However, all authors agree that T1 of the giraffe appears to be a transitional vertebra, because of its short spinous process compared to other thoracic vertebrae. Mitchell and Skinner ( 2003) and Badlangana and Adams ( 2009) criticized this idea based on the articulation of a rib on the giraffe's T1 that attaches directly to the sternum-the defining characteristic of a thoracic vertebra. In contrast to a homeotic variation (i.e., the transformation of one morphology into another), a change in the number of segments is referred to as a meristic variation (Bateson, 1894). He proposed that the giraffe escapes from the “rule of seven” and possesses eight cervical vertebrae with insertion of an additional vertebra between C2 and C6 (Solounias 1999). Solounias ( 1999) reinvestigated Lankester's observations and noted that many anatomical characters of the giraffe are located one vertebra posteriorly compared to other mammals (e.g., roots of the brachial plexus and insertion of thoracic longus colli muscles). Lankester's “functional elongation hypothesis” has been substantiated by recent evidence for unique musculoskeletal features in agreement with increased mobility at the giraffe's cervicothoracic transition (Gunji and Endo 2016).
The author found that in the okapi and other ungulates, the articulation between the seventh cervical (C7) and T1 changes (from laterally facing zygapophyseal facets to medially facing zygapophyseal facets), whereas in the giraffe this change in the articulation pattern occurs between T1 and T2. Specifically, Lankester ( 1908) qualitatively compared the cervical shape of giraffes to other mammal species. This “functional elongation hypothesis” posits that while maintaining a count of seven cervicals, the first thoracic vertebra (T1) has been functionally incorporated into the giraffe neck despite maintaining its thoracic (i.e., rib-bearing) identity. Lankester ( 1908) was the first to note structural differences of the cervicothoracic transition in giraffes in comparison to other mammals and proposed a functional elongation of the neck. Goethe ( 2012) and Owen ( 1866) already were familiar with the puzzling observation and maintenance of just seven but extraordinary elongate cervicals in the giraffe. Its exceptional length is achieved while adhering to the mammalian “rule of seven” cervical vertebrae (Flower and Gadow 1885 Simmons and Scheepers 1996 Mitchell and Skinner 2003 Van Sittert et al. The neck of the giraffe ( Giraffa camelopardalis: Mammalia, Ruminantia) is an icon of evolutionary biology. Additionally, other factors related to the unique morphology of the giraffe's cervicothoracic transition such as neck posture and intervertebral stability are discussed and should be considered in future studies of giraffe neck evolution. Quantitative range of motion (ROM) analysis at the cervicothoracic transition in ruminants and camelids confirms the “functional elongation hypothesis” for the giraffe in terms of increased mobility, especially with regard to dorsoventral flexion/extension. Other giraffids (okapi and extinct Sivatherium) did not exhibit “cervicalized” T1 morphology. In contrast, we demonstrate a “thoracalization” of C7 for the European bison. However, we find a unique “cervicalization” of the giraffe's T1. Digital bone models of the cervicothoracic transition were subjected to 3D geometric morphometric analysis revealing how the shape of the seventh cervical (C7) has converged in several long-necked species. We test this “functional elongation hypothesis” by combining phylogenetically informed analyses of neck length, three-dimensional (3D) vertebral shape, and of the functional significance of shape differences across a broad sample of ruminants and camelids.
Although giraffes maintain the usual mammalian cervical number of seven vertebrae, their first thoracic vertebra (T1) exhibits aberrant anatomy and has been hypothesized to functionally elongate the neck.
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