Figure 1.
Simplified overview of Triassic archosauriform phylogeny based upon Nesbitt [48] showing relationships of major clades.
Taxa marked with an asterisk were sampled for micro-CT scanning as part of this study. Stars indicate clades with unambiguous osteological evidence for postcranial skeletal pneumaticity (pterosaurs, neotheropods, most sauropodomorphs). The dark grey box delimits the clade (Ornithodira) for which we propose a bird-like air sac system was present. The light grey box delimits the minimum clade for which Gower [34] suggested postcranial skeletal pneumaticity might be present.
Figure 2.
Holotype of the ctenosauriscid poposauroid Hypselorhachis mirabilis (NHMUK R16586, dorsal vertebra; with the elongate neural spine removed).
Anterior (A), left lateral (B) and posterior (C) views, illustrating many of the typical vertebral laminae and fossae present in Triassic archosauriform vertebrae. Abbreviations: cdf, centrodiapophyseal fossa; pa, parapophysis; pcdl, posterior centrodiapophyseal lamina; pocdf, postzygapophyseal centrodiapophyseal fossa; podl, postzygodiapophyseal lamina; ppdl, paradiapophyseal lamina; prcdf, prezygapophyseal centrodiapophyseal fossa; prdl, prezygodiapophyseal lamina; prpl, prezygaparapophyseal lamina; sdf, spinodiapophyseal fossa; spof, spinopostzygapophyseal fossa; sprf, spinosprezygapophyseal fossa; sprl, spinoprezygapophyseal lamina. After Butler et al. (2009b). Scale bar equals 10 mm.
Figure 3.
Vertebrae of extant reptiles lacking postcranial skeletal pneumaticity.
A, B: Alligator mississippiensis, NHMUK RR 73.2.21.2, right lateral view (A) and transverse section (B). C–E: Alligator mississippiensis, NHMUK RR 1975.1423, transverse sections (C, E) and right lateral view (D). F–H: Chelonoidis nigra abingdoni, NHMUK RR 76.6.21.44, right lateral view (F), and transverse sections (G, H). I–K, Varanus komodoensis, NHMUK RR 1934.9.2.2, right lateral view (I, rendering of CT data) and transverse (J) and axial (K) sections. Asterisks adjacent to renderings indicate positions of sections. Abbreviation: nf, nutrient foramina. Scale bars equal 10 mm.
Figure 4.
Ostrich, Struthio camelus (NHMUK unnumbered, first rib-bearing vertebra).
Postcranial skeletal pneumaticity in an extant taxon. A, C: left (A) and right (C) lateral views. Asterisks mark the point of the cross-sections shown in B, D, and E. B, D, E: transverse sections through the vertebra. F: oblique right anterolateral view. G: cutaway of rendered model showing internal pneumatic cavities. Abbreviation: pnf, pneumatic foramen. Scale bar in A and C equals 10 mm.
Figure 5.
Rhynchosaur Stenaulorhynchus stockleyi, dorsal vertebra (NHMUK R36618).
A: right lateral view. The asterisk positioned adjacent to the anterior margin shows the approximate position of section shown in B, whereas the asterisk positioned along the dorsal margin of the element corresponds to the approximate position of the transverse section shown in C. B, C: sections through the element. Abbreviations: ctb, cortical bone; dia, diapophysis; dtb, dense trabecular bone; nc, neural canal; poz, postzygapophysis; prz, prezygapophysis; sed, sediment. Scale bar equals 10 mm.
Figure 6.
Erythrosuchus africanus, vertebrae and vertebral fragments.
NHMUK R3592, CT cross-sections (only the neural arch and the dorsal part of the centrum were scanned): A: transverse section, taken at a point level to the anterior margin of the transverse process. B: axial section through neural arch at a point level with bases of postzygapophyses. C: parasagittal section taken at point just lateral to right border of neural canal. D: left lateral view of NHMUK R3592 ‘fragment A’ (CT rendering). E: axial section of ‘fragment A’, illustrating cavity present within the neural arch. F: parasagittal section of ‘fragment A’, illustrating sediment-filled canal that runs through bone dorsomedially from the deepest part of the infradiapophyseal fossa. G: transverse section through ‘fragment B’, illustrating vacuity within left postzygapophysis. H: transverse section through ‘fragment B’, illustrating possible connection between vacuity within left postzygapophysis and the postspinal fossa. Abbreviations: cdf, centrodiapophyseal fossa; ?con, possible connection between postspinal fossa and vacuity; dtb, dense trabecular bone; for, foramen; lpoz, left postzygapophysis; nc, neural canal; pocdf, postzygapophyseal centrodiapophyseal fossa; poz, postzygapophysis; prcdf, prezygapophyseal centrodiapophyseal fossa; prz, prezygapophysis; rpoz, right postzygapophysis; sedf, sediment-infilled external fossa; spof, spinopostzygapophyseal fossa; vac, larger intertrabecular vacuities within bone. All scale bars equal 10 mm.
