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Why sauropods had long necks; and why giraffes have short necks - PubMed

  • ️Tue Jan 01 2013

Why sauropods had long necks; and why giraffes have short necks

Michael P Taylor et al. PeerJ. 2013.

Abstract

The necks of the sauropod dinosaurs reached 15 m in length: six times longer than that of the world record giraffe and five times longer than those of all other terrestrial animals. Several anatomical features enabled this extreme elongation, including: absolutely large body size and quadrupedal stance providing a stable platform for a long neck; a small, light head that did not orally process food; cervical vertebrae that were both numerous and individually elongate; an efficient air-sac-based respiratory system; and distinctive cervical architecture. Relevant features of sauropod cervical vertebrae include: pneumatic chambers that enabled the bone to be positioned in a mechanically efficient way within the envelope; and muscular attachments of varying importance to the neural spines, epipophyses and cervical ribs. Other long-necked tetrapods lacked important features of sauropods, preventing the evolution of longer necks: for example, giraffes have relatively small torsos and large, heavy heads, share the usual mammalian constraint of only seven cervical vertebrae, and lack an air-sac system and pneumatic bones. Among non-sauropods, their saurischian relatives the theropod dinosaurs seem to have been best placed to evolve long necks, and indeed their necks probably surpassed those of giraffes. But 150 million years of evolution did not suffice for them to exceed a relatively modest 2.5 m.

Keywords: Cervical vertebra; Dinosaur; Evolution; Giraffe; Neck; Sauropod.

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Figures

Figure 1
Figure 1. Necks of long-necked non-sauropods, to scale.

The giraffe and Paraceratherium are the longest necked mammals; the ostrich is the longest necked extant bird; Therizinosaurus and Gigantoraptor are the largest representatives of two long-necked theropod clades; Arambourgiania is the longest necked pterosaur; and Tanystropheus has a uniquely long neck relative to torso length. Human head modified from Gray’s Anatomy (1918 edition, fig. 602). Giraffe modified from photograph by Kevin Ryder (CC BY,

http://flic.kr/p/cRvCcQ

). Ostrich modified from photograph by “kei51” (CC BY,

http://flic.kr/p/cowoYW

). Paraceratherium modified from Osborn (, figure 1). Therizinosaurus modified from Nothronychus reconstruction by Scott Hartman. Gigantoraptor modified from Heyuannia reconstruction by Scott Hartman. Arambourgiania modified from Zhejiangopterus reconstruction by Witton & Naish (2008, figure 1). Tanystropheus modified from reconstruction by David Peters. Alternating blue and pink bars are 1 m tall.

Figure 2
Figure 2. Full skeletal reconstructions of selected long-necked non-sauropods, to scale.

1, Paraceratherium. 2, Therizinosaurus. 3, Gigantoraptor. 4, Elasmosaurus. 5, Tanystropheus. Elasmosaurus modified from Cope (1870, plate II, figure 1). Other image sources as for Fig. 1. Scale bar = 2 m.

Figure 3
Figure 3. Necks of long-necked sauropods, to scale.

Diplodocus, modified from elements in Hatcher (1901, plate 3), represents a “typical” long-necked sauropod, familiar from many mounted skeletons in museums. Puertasaurus, Sauroposeidon, Mamenchisaurus and Supersaurus modified from Scott Hartman’s reconstructions of Futalognkosaurus, Cedarosaurus, Mamenchisaurus and Supersaurus respectively. Alternating pink and blue bars are one meter in width. Inset shows Fig. 1 to the same scale.

Figure 4
Figure 4. Extent of soft tissue on ostrich and sauropod necks.

1, Ostrich neck in cross section from Wedel (2003, figure 2). Bone is white, air-spaces are black, and soft tissue is grey. 2, Hypothetical sauropod neck with similarly proportioned soft-tissue. (Diplodocus vertebra silhouette modified from Paul, 1997, figure 4A). The extent of soft tissue depicted greatly exceeds that shown in any published life restoration of a sauropod, and is unrealistic. 3, More realistic sauropod neck. It is not that the soft-tissue is reduced but that the vertebra within is enlarged, and mass is reduced by extensive pneumaticity in both the bone and the soft-tissue.

Figure 5
Figure 5. Simplified myology of the sauropod neck, in left lateral view, based primarily on homology with birds, modified from Wedel & Sanders (2002, figure 2).

Dashed arrows indicate muscle passing medially behind bone. A, B. Muscles inserting on the epipophyses, shown in red. C, D, E. Muscles inserting on the cervical ribs, shown in green. F, G. Muscles inserting on the neural spine, shown in blue. H. Muscles inserting on the ansa costotransversaria (“cervical rib loop”), shown in brown. Specifically: A. M. longus colli dorsalis. B. M. cervicalis ascendens. C. M. flexor colli lateralis. D. M. flexor colli medialis. E. M. longus colli ventralis. In birds, this muscle originates from the processes carotici, which are absent in the vertebrae of sauropods. F. Mm. intercristales. G. Mm. interspinales. H. Mm. intertransversarii. Vertebrae modified from Gilmore (1936, plate 24).

Figure 6
Figure 6. Basic cervical vertebral architecture in archosaurs.

