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The scaling of vertebrae
The lengths of the vertebral bodies from T2 to L5 scaled similarly from foetus to adult. In contrast, a significant increase in the elongation rate occurs in the C2–C7 vertebrae as well as in the first thoracic vertebra after birth. This postnatal elongation rate is similar from C2 to C7. However, it should be borne in mind that foetal exponents, due to the small foetal sample size, have rather wide confidence intervals. Therefore, smaller deviations from isometry might be hidden especially in the foetal sample.
The remarkable postnatal elongation of cervical vertebrae is well illustrated when it is expressed as a percentage of trunk length: Although actually decreasing in the last trimester foetus to ca. 112%, once born, it increased to 122% after 6 months (Table 2.5). An advantage of an increased postnatal elongation rate may be linked to the mechanics of normal parturition. Giraffes, similar to other ungulates, are born in cranial presentation, dorsal position, and unbent head and legs with the forelegs extended cranial to the head (Dagg & Foster 1976). If the foetal giraffe had a neck length to foreleg length ratio of that of the adult animal (which is ca. 1.09 compared with ca. 0.72 in the foetus; (Mitchell et al. 2009a), the neck and head would extend beyond the forelegs during parturition, which may pose an increased risk of elbow flexion and ultimately dystocia. The mechanism that triggers differential elongation of the cervical vertebrae postnatally is unknown and needs further investigation.
Previous authors (Lankester 1908; Solounias 1999) alluded to T1 being adapted to the giraffe body form. Building on their findings, the scaling of T1 was shown in this study to scale with positive allometry (0.436), similar to the common slope of the cervical vertebrae (0.455). Therefore, because there is: a) Lack of significant difference between the scaling of T1 vs. cervical vertebrae and b) A significant difference between T1 and the T2–T14 thoracic series, it is suggest T1 is ‘‘cervicalised’’ with regards to its scaling exponent. In terms of scaling, therefore, T1 is a transitional vertebrae and Lankester’s (1908) finding of cervical homeosis is supported. Nevertheless, T1 starts out shorter than cervical vertebrae and never reaches the remarkable length of the cervical vertebrae (Figure 2.6). Because, in giraffes, the point of articulation between the cervical and thoracic vertebrae has shifted caudally to lie between T1 and T2 rather than between C7 and T1, which is the typical ruminant pattern, all seven cervical vertebrae contribute to neck length, whereas in other ruminant species only six do (Solounias 1999, Figure 2.8). However, given positive allometry of T1 length, it can perhaps be argued that even T1 indirectly contributes to neck length by increasingly shifting the C1–C7 vertebrae cranially. T1 does however never reach the remarkable length of the cervicals, and also does not contribute to neck length on the cervical axis but rather from the thoracic axis (Figure 2.8).
Therefore, this study showed that the giraffe evolved an elongated neck within the constraints of seven cervical vertebrae, thereby confirming the stance of Badlangana et al. (2009) and Mitchell & Skinner (2003) but disagreeing with Solounias (1999). The elongation is brought about by increasing the rate of elongation in the cervical (C2‐C7) vertebrae postnatally and by shifting the function of C7 to T1.
The cervicothoracic delineation
The study of the giraffe’s fascinating shape has recently started to incorporate genetic and molecular models to solve controversy over its evolutionary process, developmental constraints, and segmental identity of vertebrae (Badlangana et al. 2009). It has been suggested that the extraordinary length of the cervical vertebrae could be explained by the caudal shifting of Hox gene expression in the presomatic mesoderm during embryonic development. This would then result in a larger proportion of axial skeleton being devoted to cervical vertebrae, in turn leading to seven elongated cervical vertebrae (Badlangana et al. 2009). However, if somite (embryonic segments containing the pre‐ cursors of vertebrae) sizes remain similar to those of other mammals, a caudal shift in the expression boundaries of Hox genes expressing thoracic delineation (e.g. the Hoxc‐6 gene; (Burke et al.1995; Gaunt 1994) would lead to an increased number of cervical vertebrae, and not increased vertebral length.
