SOME LIKE IT HOT: THE SMITHIAN DIVERSIFICATION-EXTINCTION MODEL

Ammonoid underwent a protracted diversification during the early Smithian through addition of new families and first peaked during the middle Smithian through the evolution of extremely short-lived species. Only a few species-poor families survived into the late Smithian, among which the xenoceltitids provide the rootstock for the next ammonoid radiation that started from the early Spathian onwards. Similarly, conodonts reached their highest Triassic generic diversity during the middle Smithian, but only a very few species made it through the end-Smithian crisis. Apparently the slower evolving marine benthic faunas show no comparable concomitant set back, although the amount of high resolution data is still insufficient. On land, floras underwent a drastic ecological turnover with a middle Smithian spore peak comparable to the end-Permian one. It was followed by an abrupt, global change from hygrophytic to xerophytic associations across the Smithian-Spathian boundary. The global carbon isotope record displays a marked negative peak during the middle Smithian, followed by an abrupt positive shift in the late Smithian. In the Tethys the oxygen isotope record from biogenic phosphate follows the same trends as the carbon isotope record and indicates a temperature drop of ca. 7.5°C close to the Smithian-Spathian boundary. Hence, the first biotic reaction to the warmest and highest carbon dioxide concentration of the middle Smithian episode was a dramatic acceleration of the evolutionary turnover rates of ammonoids and conodonts in the ocean and a massive predominance of pteridophytes on land. We hypothesize that the late Smithian global positive shift of the carbon cycle and ensuing cooling were triggered by the overshooting abundance of primary producers whose oxidation depleted the oxygen content of the upper water column, thus leading in a second step to a diversity crash of consumers such as ammonoids and conodonts. The widespread deposition of organic rich sediments during the late Smithian strongly supports this model.