Early Triassic

The Permian-Triassic extinction event spawned the Triassic period. The massive extinctions that ended the Permian period and Paleozoic era caused extreme hardships for the surviving species.

The Early Triassic epoch saw the recovery of life after the biggest mass extinction event of the past, which took millions of years due to the severity of the event and the harsh Early Triassic climate.[6] Many types of corals, brachiopods, molluscs, echinoderms, and other invertebrates had disappeared. The Permian vegetation dominated by Glossopteris in the southern hemisphere ceased to exist.[1] Other groups, such as Actinopterygii, appear to have been less affected by this extinction event[7] and body size was not a selective factor during the extinction event.[8][9] Different patterns of recovery are evident on land and in the sea.

Early Triassic faunas lacked biodiversity and were relatively homogeneous due to the effects of the extinction. The ecological recovery on land took 30 million years.[10] The climate during the Early Triassic epoch (especially in the interior of the supercontinent Pangaea) was generally arid, rainless and dry and deserts were widespread; however the poles possessed a temperate climate. The pole-to-equator temperature gradient was temporally flat during the Early Triassic and may have allowed tropical species to extend their distribution poleward. This is evidenced by the global distribution of ammonoids.[11] The mostly hot climate of the Early Triassic may have been caused by late volcanic eruptions of the Siberian Traps, which had probably triggered the Permian-Triassic extinction event and accelerated the rate of global warming into the Triassic. Studies suggest that Early Triassic climate was volatile, with relatively rapid and large temperature changes.[12][13][14]

The Putorana plateau is composed of basalt rocks of the Siberian Traps.

The most common land vertebrate was the small herbivorous synapsid Lystrosaurus. Interpreted as a disaster taxon, Lystrosaurus had a wide range across Pangea. In the southern part of the supercontinent, it co-occurred with the non-mammalian cynodonts Galesaurus and Thrinaxodon, early relatives of mammals. First archosauriforms appeared, such as Erythrosuchus (Olenekian-Ladinian).[15] This group includes the ancestors of crocodiles and dinosaurs (including birds). Fossilized foot prints of dinosauromorphs are known from the Olenekian.[16]

The flora was gymnosperm dominated at the onset of the Triassic, but changed rapidly and became lycopod dominated (e.g. Pleuromeia) during the Griesbachian-Dienerian ecological crisis. This change coincided with the extinction of the Permian Glossopteris flora.[1] In the Spathian subage, the flora changed back to gymnosperm and pteridophyte dominated.[17] These shifts reflect global changes in precipitation and temperature.[1]

Skull of Erythrosuchus

Early Triassic brittle stars (echinoderms)

In the oceans, the most common Early Triassic hard-shelled marine invertebrates were bivalves, gastropods, ammonites, echinoids, and a few articulate brachiopods. First oysters appeared in the Early Triassic. They grew on the shells of living ammonoids.[18] Microbial reefs were common, possibly due to lack of competition with metazoan reef builders as a result of the extinction.[19] However, transient metazoan reefs reoccurred during the Olenekian wherever permitted by environmental conditions.[20] Ammonoids show blooms followed by extinctions during the Early Triassic.[21]

Aquatic vertebrates diversified after the extinction.

Fishes: Typical Triassic ray-finned fishes, such as Australosomus, Birgeria, Bobasatrania, Boreosomus, Pteronisculus, Parasemionotidae and Saurichthys appeared close to the Permian-Triassic boundary, whereas neopterygians diversified later during the Triassic.[8] Many species of fish had a global distribution during the Early Triassic. Coelacanths show a peak in their diversity and new modes of life (Rebellatrix). Chondrichthyes are represented by Hybodontiformes like Palaeobates, Omanoselache, Lissodus, some Neoselachii, as well as last survivors of Eugeneodontida (Caseodus, Fadenia).

Amphibians: Relatively large, marine temnospondyl amphibians, such as Aphaneramma or Wantzosaurus, were geographically widespread during the Induan and Olenekian ages. The fossils of these crocodile-shaped amphibians were found in Greenland, Spitsbergen, Pakistan and Madagascar.

Hupehsuchus fossil in the Paleozoological Museum of China

Reptiles: In the oceans, first marine reptiles appeared during the Early Triassic.[22] Their descendants ruled the oceans during the Mesozoic. Hupehsuchia, Ichthyopterygia and sauropterygians are among the first marine reptiles to enter the scene in the Olenekian (e.g. Cartorhynchus, Chaohusaurus, Utatsusaurus, Hupehsuchus, Grippia, Omphalosaurus, Corosaurus). Other marine reptiles such as Tanystropheus, Helveticosaurus, Atopodentatus, placodonts or the thalattosaurs followed later in the Middle Triassic.[22] The Anisian aged ichthyosaur Thalattoarchon was one of the first marine macropredators capable of eating prey that was similar in size to itself, an ecological role that can be compared to that of modern orcas.[23]

Early Triassic and Anisian marine predators:[22] 1. Wantzosaurus, 2. Fadenia, 3. Saurichthys, 4. Rebellatrix, 5. Hovasaurus, 6. Birgeria, 7. Aphaneramma, 8. Bobasatrania, 9. Hybodontiformes, 10. Mylacanthus, 11. Tanystropheus, 12. Corosaurus, 13. Ticinepomis, 14. Mixosaurus, 15. Cymbospondylidae, 16. Neoselachii, 17. Omphalosaurus skeleton, 18. Placodus

An important extinction event occurred during the Olenekian age of the Early Triassic, near the Smithian and Spathian subage boundary. The main victims of this Smithian-Spathian boundary extinction event[24] were ammonoids and conodonts, as well as a few Palaeozoic species that survived the Permian-Triassic extinction event. Marine reptiles, such as ichthyopterygians and sauropterygians, diversified after the extinction.

The flora was also affected significantly. It changed from lycopod dominated (e.g. Pleuromeia) during the Dienerian and Smithian subages to gymnosperm and pteridophyte dominated in the Spathian.[25][26] These vegetation changes are due to global changes in temperature and precipitation. Conifers (gymnosperms) were the dominant plants during most of the Mesozoic. Until recently the existence of this extinction event about 249.4 Ma ago[27] was not recognised.[28]

The Smithian-Spathian boundary extinction was linked to late eruptions of the Siberian Traps, which resulted in climate change.[12] 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 40 °C (104 °F) in the tropics during the Smithian.[29] The extinction itself occurred during a subsequent drop in global temperatures in the latest Smithian; however, temperature alone cannot account for the Smithian-Spathian boundary extinction, because several factors were at play.[14][30]

In the ocean, many large and mobile species moved away from the tropics, but large fish remained[31], and amongst the immobile species such as molluscs, only the ones that could cope with the heat survived; half the bivalves disappeared.[32] On land, the tropics were nearly devoid of life.[13] Many big, active animals only returned to the tropics, and plants recolonised on land 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)[33], which suggest that food webs were complex and comprised several trophic levels.