Life was still recovering from the severe
end-Permian mass extinction. During the Olenekian, the
flora changed from
lycopod dominated (e.g.
Pleuromeia) to
gymnosperm and
pteridophyte dominated. These vegetation changes are due to global changes in temperature and
precipitation.
Conifers (
gymnosperms) 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 (see below) at the end of the Smithian subage.
Ray-finned fishes largely remained unaffected by the Permian-Triassic extinction event.
Coelacanths show their highest post-
Devonian diversity during the Early Triassic. Many fish genera show a
cosmopolitan distribution during the
Induan and Olenekian, such as
Australosomus,
Birgeria,
Parasemionotidae,
Pteronisculus,
Ptycholepidae,
Saurichthys and
Whiteia. This is well exemplified in the
Griesbachian (early
Induan) aged fish assemblages of the
Wordie Creek Formation (East
Greenland), the
Dienerian (late
Induan) aged assemblages of the
Middle Sakamena Formation (
Madagascar),
Candelaria Formation (
Nevada, United States), and
Mikin Formation (
Himachal Pradesh, India), and
Daye Formation (
Guizhou, China), and the Smithian aged assemblages of the
Vikinghøgda Formation (
Spitsbergen, Norway), and
Thaynes Group (western
United States), and several Early Triassic layers of the
Sulphur Mountain Formation (western
Canada). Ray-finned fishes diversified after the mass extinction and reached peak diversity during the
Middle Triassic. This diversification is, however, obscured by a
taphonomic megabias (Spathian-Bithynian Gap, SBG) during the late Olenekian and early middle
Anisian. The earliest large durophagous neopterygian is known from the SBG, suggesting an early onset of the Triassic actinopterygian revolution. Olenekian
chondrichthyan fishes include
hybodonts and
neoselachians, but also a few surviving lineages of
eugeneodontid holocephalians, 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. Their fossils are found in
Greenland,
Spitsbergen,
Pakistan and
Madagascar. Others, such as
Trematosaurus, inhabited freshwater environments and were less widespread. The first marine reptiles appeared during the Olenekian. and other nearby sites in Idaho and
Nevada. 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. Such diverse assemblages show that organisms diversified wherever and whenever climatic and environmental conditions ameliorated.
Smithian–Spathian boundary event disaster taxa that survived the
Permian–Triassic extinction event and flourished in the newly vacated
niches during immediate aftermath of the Great Death;
ammonoids,
conodonts and
radiolarians in particular suffered drastic biodiversity losses,
Marine reptiles, such as
ichthyopterygians and
sauropterygians, diversified after the extinction. was not recognised. The Smithian–Spathian boundary extinction was linked to late eruptions of the
Siberian Traps, which released warming
greenhouse gases, resulting in global warming and in acidification, both on land and in the ocean. A large spike in
mercury concentrations relative to total organic carbon, much like during the Permian-Triassic extinction, has been suggested as another contributor to the extinction, although this is controversial and has been disputed by other research that suggests elevated mercury levels already existed by the middle Smithian. Prior to the Smithian-Spathian Boundary extinction event, a flat
gradient of latitudinal species richness is observed, suggesting that warmer temperatures extended into higher
latitudes, allowing extension of geographic ranges of species adapted to warmer temperatures, and displacement or extinctions of species adapted to cooler temperatures.
Oxygen isotope studies on conodonts have revealed that temperatures rose in the first 2 million years of the Triassic, ultimately reaching
sea surface temperatures of up to in the tropics during the Smithian. The extinction itself occurred during a subsequent drop in global temperatures (ca. 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. after which a gradual biotic recovery took place over the early and middle Spathian, along with a decline in continental weathering and a rejuvenation of ocean circulation. In the ocean, many large and mobile species moved away from the
tropics, but large fish remained, and amongst the immobile species such as
molluscs, only the ones that could cope with the heat survived; half the
bivalves disappeared. Conodonts decreased in average size as a result of the extinction. On land, the tropics were nearly devoid of life, with exceptionally arid conditions recorded in Iberia and other parts of Europe then at low latitude. Many big, active
animals returned to the tropics, and plants recolonised on land, only when temperatures returned to normal. There is evidence that life had recovered rapidly, at least locally. This is indicated by sites that show exceptionally high biodiversity (e.g. the earliest Spathian
Paris Biota), which suggest that
food webs were complex and comprised several
trophic levels. == Notable formations ==