During the Carnian Stage in the late Triassic Period, around 237 to 227 million years ago, three major events occurred: the Wrangellia eruptions, the Carnian Pluvial Episode (CPE), and the emergence and diversification of dinosaurs. The Wrangellia eruptions, which formed the Wrangellia basalt floods in the eastern Panthalassic Ocean, were major tectonic events that caused a huge release of greenhouse gases. Around the same time, there was a shift to a warmer and more humid climate, known as the CPE, compared to the dry climate of the rest of the Triassic Period. It is hypothesised that the Wrangellia eruptions lead to the CPE and that this change in climate allowed for the emergence of dinosaurs or their close relatives. This is a tempting and logical conclusion; but more evidence is needed to show that this sequence of events is not simply coincidence, and more precise dating is needed to cement the timescale.
A 2006 study in Italy found evidence of land and marine diversification in Tethyan deposits from the Carnian Stage. Using uranium-lead dating, the authors were able to determine an age of about 230 million years ago. Uranium-lead dating is useful because, as Lisa Potter explains, crystallised zircon from an eruption can integrate uranium, and as time passes the uranium decays into lead. Since lead cannot be trapped, all the lead present must have once been uranium, thus the ratio of uranium to lead present can be used to give a precise age. This study provided an age constraint for the CPE, however little paleoclimatic data has been collected outside these Tethyan deposits. This presents a gap in the picture because Tethyan deposits are only found in North America, Europe north of the Alps, and Asia north of the Himalayan mountain range. During the Triassic, Pangea was separating into Laurasia (in the northern hemisphere) and Gondwana (in the southern hemisphere), and water flooded in between the two landmasses to form the Tethys Ocean. All Tethyan deposits are from the northern land of Laurasia, therefore, paleoclimatic and geochronologic information is needed for Gondwana. With this limited evidence, it was uncertain if the CPE occurred on a global scale.
Adriana Mancuso, Cecilia Benavente, Randall Irmis, and Roland Mundil set out to determine if the CPE extended to Gondwana in a recent study. They focused on the Ischigualasto-Villa Union (IVU) Basin, specifically Los Rastros Formation, in Argentina. This region was chosen because it contains fossils, a volcanic ash layer, and lake sediment, which provide the basis for paleoclimatic analysis. The sediment was analysed for evidence of a warmer and more humid climate compared to that of the surrounding strata (the layers of sediment laid down in the eras before and after), and searched for evidence of early dinosaur fossils.
The analysis of various factors of the lake sediment, such as the types of clay present, and proportion of different isotopes of carbon and oxygen, led to the conclusion that there was a humid, warmer bout compared to the flanking strata. To determine if this duration of warmer climate in Gondwana matched the CPE in the Northern hemisphere, zircon present in the volcanic ash layer was dated. The analysed zircon from the Los Rastros formation dated the layer of sediment at circa 234 million years ago. The age of the zircon concurs with that found in Tethys regions for the CPE.
We now have good evidence for a global shift in climate around the time of the emergence of dinosaurs, but the question of whether the CPE facilitated the dawn of the dinosaurs is a complex one. Mancuso and colleagues did find traces of dinosaurs in the various locations sampled that are located in strata above and close to the ash layers from the Los Rastros formation. However, it is difficult to ascertain that the tracks actually belong to early dinosaurs and not early indirect relatives that did not give rise to the dinosaurs because there are not many characteristics that allow discernment. The evidence currently presented is not enough to confirm that the CPE led to the diversification of dinosaurs: more analysis needs to be done on the fossils to determine if they are truly of early dinosaurs, and more precise dating is needed to determine how close their age is to that of the CPE.
Though this data does provide evidence that the CPE extended into the Southern Hemisphere, as the dating of the zircon in Gondwana aligns with the time interval of the Tethys region, there needs to be more precise dating in other areas of Gondwana to strengthen the link before we credit this climate change with the dawn of the dinosaurs.
Written by Arianna Schneier and edited by Ailie McWhinnie.
Arianna’s thoughts… Currently, there is no evidence based conclusion on the emergence of dinosaurs having a definitive link with the Carnian Pluvial Episode (CPE). It will be interesting to see if there is evidence of CPE happening in other areas in Gondwana and how the paleoclimatic results compare between various regions.
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