Extended Data Fig. 3: The geography, geology, taphonomy, and fossil collecting at the Egg Mountain locality (Museum of the Rockies site TM-006).

a, Locality map of the field area at the Willow Creek Anticline, Two Medicine Formation near Choteau, Montana, USA. The Egg Mountain locality (EM) is indicated with an arrow. b, Stratigraphic section measured at Egg Mountain. See Supplementary Discussion for detailed discussion of Egg Mountain geology. c, Close up of Units 1–3, which were quarried from 2010 to 2016. These sections are subdivided into 12 separate jackhammer passes (JHP). JHPs are stratigraphically parallel cuts that proceeded at ~10 cm-deep intervals through the quarry. The JHPs that produced F. primaevus fossils are indicated by the skeleton illustrations. The ichnofauna (see legend below) indicate that Egg Mountain sediments were subaerially exposed. d, Photograph of Egg Mountain quarry taken from the north. Note the thick rusty limestones in the foreground, which poke through the siltstone in the main quarry towards the southern quarry wall. A smaller discontinuous limestone lens is visible in the east wall of the main quarry. e–g, Representative dinosaur specimens from Egg Mountain. e, Tyrannosaurid tooth showing the longitudinal fracturing common at the site. f, Shed hadrosaur tooth. One of the most common fossils are shed hadrosaur teeth of small dimensions. g, Unidentified hadrosaur bone, one of the few large bones recovered from the site. This specimen is from a partial, associated hadrosaur skeleton. In contrast to the bones of small mammals and squamates from the site, these large bones are in poor condition showing surface corrosion and numerous breaks. The close position of the fragments suggests that the breakdown of these bones occurred post depositionally, either subaerially or within the soils. h, Grid system of the main quarry (MQ) set up 11 m north by 9 m west (N1–11/W5–13); each quadrant is one square-meter. Yellow square indicates the portion of the quarry where F. primaevus fossils were found. Pins labeled 1–11 indicate where specific F. primaevus specimens were found (corresponding to Fig. 2 in the main text), pins labeled 8₁ and 8₂ represent the location of the two collections that make up MOR 10908. i, MOR 10908 as found in situ in the field. Skull (MOR 10908 A) visible in right lateral view at top left of image with its pelvic girdle visible in lower left. Portions of its vertebral column were lost in missing block to left. Postcrania of other individuals are partially visible to the right of this specimen, including the impression of a femur. Orientation as in Fig. 1j in the main text. Parts a–d and h were modified from ref. ²⁹. Abbreviations: F, femur; Ls, micritic limestone; MQ, main quarry; Pg, pelvic girdle; Sk, skull; Sltst, siltstone; UQ, upper quarry. Scale bars = 1 cm (e), 0.5 cm (f), and 2 cm (g, i).