Fig. 3. 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, left dorsal rib "Rib B". A1, the whole rib, posterior face in proximal view. Foreshortening makes the shaft look shorter and narrower than it actually is: the position of the rib between two shelves makes it impossible to photograph in true posterior view; A2, close-up of the pneumatic opening in the tuberculum in medial view, with anterior to the bottom; A3, red-cyan anaglyph of the same, indicating the form and depth of the fossa. Scale bars provide only a rough indication of the size of the elements: see the text for measurements.