Tanis is a private paleontological site in southwestern North Dakota, United States. It is part of the heavily studied Hell Creek Formation, a geological region renowned for many significant fossil discoveries from the Upper Cretaceous and lower Paleocene. Based solely on the Tanis team's publications, the Tanis is unique in that it appears to record, in detail, concrete evidence of the direct effects of the giant Chicxulub asteroid impact which struck the Gulf of Mexico 65.5 million years ago, and wiped out almost all dinosaurs and many other species at the end of the Mesozoic. The extinction event caused by this impact began the Cenozoic, in which mammals—including humans—eventually came to dominate life on Earth.

The site was originally discovered in 2008 by University of North Georgia Professor Steve Nicklas and field paleontologist Rob Sula. Their team successfully removed fossil field jackets that contained articulated sturgeons, paddlefish, and bowfins. These fossils were delivered for research to the Field Museum of Natural History in Chicago. Recognizing the unique nature of the site, Nicklas and Sula brought in Robert DePalma, a University of Kansas graduate student, to perform additional excavations. The site was systematically excavated by Robert DePalma over several years beginning in 2012, working in near-total secrecy. Key findings were presented in two conference papers in October 2017. The full paper introducing Tanis was widely covered in worldwide media on 29 March 2019, in advance of its official publication three days later. The co-authors included Walter Alvarez and Jan Smit, both renowned experts on the K–Pg impact and extinction.

At Tanis, unlike any other known Lagerstätte site, it appears specific circumstances allowed for the preservation of moment-by-moment details caused by the impact event. These include finds which allow examination of the direct effects of the impact on plants and animals alive at the time of the large impact some 3,000 km (1,900 mi) distant. The events at Tanis occurred too soon after impact to be caused by the megatsunamis expected from any large impact near large bodies of water. Instead, much faster seismic waves from the magnitude 10 – 11.5 earthquakes probably reached the Hell Creek area as soon as ten minutes after the impact, creating seiche waves between 10–100 m (33–328 ft) high in the Western Interior Seaway. The site formed in part of a bend in an ancient river on the westward shore of the seaway, and was flooded with great force by these waves, which carried sea, land, freshwater animals and plants, and other debris several miles inland. The seiche waves exposed and covered the site twice, as millions of tiny microtektite droplets and debris from the impact were arriving on ballistic trajectories from their source in what is now the Yucatán Peninsula.

Reported findings include:

Analysis of the fish skeletons found them to be in the spring phase of their annual cyclical changes, implying that the impact had occurred in spring. Depalma et al. (2021) opted for a spring-summer range, but During et al. (2024) reevaluated and criticized this study based on its lack of primary data, unidentified laboratory for the analyses, insufficient methods for accurate replication and problematic isotopic graphs with irregular data and error bars.

The hundreds of fish remains are distributed by size, and generally show evidence of tetany (a body posture related to suffocation in fish), suggesting the suffocation of an entire population. Fragile remains spanning the layers of debris show the site was laid down in a single brief event.

The exceptional nature of the findings and conclusions led some scientists to await further scrutiny before agreeing that the discoveries at Tanis had been correctly understood, further exacerbated by concerns over the reliability of data with researchers racing to claim credit for findings.

The Cretaceous–Paleogene ("K-Pg" or "K-T") extinction event around 65.5 million years ago wiped out almost dinosaurs and many other species. Proposed by Luis and Walter Alvarez, it is now widely accepted that the extinction was caused by a huge asteroid or bolide that impacted Earth in the shallow seas of the Gulf of Mexico, leaving behind the Chicxulub crater. The impactor tore through the Earth's crust, creating huge earthquakes, giant waves, and a crater 180 kilometers (112 mi) wide, and blasted aloft trillions of tons of dust, debris, and climate-changing sulfates from the gypsum seabed, and it may have created firestorms worldwide. With the exception of some ectothermic species such as the ancestors of the modern leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kg (55 lb) survived. It marked the end of the Cretaceous period and the Mesozoic Era, opening the Cenozoic Era that continues today.