Somites are formed periodically from the presomatic mesoderm by the processes of a segmentation clock interacting with a maturation wave or wavefront (Dequéant & Pourquié 2008). The somite sizes are determined by the interaction of these two parameters, which vary during axis production. It is possible that, in the giraffe, these parameters may interact differently to other mammals to produce larger somites anteriorly (Pourquié, 2010, personal communication). If not, then differential elongation of the cervical vertebrae must occur very early after the vertebrae formed, with similar growth to other vertebrae during the rest of the foetal period, and again differential growth postnatally. Indeed, in this study it has been shown that the disproportionate elongation of the neck occurs mostly postnatally, long after determination of somite sizes and the Hox genes have exerted their effect on the identity of the somites.
Chapter 1 Introduction .
1.1 On the biology of giraffes .
1.1.1 Phylogeny and Classification .
1.1.2 Distribution and morphotypes
1.1.3 Giraffe general morphology .
1.1.4 The physiological and anatomical adaptations of being tall
1.1.5 Skeletal anatomy and physiology
1.1.6 Drives for a long neck .
1.2 On bone biology
1.2.1 Scaling of bone
1.3 Objectives, research questions and hypotheses
1.4 General materials and methods
1.4.1 Experimental/ observational design
1.4.2 Obtaining samples
1.4.3 Measurements taken
1.4.4 Data analyses
1.5 Outline of thesis
Chapter 2 On the scaling of the vertebral column of Giraffa camelopardalis
2.1 Introduction
2.2 Materials and methods
2.2.1 Preparation and measurement ..
2.2.2 Data analysis
2.3 Results
2.3.1 Description of study sample
2.3.2 Sexual dimorphism
2.3.3 Scaling of exponents of vertebral lengths with regard to body mass
2.3.4 Scaling exponents of vertebral widths, heights and spinous processes vs. body mass
2.3.5 Comparison of Lengths, widths and heights at different body masses
2.4 Discussion
2.4.1 The scaling of vertebrae .
2.4.2 The cervicothoracic delineation
2.4.3 Sexual dimorphism in scaling
2.5 Conclusion
Chapter 3 On the scaling of the appendicular skeleton of Giraffa camelopardalis
3.1 Introduction
3.2 Materials and methods
3.2.1 Sampling
3.2.2 Body mass
3.2.3 Bone preparation
3.2.4 Bone measurements
3.2.5 Scaling mod
3.2.6 Statistics
3.3 Results
3.3.1 Sample description
3.3.2 Sexual dimorphism
3.3.3 Growth patterns
3.4 Discussion .
3.4.1 Pre‐ and postnatal growth differences
3.4.2 Sexual dimorphism
3.4.3 Long bone length vs. body mass
3.4.4 Increase in diameter and circumference with regard to body mass
3.4.5 Diameter and circumference vs. length
3.4.6 Cross sectional area .
3.4.7 Practical application of giraffe allometric equations
3.5 Conclusion
Chapter 4 On reconstructing Giraffa sivalensis, an extinct giraffid from the Siwalik Hills, India
4.1 Introduction
4.2 Materials and methods
4.2.1 Studied material and dimensions measured
4.2.2 Statistical analyses
4.2.3 Assumptions made
4.3 Results
4.3.1 Dimensions measured
4.3.2 Predictions based on vertebra OR39747
4.3.3 Predictions based on long bone dimension
4.3.4 Predictions based on dental dimensions .
4.4 Discussion
4.4.1 Vertebral identity of OR39747 .
4.4.2 Ontogenetic and interspecific scaling models
4.4.3 Neck length and reaching height
4.4.4 Body mass .
4.4.5 G. sivalensis palaeoart— notes on palaeoenvironment, body shape and skin colou
4.5 Conclusion
Chapter 5 General discussion
5.1 Introduction and salient findings
5.2 Research questions
5.3 Value of study
5.3.1 Perspective on the evolution of the giraffe
5.3.2 Size estimates for G. sivalensis
5.3.3 Determining the population structure of giraffes that succumbed during a drought
5.4 Limitations of study
5.4.1 Small foetal sample sizes
5.4.2 Lack of data on other tax
5.4.3 Lack of certainty regarding the origin of fossil specimens of G. sivalensis .
5.4.4 Cross sectional areas
5.4.5 The effect of soft tissues or cartilage loss
5.5 Possible avenues for future research
References
Appendices
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Ontogenetic allometry of the postcranial skeleton of the giraffe (Giraffa camelopardalis), with application to giraffe life history, evolution and palaeontology
