Clade-dependent size response of conodonts to environmental changes during the late Smithian extinction

Abstract

In contrast to the benthos whose taxonomic recovery was lagging way behind that of the nekton, highly diverse Smithian conodont and ammonoid faunas were profoundly decimated during the late Smithian, ca. 2 Myr after the Permian-Triassic boundary mass extinction. As body size reduction is a common evolutionary response to heavy environmental stress, we investigate how the size of the P1 elements of different conodont clades responded during the late Smithian crisis based on three sections from the northern Indian Margin. A major and world-wide positive carbon excursion is also a consistent signature of the late Smithian time interval. Where adequate ammonoid biochronological control is lacking (i.e., Kashmir), a new carbonate carbon isotope record provides an independent age proxy for correlations with southern Tibet and the Salt Range. Assuming a positive correlation between the size of P1 elements and body size, we confirm that segminate conodonts underwent a size decrease during the late Smithian. However, segminiplanate conodonts consistently displayed a substantial size increase during the same time interval, thus highlighting clade specific, diverging answers and precluding any simplistic generalization of size responses to the same stress event. Additionally, a moderate but consistent size increase during the early Spathian is documented for both clades, thus obscuring any simple general relation between temperature stress and size among conodonts. Comparison of size time series with the oxygen isotopic composition of conodonts (a proxy for sea-water temperature) suggests that only the size of segminate P1 elements may correlate positively with temperature. Although at a slower pace, the size of segminiplanate P1 elements continued to increase during the early Spathian, when temperature rose again after the late Smithian cooling event. Therefore, temperature alone cannot explain the size variations of segminiplanate conodonts. The late Smithian was also a time of increasing burial of organic matter on continental shelves, but lateral variations of this factor also obscure any relation with the documented size changes. Last but not least, the stratigraphically more comprehensive study in Nammal reveals a segminiplanate gap during the middle Smithian thermal high. The biogeographical expansion of this clade towards the low latitudes during the Griesbachian, the early Smithian, the late Smithian and the early Spathian apparently occurred during the coolest intervals of the Early Triassic, in agreement with their supposed cooler habitat.