Figure 1. Mounted cast skeleton of Barosaurus referred specimen AMNH 6341, in the entrance hall of the American Museum of Natural History. Homo sapiens (MPT) for scale. Photograph by MJW.

Figure 2. Posterior cervical vertebrae (C13) of diplodocine sauropods in lateral view, showing proportional differences. Top left, Kaatedocus, from Tschopp and Mateus (2012: unnumbered supplementary figure tjsp_a_746589_sup_30912151.tif); top right, Barosaurus referred specimen AMNH 6341, left lateral, reversed (photo by MJW); bottom left, Diplodocus, from Hatcher (1901: plate III); bottom right, Diplodocus elongated by 33%. Kaatedocus and Barosaurus scaled to the same centrum length as original Diplodocus and elongated Diplodocus respectively. In lateral view, the widely assumed similarity between the cervicals of Barosaurus and elongated Diplodocus is largely borne out: the principal differences in Barosaurus are the less prominent and more posteriorly positioned neural spine, the more prominent ventrolateral flanges, and the reduced pneumatic fossa in the centrum.

Figure 3. Barosaurus lentus holotype YPM 429, vertebra R, C?15. Top row, left to right: posterior, dorsal and anterior views; middle row: right lateral view; bottom row: ventral view, from Lull (1919: plate II). Note the apparently very low, undivided neural spine, forward-shifted neural arch, broad prezygapophyses, broad, wing-like lateral laminae, and great width across the diapophyses and across the parapophyses.

Figure 4. Diplodocid posterior vertebrae in dorsal view, scaled to equal total length. Left column, from top to bottom: Apatosaurus ajax Tokyo specimen NSMT-PV 20375, C12; Barosaurus lentus holotype YPM 429, vertebra R (C?15). Right column, from top to bottom: Apatosaurus ajax holotype YPM 1860, C?11; Suuwassea emilieae holotype ANS 21122, C7; Kaatedocus siberi holotype SMA 004, C13. Diplodocus is not pictured due to the lack of published illustrations. The vertebrae of Apatosaurus and Barosaurus are proportionally much wider than those of Suuwassea and Kaatedocus, and the bifurcation of the neural spine is far wider in Apatosaurus than in Kaatedocus.

Figure 5. Giraffatitan brancai lectotype MB.R.2180, fifth cervical vertebra. Top row: dorsal view, with anterior to the left. Bottom row (from left to right): anterior, left lateral and posterior views.

Figure 6. Barosaurus lentus holotype YPM 429, Vertebra Q (C?13). Top row: left ventrolateral view. Middle row, from left to right: anterior view, with ventral to the right; ventral view; posterior view, with ventral to the left. Bottom row: right lateral view, inverted. Inset shows diapophyseal facet on right side of vertebra, indicating that the cervical ribs were unfused in this individual despite its great size. Note the broad, flat prezygapophyseal facet visible in anterior view.

Figure 7. Barosaurus lentus holotype YPM 429, Vertebra S (C?12). Left column from top to bottom: dorsal, right lateral and ventral views; right column: anterior view. Inset shows displaced fragment of broken prezygapophysis. Note the narrow span across the parapophyses in ventral view, and the lack of damage to the ventral surface of the centrum which would indicate transverse crushing.

Figure 8. Barosaurus lentus holotype YPM 429, cervical vertebrae in ventral view. From top to bottom: vertebra R (from Lull 1919: plate II), vertebra Q , vertebra S. Probably from more posterior to more anterior.

Figure 9. Partial reconstruction of the Barosaurus lentus holotype YPM 429, cervical vertebra R, approximating its undamaged state by allowing for dorsoventral crushing, shearing and loss of some extremities. Anterior and posterior views scaled to 125% of uncorrected width and 80% of uncorrected height. Dorsal view scaled to 80% of uncorrected height; condyle moved forward and cotyle scaled to 50% of uncorrected width to allow for shearing. Lateral view scaled to 125% of uncorrected height, and sheared backwards 15 degrees. Metapophyses and postzygapophyses drawn in multiple views based on vertebrae Q and S and AMNH 6341 material.

Figure 10. Barosaurus AMNH 6341, cervical vertebrae in dorsal view, to scale. Left column, from top to bottom: C9–C12. Right column, from top to bottom: C14–C16. Extensive image manipulation was necessary to bring out the information in these photographs, due to to poor photography conditions. C16 is sheered to the right, so the aspect is slightly left dorsolateral rather than true dorsal. C8 is on display in the gallery with these vertebrae, but the structure of the display makes it impossible to photograph in dorsal view. C13 is on a shelf in collections, apart from the other cervicals, and we were not able to photograph it in dorsal view.

Figure 11. Barosaurus AMNH 6341, cervical vertebrae C8–C16 in dorsal (where available) and lateral views, to scale. Lateral views except C13 from McIntosh (2005: fig. 2.1).

Figure 12. Similarities between Barosaurus lentus holotype YPM 429, cervical vertebra R (C?15, left) and referred specimen AMNH 6341, C15 (right). Green brackets show width of prezygapophyseal rami, omitting apparent reconstruction on left anterolateral corner of YPM 429. Dark red outlines indicate margins of diapophyseal wings. Dark blue outlines show posterior fillets of diapophyseal wings. Light red “X” on AMNH 6341 indicates base of metapophyses, extended from prezygadiapophyseal and postzygadiapophyseal laminae and forming a diagonal cross similar to that of vertebra R. Prezygapophyseal facets of AMNH 6341 highlighted in yellow: the right facet is fairly clear in the photograph (see Figure 10); the exact margin of the left facet is less certain. Zygapophyseal facets cannot be directly recognised in vertebra R due to poor preservation and overzealous reconstruction.

Figure 13. Attachments of the lateral flexor muscles of the neck in Kaatedocus and Barosaurus. On the left, C11 of Kaatedocus siberi holotype SMA 0004 (traced from Tschopp and Mateus 2012: fig. 10C2) in dorsal (top) and right lateral (bottom) views, with simplified versions of the lateral flexor muscles included, based on those of birds (see Wedel and Sanders 2002, and Taylor and Wedel 2013). The M. longus colli dorsalis and M. cervicalis ascendens insert together on the epipophysis (= torus dorsalis of birds), and the M. flexor colli lateralis and M. longus colli ventralis (ventral and medial, not shown) insert together on the cervical rib. The pre-epipophysis (sensu Tschopp and Mateus 2012) and the head of the cervical rib may have served as expanded attachments for M. cervicalis ascendens and M. flexor colli lateralis, respectively. The actual muscles were probably much more complex than those drawn here, with numerous slips connecting multiple vertebrae: for a similar condition in birds, see Zweers et al. (1987) and van der Leeuw et al. (2001: fig. 2). On the right, C15 of Barosaurus AMNH 6341, scaled to the same total length as C11 of Kaatedocus. Actual total lengths for the two vertebrae are 840 mm for C15 of Barosaurus (McIntosh 2005: table 2.1) and 324 mm for C11 of Kaatedocus (Tschopp and Mateus 2012: table 1). In Barosaurus, the ansae costotransversariae or cervical rib loops are taller, wider and more posteriorly located than in Kaatedocus, providing a larger attachment area for the lateral flexor muscles (blue arcs) and lending them greater mechanical advantage (red lines). In this respect, Barosaurus is more similar to Apatosaurus than to the narrow-necked Diplodocus, although the cervical ribs of Barosaurus are much less robust than those of Apatosaurus.