Figure 7.
Phytosauria indet., vertebrae.
A, B: NHMUK OR38072, dorsal vertebra in anterior (A) and posterior (B) views (photographs). C-F: SMNS unnumbered, dorsal vertebra in anterior (C) and right lateral (D) views with sections through the specimen (E, F). Abbreviations: acdl, anterior centrodiapophyseal lamina; cdf, centrodiapophyseal fossa; dtb, dense trabecular bone; nc, neural canal; pcdl, posterior centrodiapophyseal lamina; pocdf, postzygapophyseal centrodiapophyseal fossa; podl, postzygodiapophyseal lamina; poz, postzygapophysis; prcdf, prezygapophyseal centrodiapophyseal fossa; prdl, prezygodiapophyseal lamina; prz, prezygapophysis; spof, spinopostzygapophyseal fossa; sprf, spinosprezygapophyseal fossa. All scale bars equal 10 mm.
Figure 8.
Paratypothoracine aetosaur, anterior dorsal vertebra (NHMUK OR38070).
A, B: dorsal vertebra in anterior (A) and posterior (B) views. Note that there is a large volume of sediment adhering to the posteroventral surface of the left transverse process. The left transverse process was incompletely scanned and so is artificially truncated at a point just distal to the parapophysis. C: CT slice showing transverse section (in anterior view) immediately anterior to the base of the postspinal fossa. The left of the two foramina within the postspinal fossa is visible, and is surrounded by dense trabecular bone. D: CT slice showing section through the neural canal. The position of the left foramen within the postspinal fossa is marked. Note that in both CT slices intertrabecular spaces are mineral-infilled; pore spaces in the sediment immediately adjacent to the external surface of the bone are also infilled. Abbreviations: acpl, anterior centroparapophyseal lamina; for, foramen; ipmn, mineral-infilled pore spaces within sediment adjacent to the bone; nc, neural canal; pa, parapophysis; podl, postzygodiapophyseal lamina; poz, postzygapophysis; prdl, prezygadiapophyseal lamina; prpl, low ridge forming incipient prezygaparapophyseal lamina; prz, prezygapophysis; sed, sediment; spof, spinopostzygapophyseal fossa; sprf, spinosprezygapophyseal fossa. All scale bars equal 10 mm.
Figure 9.
Bromsgroveia, dorsal vertebra (BIRUG 2473).
A: left lateral view. Note that the asterisks positioned along the dorsal margin of the element correspond to positions of transverse sections shown in C–F, while the asterisk positioned adjacent to the posterior margin shows the approximate position of axial sections G and H. B: close-up of deepest part of centrodiapophyseal fossa in left lateral view showing the positions of ‘foramen 2’ and ‘foramen 3’. C: transverse section through the element close to its anterior end. Note that the centrum, neural arch pedicel, and prezygapophysis are composed of dense trabecular bone. D: transverse section through the element close to mid-length. Note the presence of a relatively large sediment-infilled intertrabecular space (siv) in the neural arch. E: transverse section through the element close to the posterior end. Relatively large paired sediment-filled intertrabecular spaces are present in the neural arch dorsal to the neural canal and are separated from one another by a bony midline septum. F: tranverse section through the element close to the posterior end. G: axial section. Note that ‘foramen 4’ extends into a sediment-infilled canal that is connected to ‘foramen 3’. H: axial section, positioned slightly dorsal to the section shown in G. ‘Foramen 1’ also extends into a sediment-infilled canal that is connected to ‘foramen 2’. I: sagittal section (anterior end of the specimen is towards the right). Two relatively large sediment-infilled intertrabecular spaces (siv) are visible in the neural arch – the posterior space corresponds to that shown in E, and the anterior space corresponds to that shown in D. Abbreviations: cdf, centrodiapophyseal fossa; ctb, cortical bone; dia, diapophysis; dtb, dense trabecular bone; for1, for2, for3, for4, foramina; fs, fossa; nc, neural canal; pa, parapophysis; pcdl, posterior centrodiapophyseal lamina; pocdf, postzygapophyseal centrodiapophyseal fossa; podl, postzygodiapophyseal lamina; poz, postzygapophysis; ppdl, paradiapophyseal lamina; prcdf, prezygapophyseal centrodiapophyseal fossa; prdl, prezygodiapophyseal lamina; prpl, prezygaparapohyseal lamina; prz, prezygapophysis; sed, sediment; siv, sediment-infilled vacuity; spt, bony septum. All scale bars equal 10 mm with the exception of B, which is equal to 5 mm.
