Neck modularity in mammals
Recent studies have suggested that despite the constancy of cervical vertebrae number, the functional and morphology diversity in mammals seems as high as in birds, where this number is highly variable. Arnold (2021) attributed this diversity to the only recently appreciated modularity of the mammalian neck. A particular phenotype can often be decomposed into modular parts. Each module consists of parts which interact with each other, with relatively little interaction with other parts contributing to the phenotype.
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Shortly after the origin of amniotes, the synapsids (including the ancestor of mammals) already showed a more conserved axial configuration than the non-synapsids (Muller 2010). The variability of the latter continues to be seen in the necks of birds where there is only gradual variation between adjacent vertebrae, with different ranges of motion associated with different number of vertebrae,
In contrast, in extant mammals there are distinct transitions at various places in the neck. In terms of overall shape, there appear to be 4 distinct regions: C1-the atlas, C2-the axis, C3 through C5, and C6 and C7. These regions correspond to regions with distinct patterns of expression of genes from the Hox 4, 5, and 6 families.
Arnold (2021) studied synapsid fossils starting in the late Permian (>250 mya) to determine when changes in vertebrae structure occurred. He and others have concluded that the constraint on cervical vertebrae number occurred around the time of the great extinction at the Permian/Triassic boundary. He analyzed modularity in terms of 7 aspects of development, structure, and function. Five of the patterns were consistent with the above discussed boundaries of Hox gene expression.
Regarding muscular-skeletal structures there was one deviation: the muscles attached to C5 through C7 appear to act as a module. Different modularity was observed in the axial length of the vertebrae relative to body size, which appears to reflect later growth rates rather than initial size. Long-necked species have relatively long C3 through C6, but short C1, C2, and C7, while many fossorial (living underground) are the opposite, and large-headed and fully-aquatic species have a massive C1 and short C3 through C6.