The Lower Triassic series is coeval with the Scythian stage, which is today not included in the official timescales but can be found in older literature. In Europe, most of the Lower Triassic is composed of Buntsandstein, a lithostratigraphic unit of continental red beds.
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. 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. Other groups, such as Actinopterygii, appear to have been less affected by this extinction event and body size was not a selective factor during the extinction event. 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.
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. 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.
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.Microbial reefs were common, possibly due to lack of competition with metazoanreef builders as a result of the extinction. However, transient metazoan reefs reoccurred during the Olenekian wherever permitted by environmental conditions.Ammonoids show blooms followed by extinctions during the Early Triassic.
Aquatic vertebrates diversified after the extinction.
The Smithian-Spathian boundary extinction was linked to late eruptions of the Siberian Traps, which resulted in climate change.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. 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.
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. On land, the tropics were nearly devoid of life. 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), which suggest that food webs were complex and comprised several trophic levels.
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