Dienerian (Early Triassic) ammonoids from the Northern Indian Margin
Erschienen 2019 bei Wiley-Blackwell
Dienerian (Early Triassic) ammonoids and the Early Triassic
biotic recovery: a review
DAVID WARE AND HUGO BUCHER
It has been estimated that about 90% of all marine species disappeared during the end-Permian mass extinction (Raup & Sepkoski 1982). It is the biggest known biodiversity crisis in the history of Phanerozoic life, and it led to the replacement of typical Palaeozoic faunas by typical modern communities (Sepkoski 1984). The recovery which followed in the Early Triassic is an intensively studied topic. This recovery is traditionally considered as delayed in comparison with other mass extinctions (Erwin 1998, 2006) as several major marine clades such as corals (Stanley 2003), foraminifers (Tong & Shi 2000) or radiolarians (Racki 1999) recovered only in the late Spathian (Early Triassic) or in the Anisian (Middle Triassic), ca. 5 My after the Permian-Triassic boundary. This delay is interpreted as the consequence of persisting anoxic conditions (Wignall& Twitchett 2002) and unstable environmental conditions during the entire Early Triassic (Payne et al . 2004). However, several recent studies suggest a more complex scenario, with pulses of recovery interrupted by periods of additional extinctions. For example, conodonts (Orchard 2007; Goudemand et al . 2008) first underwent an important turnover at the Griesbachian-Dienerian boundary, followed by an explosive radiation in the early-middle Smithian, a dramatic extinction in the late Smithian, and another radiation during the early Spathian. Ammonoids also recovered very fast compared to other groups, reaching pre-extinction levels of diversity already during the Smithian ( Fig. 1 ; Brayard et al . 2009). Hofmann et al . (2014) showed that benthic ecosystems started to recover already in the Griesbachian, but this recovery has been interrupted by a return to harsh environmental conditions (e.g. anoxia, warm temperatures) during the Dienerian. Recovery of the benthos resumed during the Smithian. Based on palynological and carbon isotopes analysis, Hermann et al . (2011a,b, 2012a,b) and Schneebeli-Hermann et al . (2012, 2015) contradicted the idea of persistent widespread anoxia and showed that this anoxia was restricted to the middle-late Dienerian and late Smithian. Late Permian and Early Triassic ecological crises of terrestrial plants also immediately predate extinction crises of marine organisms, and the Dienerian diversity low is no exception as documented by Hochuli et al . (2016).
Many studies addressing the recovery are based on insufficiently resolved age controls. The construction of a detailed time-scale for the Early Triassic is the cornerstone on which any study addressing this biotic recovery must be based. Ovtcharova et al . (2006) and Galfetti et al . (2007) established a duration of ca. 4.5 Myr for the Early Triassic and showed that the four Early Triassic ages were of very uneven duration, the Spathian representing more than half of this interval ( Fig. 2 ). Galfetti et al . (2007) obtained a maximal duration of ca. 1.4 ± 0.4 Myr for the Griesbachian-Dienerian time interval. No duration of the Dienerian alone is available, but it can be reasonably assumed that it is <1 Myr. A new generation of high-resolution U-Pb ages for the Permian-Triassic boundary (Burgess et al . 2014) and for the Early-Middle Triassic boundary (Ovtcharova et al . 2015) indicate a duration of 4.83 ± 0.19 Myr for the Early Triassic. However, the respective duration of each of the four Lower Triassic substages may not be significantly changed because these new U-Pb ages are consistently younger than those of the previous generation.
Fig. 1 . Total generic richness (black bold line, all ammonoids; grey lines, major ammonoid groups) and mean Chao2 estimate of the overall generic richness
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