Tozer's nomenclature was largely derived from Mojsisovics's work, who coined most of the Triassic stages and substages, but he redefined them using North American sites.

The Smithian is defined by the Arctoceras bloomstrandi ammonoid zone (contains Euflemingites romunderi and Juvenites crassus) and the overlying Meekoceras gracilitatis and Wasatchites tardus subzones.

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.

[16][17] Many fish genera show a cosmopolitan distribution during the Induan and Olenekian, such as Australosomus, Birgeria, Parasemionotidae, Pteronisculus, Ptycholepidae, Saurichthys and Whiteia.

[32] Olenekian chondrichthyan fishes include hybodonts and neoselachians,[24][33][34] but also a few surviving lineages of eugeneodontid holocephalians,[35] a mainly Palaeozoic group that went extinct during the Early Triassic.

Marine temnospondyl amphibians, such as the superficially crocodile-shaped trematosaurids Aphaneramma and Wantzosaurus, show wide geographic ranges during the Induan and Olenekian ages.

[36] Hupehsuchia, Ichthyopterygia and Sauropterygia are among the first marine reptiles to enter the scene (e.g. Cartorhynchus, Chaohusaurus, Utatsusaurus, Hupehsuchus, Grippia, Omphalosaurus, Corosaurus).

[38] The Paris Biota was deposited in the wake of the SSBM and it features at least 7 phyla and 20 distinct metazoan orders, including leptomitid protomonaxonid sponges (previously only known from the Paleozoic), thylacocephalans, crustaceans, nautiloids, ammonoids, coleoids, ophiuroids, crinoids, and vertebrates.

[36] The terrestrial flora was also affected significantly, changing from lycopod-dominated (e.g. Pleuromeia) during the Dienerian and Smithian subages to gymnosperm- and pteridophyte-dominated in the Spathian.

8°C over a geologically short period) in the latest Smithian; however, temperature alone cannot account for the Smithian-Spathian boundary extinction, because several factors were at play.

[12][46] An alternative explanation for the extinction event hypothesises the biotic crisis took place not at the Smithian-Spathian boundary but shortly before, during the Late Smithian Thermal Maximum (LSTM), with the Smithian-Spathian boundary itself being associated with cessation of intrusive magmatic activity of the Siberian Traps,[57] along with significant global cooling,[58][59] after which a gradual biotic recovery took place over the early and middle Spathian,[57] along with a decline in continental weathering[60] and a rejuvenation of ocean circulation.

[63] On land, the tropics were nearly devoid of life,[64] with exceptionally arid conditions recorded in Iberia and other parts of Europe then at low latitude.

This is indicated by sites that show exceptionally high biodiversity (e.g. the earliest Spathian Paris Biota),[37][38] which suggest that food webs were complex and comprised several trophic levels.