Trix is a Tyrannosaurus rex specimen excavated in 2013 in Montana, United States by a team of paleontologists from the Naturalis Biodiversity Center in Leiden, the Netherlands and Black Hills institute of Geological Research in South Dakota.

In 2012, Naturalis Biodiversity Center at Leiden, the largest natural history museum of the Netherlands, planned to open a new exhibition hall in 2017.

In order to increase the structural number of visitors from 300,000 to 400,000 per annum, the management decided to try and procure an authentic Tyrannosaurus skeleton, preferably one excavated by the museum itself.

In September 2012, a museum delegation travelled to the US to contact the Black Hills Institute, a company that had been involved in nine Tyrannosaurus excavations.

Senior Naturalis paleontologist John de Vos immediately visited the site and identified the remains as those of Tyrannosaurus.

The excellent preservation had been caused by the skeleton being surrounded by a three metres thick sandstone lens with a high chalk content, neutralising damaging acids.

On 5 September, paleontologist Philip Manning of the University of Manchester performed a lidar-laserscan of the site surface to precisely determine the position of all bones.

According to Peter Larson, director of the Black Hills Institute, Trix is the third most complete Tyrannosaurus found, after Sue and Stan.

The right lower leg bone and one of the ribs show evidence of distorted growth, suggesting a fracture or infection.

The sacrum and the first tail vertebra show uneven development which could suggest either a birth defect or an injury suffered at a very young age.

It is the only Tyrannosaurus specimen alongside MOR 980 (Peck's Rex) in which the mount includes the real skull fossilized bones.

[17] Also some of the actual gastralia or belly ribs were used, which had been found in articulation with the remainder of the ribcage, allowing for a particularly realistic restoration of the thorax.

On 23 June 2016 it was announced that the name overwhelmingly chosen was "Trix", as both an allusion to "T-rex" and former Queen Beatrix of the Netherlands.

[20] T. rex in Town is a temporary itinerant exhibition that happened for the first time from 10 September 2016 to 5 June 2017 in the Naturalis Biodiversity Center in Leiden, The Netherlands.

After travelling through different European countries in 2017, 2018 and 2019, T. rex in Town last visited Glasgow, Scotland, from 18 April 2019 to 31 July 2019 at the Kelvin Hall building.

[22][23][24] After that, the specimen returned to the Naturalis museum, to be installed on its final allocated location, a special hall that was under construction during Trix's European tour.

After a festive public reception on the city square of Leiden on 26 August, led by Dutch wildlife expert and television personality Freek Vonk, the skeleton was reassembled by Pete Larson for exhibition.

In 2021, a study of Trix by Pasha van Bijlert and Knoek van Soest concluded that when the energy use during a walking phase was minimized by optimizing the elastic energy in a swaying tail, the resulting step frequency indicated a preferred walking speed of about 5 km/h, less than previously assumed for large theropods.

[28] During its initial negotiations with Murray, Naturalis obtained an option of purchase, the eventual buying price to be determined by the completeness of the skeleton.

When more and more bones surfaced, it became clear that the museum had endeavoured on a project of considerable cost, estimated at seven million euros in November 2013.

Naturalis tried to increase the public awareness of the specimen, a crucial factor in attaining the projected number of visitors.

Part of the campaign was a dinosaur sticker book action by the Dutch largest supermarket chain, Albert Heijn, centred on the new skeleton and using Wikipedia images.

In September 2016 Dutch media reported that Trix's arrival in Leiden had resulted in €1.3 million of "free publicity" for the museum.