Dinosaurs Spring to Extinction: Springtime pinpointed as the season for dinosaur extinction

An international team led by researchers from The University of Manchester today published in Scientific Reports a groundbreaking study that sheds new light on the timing associated with the dinosaur-killing asteroid impact that occurred 66 million years ago.

The study, “Seasonal calibration of the end-Cretaceous Chicxulub Impact Event”, provides new evidence that helps us to understand the significance of the timing for the events that brought an end to the dinosaurs—and 75% of life on Earth. Spring time, a season associated with new life, saw the death of an ~160 million year old dynasty and changed the course of evolution of life on Earth.

A team led by University of Manchester PhD student Robert DePalma examined the Tanis research site in North Dakota, USA, one of the most highly detailed Cretaceous-Paleogene (KPg) boundary sites in the world, to understand the inner workings of the extinction event.

DePalma said: “This project has been a huge undertaking but well worth it. For so many years we’ve collected and processed the data, and now we have compelling evidence that changes how we think of the KPg event, but can simultaneously help us better prepare for future ecological and environmental hazards”.

“Extinction can mark the end of a dynasty, but we must not forget that our own species might not have evolved if it weren’t for the impact and the timing of events that saw the end of the dinosaurs".

Previous research has clearly documented the cataclysmic Chicxulub asteroid impact that hit the Yucatan Peninsula 66 million years ago. The impact triggered the most infamous extinction event in Earth’s history, dramatically changing global biomes in ways that directly relate to our current global ecological crisis. What remains unknown are the finer details of what happened in between impact and the resulting crash in global ecosystems.

Using multiple lines of evidence including radiometric dating, stratigraphy, fossilised remains of biological marker species, and a distinctive capping layer of iridium-rich clay, DePalma’s team previously documented that the new site that has been named Tanis, contains the only impact-caused vertebrate mass-death assemblage at the KPg boundary. The site was unequivocally dated from end of the Cretaceous, within the first hours of the Chicxulub impact (DePalma et al., 2019).

The same study documented that a massive surge of water, associated with vast earthquakes triggered by the impact, was the cause for the rapidly deposited sediments that locked-in the evidence used in this study. The densely packed tangle of plants, animals, trees, and impact ejecta has enabled an unprecedented opportunity to refine details on the KPg event, the biota that succumbed to it, and the environment in which they lived.

However, time of year plays an important role in many biological functions— reproduction, feeding strategies, host-parasite interactions, seasonal dormancy, breeding patterns, to name a few. It is hence no surprise that the time of year for a global-scale hazard can play a big role in how harshly it impacts life. The seasonal timing of the Chicxulub impact has therefore been a critical question for the story of the end-Cretaceous extinction. Until now the answer to that question has remained unclear.

Since 2014, the interdisciplinary team have applied a combination of traditional and cutting-edge techniques to piece together evidence that has enabled the identification of the season in which the Chicxulub impact event took place. Robert DePalma, lead author on the study, said: “The work at Tanis has been enriching and incredibly enjoyable. That solid connection to prehistory provides our whole team an opportunity of a lifetime".

The team used growth lines that were preserved in the fossil bones of fish that died in the powerful impact-triggered event at Tanis site. These growth lines provide a unique record of the fish life histories, and enabled the season during which they died to be deduced from the bony growth pattern. The unique structure and pattern of the growth lines, similar to a barcode, provide the evidence that all of the fossil fish examined died during the Spring-Summer growth season. Isotopic analysis of the growth lines in the fish bones provided independent confirmation of this, showing a yearly seasonally driven oscillation that also terminated during the Spring-Summer growth.

The team used multiple additional lines of evidence to verify the isotopic data. The examination of juvenile fossil fish, aided in part by cutting-edge Synchrotron-Rapid-Scanning X-Ray Fluorescence (SRS-XRF) carried out at the Stanford Synchrotron Radiation Lightsource (SSRL), provided a novel way of dating the seasonal variation observed in fossils from the deposit. By comparing the sizes of the youngest fish to the growth rates of analogous modern fish, the team predicted how long after hatching they were buried. Comparing this to known modern spawning seasons enabled the team to deduce the seasonal range that was represented by the deposit at Tanis— Spring to early Summer.

Loren Gurche, a co-author on the study, said: “Animal behavior can be a pretty powerful tool, so we overlapped even more evidence, this time of seasonal insect behavior, such as leaf mining and mayfly activity. They all matched up…everything points to the fact that the impact happened during the northern hemisphere equivalent of Spring to Summer months".

The study provides long-awaited evidence that helps confirm previous studies on the timing of the KPg mass-extinction. The multiple lines of evidence discovered in this study are essential to reconstructing the causes of post-impact biotic response and extinction patterns. That knowledge is not only pertinent to scientists and natural historians but is directly applicable to life today. The fossil record is a key to understanding the response of life to global-scale hazards, whether they be past, present or future. Professor Phil Manning, co-author on the study, stated: "The hindsight that the fossil record provides can yield critical data, which can be applied today, so that we might plan for tomorrow".