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Fossil Evidence and Stages of Elongation of the Giraffa Camelopardalis Neck

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Fossil Evidence and Stages of Elongation of the Giraffa Camelopardalis Neck

Melinda Danowitz et al. R Soc Open Sci.

Abstract

Several evolutionary theories have been proposed to explain the adaptation of the long giraffe neck; however, few studies examine the fossil cervical vertebrae. We incorporate extinct giraffids, and the okapi and giraffe cervical vertebral specimens in a comprehensive analysis of the anatomy and elongation of the neck. We establish and evaluate 20 character states that relate to general, cranial and caudal vertebral lengthening, and calculate a length-to-width ratio to measure the relative slenderness of the vertebrae. Our sample includes cervical vertebrae (n=71) of 11 taxa representing all seven subfamilies. We also perform a computational comparison of the C3 of Samotherium and Giraffa camelopardalis, which demonstrates that cervical elongation occurs disproportionately along the cranial-caudal vertebral axis. Using the morphological characters and calculated ratios, we propose stages in cervical lengthening, which are supported by the mathematical transformations using fossil and extant specimens. We find that cervical elongation is anisometric and unexpectedly precedes Giraffidae. Within the family, cranial vertebral elongation is the first lengthening stage observed followed by caudal vertebral elongation, which accounts for the extremely long neck of the giraffe.

Keywords: Giraffidae; Samotherium; cervical vertebrae; elongation; neck.

Figures

Figure 1.
Figure 1.
Labelled C3 Okapia johnstoni (AMNH 51215) vertebra depicting terminology used for describing cervical anatomy as well as for elongation characters. (a) Dorsal view, (b) cranial view, (c) lateral view, and (d) caudal view.
Figure 2.
Figure 2.
Demonstration of a dorsal view of a representative C3 vertebra for all giraffids evaluated in this study. Each specimen is isometrically scaled so that all specimens are of equal length. The most primitive giraffid (Canthumeryx sirtensis) and sister taxon (Prodremotherium elongatum) comprise the bottom row; the giraffids with secondarily shortened C3 vertebrae (Okapia johnstoni, Giraffokeryx punjabiensis, Sivatherium giganteum and Bramatherium megacephalum) comprise the middle row; the giraffids with progressive vertebral elongation comprise the top row (Samotherium major, Palaeotragus rouenii, Giraffa sivalensis and Giraffa camelopardalis). *As there are no known C3 vertebrae of Bohlinia attica, a representative C2 of this taxon is illustrated in the black square.
Figure 3.
Figure 3.
Schematic of measurements used to quantify cervical vertebral elongation. Giraffa camelopardalis C3 (AMNH 82001) in (a) dorsal and (b) lateral views. The light blue lines represent maximum length (distance between the cranial and caudal articular facets. The green line represents minimal width. The red line represents the portion of the vertebra caudal to the caudal opening of the foramen transversarium.
Figure 4.
Figure 4.
Computational model accounts for the third cervical vertebra change between Samotherium (blue) and modern giraffe (red). (a) Aligned C3 of Samotherium and G. camelopardalis show marker placement corresponding to the cranial-most point of the cranial articular facet, caudal-most point of the caudal articular facet, cranial-most point of the cranial bulge, cranial and caudal openings of the foramen transversarium, dorsal-most point of the caudal vertebral body, ventral-most point of the caudal vertebral body, and cranial-most tip of the ventral tubercle. (b) Eight marker positions in aligned lateral views of Samotherium and G. camelopardalis C3. (c) The best fit of a uniform linear ‘stretch’ model applied to the Samotherium fitted to the modern giraffe marker position. The best-fit uniform linear ‘stretch’ coefficient is 2.0 (95% confidence interval (CI) =1.7–2.3). (d) The best fit of stretch–slide model applied to the Samotherium against the modern giraffe marker position. The model predicts that the transition from a modest to marked stretch occurs caudal to the posterior opening of the foramen transversarium 128 mm from the cranial aspect of the bone (95% CI= 92–166 mm) and the corresponding ‘stretch’ coefficients are 1.8 (95% CI= 1.5–2.1) cranial to that point and 2.9 (95% CI=2.5–3.2) caudal to that point (p-value for comparing cranial and caudal elongation coefficients was =0.003). In addition, the model required a 38 mm ‘sliding’ of dorsal with relationship to the ventral vertebra (95% CI=20–58 mm). If no sliding is included in the model, the best predicted values for the transition point and the cranial and caudal ‘stretch’ coefficients are: 122 mm (95% CI=18–226 mm), 1.7 (95% CI=1.0–2.5) and 2.7 (95% CI=1.9–3.5). (e) A schematic of a transformation used in the model. Red rectangle represents the original object. The blue rectangle represents the stretched object, where stretching to the right (caudal) of the vertical line is greater than that to the left of the line. Superposition of horizontal sliding results in the final shape, represented here by the parallelogram.
Figure 5.
Figure 5.
Cladogram with geological age and dorsal view of C3 vertebrae of taxa evaluated. Pe, Prodremotherium elongatum; Cs, Canthumeryx sirtensis; Oj, Okapia johnstoni; Gp, Giraffokeryx punjabiensis; Sg, Sivatherium giganteum; Bm, Bramatherium megacephalum; Sm, Samotherium major; Pr, Palaeotragus rouenii; Ba, Bohlinia attica; Gs, Giraffa sivalensis; Gc, Giraffa camelopardalis. (a) A modified cladogram based on previously published cladograms by Hamilton [13] and Solounias [14], with the exclusion of species not evaluated in this study. Each clade terminates in a square point corresponding to the age of the respective taxon in millions of years (Ma). The dorsal view of a C3 vertebra for each taxon is demonstrated (excluding Bohlinia as there are no known C3 fossils). Each specimen is isometrically scaled so that all vertebrae are of equal length. (b) Silhouettes of O. johnstoni, S. major and G. camelopardalis (left to right) are provided to give a comprehensive image of a long-necked, short-necked and intermediate-necked individual.
Figure 6.
Figure 6.
Radar plot shows cranial and caudal elongation of giraffid C3 vertebrae. Pe, Prodremotherium elongatum; Cs, Canthumeryx sirtensis; Oj, Okapia johnstoni; Gp, Giraffokeryx punjabiensis; Sg, Sivatherium giganteum; Bm, Bramatherium megacephalum; Sm, Samotherium major; Pr, Palaeotragus rouenii; Ba, Bohlinia attica; Gs, Giraffa sivalensis; Gc, Giraffa camelopardalis. Y-axes represent giraffid species. X-axes demonstrate the degree of elongation of a given character; the centre of the plot represents the shortened state while the lateral portion of the plot represents the elongation state. Each star plot represents a specific vertebral characteristic. (a) Radar plot depicting the cranial elongation characters. S. major, P. rouenii, G. sivalensis and G. camelopardalis exhibit the greatest degree of cranial elongation. (b) Radar plot depicting the caudal elongation characters. A caudal pull on C3 can be seen in G. sivalensis, and is most pronounced in G. camelopardalis.

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