Fig. 2. Sauropod dinosaur Brachiosaurus altithorax Riggs, 1903, holotype FMNH PR 25107 from Dinosaur Quarry No. 13 near Grand Junction, Colorado, dating to the Kimmeridgian–Tithonian ages of the Late Jurassic, right dorsal rib "Rib A" in posterior view with proximal to the left. A1, the whole proximal half of the rib; a distal portion also exists, of similar length but without features relevant to this study; A2, close-up of the tuberculum, highlighting the complex network of support structures that show signs of speculative reconstruction. Circles highlight two possible sites of the "second tubercle" referred to by Riggs (1901: 549, 1903: 303, 1904: 239) based on Marsh's illustration (1896: figs. 7, 8), reproduced here in Fig. 4; A3, close-up of the pneumatic foramen in the shaft of the rib, showing natural bone texture around the margin and no indication of breakage. Scale bars provide only a rough indication of the size of the elements: see the text for measurements.