Volcanism and the Rise of the Dinosaurs

Global paleogeographic setting during the Late Triassic. From X. Fu et al., 2020.

The Carnian Pluvial Episode (CPE), dated to 234–232 Ma, was a time of major turnover in the oceans and on land. It was characterized by global warming, ocean acidification, mega-monsoonal conditions, and a generalised increase in rainfall. In the marine sedimentary basins of the Tethys realm, an abrupt change of carbonate factories and the establishment of anoxic conditions mark the beginning of the climate change. This shift from arid to more humid conditions has been linked to the large-scale volcanism of the Wrangellia large igneous province (LIP). The widespread extinction caused by the CPE was followed by the first substantial diversification of dinosaurs.

Data from Central European Basin, Wessex Basin (United Kingdom), East Greenland (37), Morocco (38), Newark Basin (North America), and the Ischigualasto Basin (Argentina), indicate that increased rainfall resulted in the widespread expansion of the endorheic basins during the CPE. In the Jiyuan Basin, located in the southeast of the Ordos Basin, North China, an expanded Carnian lake succession offers a glimpse into this crucial interval.

 

Some representative palynological genera from the Jiyuan Basin (all scale bars = 20 μm). From Lu, Jing, et al., 2021

During the Early and Middle Triassic, North China was located in eastern Tethys. Later in the Triassic, the North China Plate collided with the South China Plate which resulted in extensive volcanism and the migration of the Ordos Basin depocenter continuously westward. High-resolution chemostratigraphy, palynological and sedimentological data indicate that four pulses of LIP volcanism were likely responsible for the global negative CIEs (carbon isotope excursion) that mark the CPE and drove major environmental changes.

The CPE also marks the first massive appearance of calcareous nannoplankton, while groups, like bryozoans and crinoids, show a sharp decline during this event. On land, palaeobotanical evidence shows a shift of floral associations towards elements more adapted to humid conditions (the palynological record across the CPE suggest at least 3–4 discrete humid pulses). During this interval, several families and orders make their first appearance: bennettitaleans, modern ferns, and conifer families (Pinaceae, Araucariaceae, Cheirolepidaceae), while key herbivorous groups such as dicynodonts and rhynchosaurs, which had represented 50% or more of faunas, disappeared.

Eoraptor lunensis, outcropping from the soil. Valle de la Luna (Moon Valley), Parque Provincial Ischigualasto, Provincia de San Juan, Argentina.

Followed the extinction of rhynchosaurs in most, or all, parts of the world, there was a burst of dinosaurian diversity in the late Carnian, represented by the upper Ischigualasto Formation and coeval units, with mostly carnivorous small- to medium-sized dinosaurs. Then, the long span of the early Norian, from 228.5–218 Ma, during which dicynodonts and sauropodomorph dinosaurs were the major herbivores. Finally, with the disappearance of dicynodonts, sauropodomorph dinosaurs became truly large in the middle and late Norian, from 218 Ma. This was followed by the extinction of basal archosaur groups during the end-Triassic mass extinction, 201 Ma, and the diversification of sauropods, larger theropods, ornithopods, and armoured dinosaurs subsequently, in the Jurassic.

 

References:

Lu, Jing, et al. Volcanically driven lacustrine ecosystem changes during the Carnian Pluvial Episode (Late Triassic), PNAS (2021). doi.org/10.1073/pnas.2109895118

X. Fu et al., A possible link between the Carnian Pluvial Event, global carbon-cycle perturbation, and volcanism: New data from the Qinghai-Tibet Plateau. Global Planet. Change 194, 103300 (2020) doi:10.1016/j.gloplacha.2020.103300

Michael J. Benton et al. The Carnian Pluvial Episode and the origin of dinosaurs, Journal of the Geological Society (2018). DOI: 10.1144/jgs2018-049

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