Smithian Ammonoids: The First Diversity Peak In Ammonoid Recovery Following the Permian-Triassic Mass Extinction

In the aftermath of the end-Permian mass extinction, ammonoids were among the fastest marine clades to recover [1]. Ongoing work in S-China [2], Tibet, India, Pakistan and Oman leads to a refined biochronologic subdivision of the Smithian [3] and allows reconstructing a high-resolution diversity time series. U-Pb ages from S-China [4-5] indicate a maximal duration of the Griesbachian-Dienerian interval of 1.4±0.4my, a duration of 0.7±0.6my for the Smithian and of ca. 3my for the Spathian.

Ammonoid diversity was lowest during the Griesbachian and increased slowly during the Dienerian. In the early Smithian (Flemingites beds) diversity rapidly increased and remained high (~30 genera per zone) until the late middle Smithian (Inyoites beds). This first diversification phase occurred no later than 2my after the PTB and ended with a major extinction in the late Smithian (Anasibirites beds, Glyptophiceras beds). A longer-lived phase of diversity rebound occurred shortly after, during the Spathian.

This global diversity pattern is closely associated with major perturbations of the global carbon cycle [6-8]. The late Smithian - early Spathian time interval is marked by a prominent positive δ¹³C shift. As indicated by drastic changes in the latitudinal gradient of generic richness of ammonoids [1] and the boreal palynological record [8], Early Triassic times experienced severe climatic changes. CO₂ degassing via volcanism of hypothetical late eruptive phases of the Siberian Traps has been proposed as a trigger for these global disturbances [5].

[1] Brayard, A. et al. 2006: PPP 239.

[2] Brayard, A. & Bucher, H. 2008: Fossils & Strata 55.

[3] Brühwiler, T. et al. 2007a: NMNH Bull. 41.

[4] Galfetti, T. et al. 2007a: EPSL 258.

[5] Ovtcharova, M. et al. 2006: EPSL 243.

[6] Brühwiler, T. et al. 2007b: SGM Geneva.

[7] Galfetti, T. et al. 2007b: PPP 243.

[8] Galfetti, T. et al. 2007c: Geology 35.