Dinosaur “mummies” preserve large areas of three-dimensional, wrinkled, desiccated integument renderings enveloping articulated skeletons. The majority of dinosaur “mummies” are “duck-billed” hadrosaurids in desiccated death poses that were buried in Late Cretaceous fluvial sandstones, such as in the Lance and Hell Creek formations of western North America, and unlike the lacustrine, marshy, or shallow marine sediments of many Konservat-Lagerstätten. Although first described in1912, the structural and chemical nature of these renderings and the steps leading to their formation have only recently begun to be explored in detail. Hypotheses about the mode and mechanism of “mummy” preservation have varied widely, from simple impressions in sediment to near-pristine retention of organic tissues or biomolecules. Two newly described “mummies”of the Late Cretaceous hadrosaurid Edmontosaurus annectens were subjected to a wide range of modern structural and chemical analyses. These specimens come from a several-kilometer region in east-central Wyoming, USA, near the top of the Maastrichtian Lance Formation that has produced multiple of the best-preserved “mummy” dinosaurs over the course of the last century. These two “mummies” preserve exceptional integument renderings, including skin/scales, tail spikes, and hooves. We characterized how the three-dimensional form of the integument is preserved and whether any original tissue structure or organic biomolecules remain. We find little evidence for any endogenous organic preservation or internal tissue structure. Instead, these “mummies” likely desiccated sub-aerially for a period of time before rapid burial and infilling of the carcass by fluvial sands. Next, organic decay induced preservation of their outermost integumentary surface largely via microbially induced clay templating (a < 1 mm, predominantly kaolinite/partly illite, external “death mask”). In places, microbially induced precipitation of iron compounds (e.g., pyrite) as stains/concretions occurred, although some of these precipitates might also appear during or be altered by late-stage weathering (e.g., iron oxides, manganese oxides). Dinosaur“mummies” are rare examples of high-fidelity mineral templating of soft tissues in Mesozoic animals, a taphonomic modebetter studied in far older, invertebrate fossils of Paleozoic or Precambrian marine Lagerstätten. The “mummies” are alsoclosely associated with fossil plant material, suggesting that the sediments were not organic-limited. Future work should characterize variation in the composition of fossilized dinosaur integument, determine if any endogenous organic or inorganic compounds from the integument preserve in other “mummies”, determine which mineral precipitates are early-stage and induced by decay versus late-stage and induced by subsurface oxidative weathering, and look for bacterial biomarkers (e.g., hopanoids) in the integument template.

Key words: Dinosauria, Edmontosaurus, clay templating, integument rendering, dinosaur mummification, taphonomy, Late Cretaceous

Evan T. Saitta [evansaitta@gmail.com; ORCID: https://orcid.org/0000-0002-9306-9060] and Laure Dussubieux [ldussubieux@fieldmuseum.org; ORCID: https://orcid.org/0000-0002-9334-8131], Integrative Research Center, Field Museum of Natural History, 1400 S Lake Shore Dr, Chicago, 60605 IL, USA. Paul C. Sereno [dinosaur@uchicago.edu; ORCID:https://orcid.org/0000-0001-7958-3701], Department of Organismal Biology & Anatomy, University of Chicago, Anatomy Bldg, 1027 E 57th St, Chicago, 60637 IL, USA; Committee on Evolutionary Biology, Chicago, IL, USA. Daniel Vidal [dvidal@uchicago.edu; ORCID: https://orcid.org/0000-0002-6054-1357] and Maria Ciudad Real [paleobiologa@gmail.com; ORCID: https://orcid.org/0009-0007-8342-0658], Grupo de Biología Evolutiva, UNED, Paseo Senda del Rey 9, 28040 Madrid, Spain. Tyler M. Keillor [tkeillor@uchicago.edu; ORCID: https://orcid.org/0009-0001-4830-2513] and Lauren Bop [conroy1@uchicago.edu; ORCID: https://orcid.org/0000-0002-2799-4792], Department of Organismal Biology & Anatomy, University of Chicago, Anatomy Bldg, 1027 E 57th St, Chicago, 60605 IL, USA. Stephanie L. Baumgart [sbaumgart@ufl.edu; ORCID: https://orcid.org/0000-0001-9534-7389], Department of Physiology and Aging, University of Florida College of Medicine, 1345 Center Drive, Gainesville, 32610 FL, USA. Aidan Burdick [aidanburdick2026@u.northwestern.edu; ORCID: https://orcid.org/0000-0002-1551-8975], Department of Earth and Planetary Sciences, Northwestern University, 2145 Sheridan Road, Evanston, 60208 IL, USA. Viktor J. Radermacher [viktor.radermacher@wits.ac.za; ORCID: https://orcid.org/0000-0001-6524-7811], Evolutionary Studies Institute, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2000, South Africa; Karoo Origins Fossil Centre, Graaff-Reinet, South Africa; Department of Earth & Environmental Sciences, University of Minnesota, Minnesota, USA. Thomas G. Kaye [tom@tomkaye.com; ORCID: https://orcid.org/0000-0001-7996-618X], Foundation for Scientific Advancement, 7023 Alhambra Dr, Sierra Vista, AZ 85658, USA. Evan Maxey [emaxey@anl.gov; ORCID: https://orcid.org/0000-0001-7519-5277], Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, 60439 IL, USA. Alexander S. Filatov [afilatov@uchicago.edu; ORCID: https://orcid.org/0000-0002-8378-1994], Department of Chemistry and James Franck Institute, University of Chicago, 5735 S Ellis Ave, Chicago, 60637 IL, USA. Xinqi Chen [xchen@northwestern.edu; ORCID: https://orcid.org/0000-0003-0413-4893], Keck Interdisciplinary Surface Science (Keck-II) Center, NUANCE Center, Northwestern University, 2145 Sheridan Road, Evanston, 60208 IL, USA.