Figure 10.
Effigia okeeffeae, cervical and dorsal vertebrae (AMNH FR 30587).
A–E, anterior cervical vertebra in right lateral view (A) and cross-section (B–E). Asterisks dorsal and ventral to the vertebra in A indicate the positions of the transverse sections shown in B–D. Asterisks to left and right of the vertebra in A indicate the position of the axial section shown in E. F–H: CT slices showing transverse sections through AMNH FR 30587, four semi-articulated dorsal vertebrae. F: section through third preserved dorsal, immediately posterior to transverse process. G: section through third preserved dorsal, immediately posterior to transverse process. H: section through second preserved dorsal, immediately anterior to most anterior extent of neural spine. Note that in both vertebrae figured the neural arch and centrum are disarticulated. Abbreviations: acpl, anterior centroparapophyseal lamina; cdf, centrodiapophyseal fossa; cen, centrum; cprl, centroprezygapophyseal lamina; dia, diapophysis; for, foramina within base of spinopostzygapophyseal fossa; nc, neural canal; pa, parapophysis; pcdl, posterior centrodiapophyseal lamina; ped, neural arch peduncle; pfo, deep fossa on posterior of neural arch; pocdf, postzygapophyseal centrodiapophyseal fossa; podl, postzygodiapophyseal lamina; prcdf, prezygapophyseal centrodiapophyseal fossa; prpl, prezygoparapophyseal lamina; prz, prezygopophysis; siv, sediment-infilled vacuity within neural arch; sp, neural spine; spof, spinopostzygapophyseal fossa. All scale bars equal 10 mm.
Figure 11.
Dinosauromorpha, cervical and dorsal vertebrae.
Silesaurus (A–D) and Scelidosaurus (E). A: ZPAL Ab III 1299, anterior cervical vertebra, tranverse CT section close to anterior end of specimen. B: ZPAL Ab III 423/6, posterior cervical vertebra, transverse section close to anterior end of specimen. C: ZPAL Ab III 423/6, posterior cervical vertebra, transverse section close to posterior end of specimen. D: ZPAL Ab III 404/4, posterior dorsal vertebra, transverse section close to posterior end of specimen. E: NHMUK R1111, anterior dorsal vertebra, transverse section through element close to midlength. Note that there is substantial heterogeneity in the distribution of trabecular bone, but there is no evidence of large pneumatic vacuities. Abbreviations: cprf, centroprezygapophyseal fossa; dia, diapophysis; nc, neural canal; ncs, neurocentral suture (unfused); nfor, nutrient foramen on lateral surface of centrum; pa, parapophysis; pocdf, postzygapophyseal centrodiapophyseal fossa; prcdf, prezygapophyseal centrodiapophyseal fossa; sdf, spinodiapophyseal fossa; siv, sediment-infilled vacuity within neural arch; spof, spinopostzygapophyseal fossa; sprf, spinosprezygapophyseal fossa. All scale bars equal 10 mm.
Figure 12.
Plateosaurus, cervical and dorsal vertebrae.
A–C: SMNS F65, cervical vertebra, left lateral view (A, CT rendering) and close-up of the right postzygapophyseal centrodiapophyseal fossa in posterolateral view showing foramina (B, CT rendering; C). D: SMNS 12950, mid dorsal vertebra, close-up of right prezygapophyseal centrodiapophyseal fossa showing cluster of foramina. Abbreviations: acdl, anterior centrodiapophyseal lamina; cdf, centrodiapophyseal fossa; dia, diapophysis; for, foramina; pa, parapophysis; pcdl, posterior centrodiapophyseal lamina; pocdf, postzygapophyseal centrodiapophyseal fossa; poz, postzygapophysis; prcdf, prezygapophyseal centrodiapophyseal fossa; prdl, prezygodiapophyseal lamina. Scale bars equal 50 mm (A) and 10 mm (C, D).