1, Seventh cervical vertebra of a turkey, Meleagris gallopavo Linnaeus, 1758, traced from photographs by MPT. 2, Fifth cervical vertebra of the abelisaurid theropod Majungasaurus crenatissimus Depéret, 1896, UA 8678, traced from O’Connor (2007, figures 8 and 20). In these taxa, the epipophyses and cervical ribs are aligned with the expected vectors of muscular forces. The epipophyses are both larger and taller than the neural spine, as expected based on their mechanical importance. The posterior surface of the neurapophysis is covered by a large rugosity, which is interpreted as an interspinous ligament scar like that of birds (O’Connor, 2007). Because this scar covers the entire posterior surface of the neurapophysis, it leaves little room for muscle attachments to the spine. 3, Fifth cervical vertebra of Alligator mississippiensis Daudin, 1801, MCZ 81457, traced from 3D scans by Leon Claessens, courtesy of MCZ. Epipophyses are absent. 4, Eighth cervical vertebra of Giraffatitan brancai (Janensch, 1914) paralectotype MB.R.2181, traced from Janensch (1950, figures 43 and 46). Abbreviations: cr, cervical rib; e, epipophysis; ns, neural spine; poz, postzygapophysis.

Figure 7
Figure 7. Disparity of sauropod cervical vertebrae.

1, Apatosaurus “laticollis” Marsh, 1879b holotype YPM 1861, cervical ?13, now referred to Apatosaurus ajax (see McIntosh, 1995), in posterior and left lateral views, after Ostrom & McIntosh (1966, plate 15); the portion reconstructed in plaster (Barbour, 1890, figure 1) is grayed out in posterior view; lateral view reconstructed after Apatosaurus louisae (Gilmore, 1936, plate XXIV). 2, “Brontosaurus excelsus” Marsh, 1879a holotype YPM 1980, cervical 8, now referred to Apatosaurus excelsus (see Riggs, 1903), in anterior and left lateral views, after Ostrom & McIntosh (1966, plate 12); lateral view reconstructed after Apatosaurus louisae (Gilmore, 1936, plate XXIV). 3, “Titanosaurus” colberti Jain & Bandyopadhyay, 1997 holotype ISIR 335/2, mid-cervical vertebra, now referred to Isisaurus (See Wilson & Upchurch, 2003), in posterior and left lateral views, after Jain & Bandyopadhyay (1997, figure 4). 4, “Brachiosaurus” brancai paralectotype MB.R.2181, cervical 8, now referred to Giraffatitan (see Taylor, 2009), in posterior and left lateral views, modified from Janensch (1950, figures 43–46). 5, Erketu ellisoni holotype IGM 100/1803, cervical 4 in anterior and left lateral views, modified from Ksepka & Norell (2006, figures 5a–d).

Figure 8
Figure 8. Sauropod cervical vertebrae showing anteriorly and posteriorly directed spurs projecting from neurapophyses.

1, Cervical 5 of Sauroposeidon holotype OMNH 53062 in right lateral view, photograph by MJW. 2, Cervical 9 of Mamenchisaurus hochuanensis holotype CCG V 20401 in left lateral view, reversed, from photograph by MPT. 3, Cervical 7 or 8 of Omeisaurus junghsiensis Young, 1939 holotype in right lateral view, after Young (1939, figure 2). (No specimen number was assigned to this material, which has since been lost. DWE Hone personal communication 2008.)

Figure 9
Figure 9. Ligament scars and pneumatic foramina in intermetapophyseal troughs.

Bifid presacral vertebrae of sauropods showing ligament scars and pneumatic foramina in the intermetapophyseal trough. 1, Apatosaurus sp. cervical vertebra OMNH 01341 in right posterodorsolateral view, photograph by MJW. 2, Camarasaurus sp. dorsal vertebrae CM 584 in dorsal view, photograph by MJW. Abbreviations: las, ligament attachment site; pfa, pneumatic fossa; pfo, pneumatic foramen.

Figure 10
Figure 10. Real and speculative muscle attachments in sauropod cervical vertebrae.

1, The second through seventeenth cervical vertebrae of Euhelopus zdanskyi Wiman, 1929 cotype specimen PMU R233a-δ (“Exemplar a”). 2, Cervical 14 as it actually exists, with prominent but very short epipophyses and long cervical ribs. 3, Cervical 14 as it would appear with short cervical ribs. The long ventral neck muscles would have to attach close to the centrum. 4, Speculative version of cervical 14 with the epipophyses extended posteriorly as long bony processes. Such processes would allow the bulk of both the dorsal and ventral neck muscles to be located more posteriorly in the neck, but they are not present in any known sauropod or other non-avian dinosaur. Modified from Wiman (1929, plate 3).

Figure 11
Figure 11. Archosaur cervical vertebrae in posterior view, Showing muscle attachment points in phylogenetic context.

Blue arrows indicate epaxial muscles attaching to neural spines, red arrows indicate epaxial muscles attaching to epipophyses, and green arrows indicate hypaxial muscles attaching to cervical ribs. While hypaxial musculature anchors consistently on the cervical ribs, the principle epaxial muscle migrate from the neural spine in crocodilians to the epipophyses in non-avial theropods and modern birds, with either or both sets of muscles being significant in sauropods. 1, Fifth cervical vertebra of Alligator mississippiensis, MCZ 81457, traced from 3D scans by Leon Claessens, courtesy of MCZ. Epipophyses are absent. 2, Eighth cervical vertebra of Giraffatitan brancai paralectotype HMN SII, traced from Janensch (1950, figure 43 and 46). 3, Eleventh cervical vertebra of Camarasaurus supremus, reconstruction within AMNH 5761/X, “cervical series I”, modified from Osborn & Mook (1921, plate LXVII). 4, Fifth cervical vertebra of the abelisaurid theropod Majungasaurus crenatissimus, UA 8678, traced from O’Connor (2007, figures 8 and 20). 5, Seventh cervical vertebra of a turkey, Meleagris gallopavo, traced from photographs by MPT.

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