs

†Hupehsuchians

†Ichthyosauriforms

†Ichthyopterygians

Therapsids

Literature