Published on February 25th, 2022 | by David Marshall

Episode 137: Tanis

The end-Cretaceous mass extinction was a cataclysmic asteroid impact that ushered in the end of the non-avian dinosaurs and forever changed the course of evolution on Earth. But what can we say about the timing of the event, other than it happened 66 million years ago?

Well, it turns out that Tanis, a relatively-recently discovered fossil site in North Dakota, is full of lines of evidence that are allowing earth scientists to piece together when the impact occurred.

In this episode, we’re joined by Melanie During, Uppsala University, who has been examining the details of the bones of fish to say more about the world either side of the event. This open-access study (from her master’s degree at VU Amsterdam) was recently published in Nature.

Tanis is a lagerstätte first reported in 2019. It was created as a result of a large seismically-induced wave called a ‘seiche’ travelling from the Western Interior Seaway up the Tanis river. The seiche was likely caused by the asteroid that caused the end-Cretaceous (K-Pg) mass extinction. Image: Jackson Leibach.
The site is full of impact spherules which are small glassy beads that would have fallen at the time like hail. Based on their composition and concentric structure, these could only have formed from molten material that cooled in low gravity. From their size, it has been estimated that spherules of this size would have taken 15-30 from the asteroid impact to have been ejected from the atmosphere, cooled and then fallen back to Earth.

This video shows the record of impact spherules in the sediments at Tanis. As they hit the water and underlying sediments, they penetrated through, leaving a ‘funnel’ shape in their wake. Digging to the bottom of this, the spherule can be found.

Tanis is filled with numerous fossils of different organisms from different environments including fresh water, salt water and terrestrial (all three of which are shown in this video!). The sediments are soft enough that fossils can be dug out with a knife!

The fishes are remarkably well-preserved. They have three dimensionality and have undergone little chemical alteration which has been key to their chemical study.
3D skull of a paddlefish from Tanis.

CT scan of a 3D paddlefish from Tanis with impact spherules stuck within the gills. Interestingly, none were in the digestive tract, so it can be determined that these fish were not living amongst the spherules for long. Googly eyes represent the front of the animal. Yellow circles represent the impact spherules.

The bones of sturgeon, with oak-leaf cross sections were thinly sliced for analysis.
These bones grow similar to how tree rings grow and also show seasonality.
Tracing the bands of rapid growth (summer) and restricted growth (winter), A pattern emerged that showed that all the fishes stopped growing in springtime.
The impact occurring in springtime (northern hemisphere) was perhaps the worst case scenario for survival as animals may have just endured the hardships or winter and were beginning to raise young. That said, the impact occurring in autumn (fall) in the southern hemisphere would have been no walk in the park either. This information may help to explain why some groups went extinct, why some didn’t and how the world was able to recover. Image: Hulkepholis by Mark Witton.
The Tanis lagerstätte is a dream come true for many palaeontologists. It has incredibly well-preserved fossils that can be dated to within 30 minutes of the end-Cretaceous impact. It will undoubtedly deliver more exciting discoveries in future as the site continues to be excavated.
Breakdown of the Tanis reconstruction from Palaeoartist Joschua Knuppe.
Lead author of the study, Melanie During.

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