DD'eDeN aka note/nickname/alas_my_loves wrote:
Covariation between the cranium and the cervical vertebrae in hominids
Mikel Arlegi cs 2022 J.hum.Evol.162:103112
doi 10.1016/j.jhevol.2021.103112
The analysis of patterns of integration is crucial for the reconstruction & understanding of how morphological changes occur in a taxonomic group throughout evolution.
These patterns are rel.constant, but both patterns & the magnitudes of integration may vary across spp.
These differences may indicate morphological diversification, in some cases related to functional adaptations to the bio-mechanics of organisms.
In this study, we analyze patterns of integration between 2 functional & developmental structures: the cranium & the cervical spine in hominids,
we quantify the amount of divergence of each anatomical element through phylogeny.
We applied these methods to 3D data from 168 adult hominid individuals, summing a total of >1000 cervical vertebrae.
We found the atlas C1 & axis C2 display the lowest co-variation with the cranium in hominids: H.sapiens, P.troglodytes & paniscus, G.gorilla & beringei, Pongo pygmaeus.
Hs show a rel.different pattern of cranio-cervical correlation vs chimps & gorillas, esp. in variables implicated in maintaining the balance of the head.
The atlas & axis show lower magnitude of shape change during evolution than the rest of the cervical vertebrae, esp. those located in the middle of the subaxial cervical spine.
Overall, results suggest that differences in the pattern of cranio-cervical correlation between humans & gorillas & chimps could reflect the postural differences between these groups.
Also, the stronger cranio-cervical integration & larger magnitude of shape change during evolution shown by the middle cervical vertebrae suggests that they have been selected to play an active role in maintaining head balance.
https://www.sciencedirect.com/science/article/abs/pii/S0047248421001640
"5. Conclusions
The major objective addressed in this study was to analyze the
degree of covariation between the cranium and the cervical
vertebrae in hominids, for which we proposed and tested five
hypotheses resulting in: First, in hominids, the atlas and axis
module show significantly lower values of magnitudes of
craniocervical integration than the rest of the cervical vertebrae.
Second, as expected, species with larger musculoskeletal features
in the dorsal neck, in this case, gorillas, show the highest degree of craniocervical correlation. Third, in contraposition to gorillas and chimpanzees, modern humans showed a different pattern of
craniocervical correlation, showing less specialized and more
diverse association among their features. Fourth, from an
evolutionary point of view, the cervical vertebrae showed different evolutionary patterns, the atlas of hominines displayed a certain
degree of homoplasy, and the subaxial cervical vertebrae a more directional-like mode of evolution. This indicates a relative
morphological and evolutionary stasis in the atlas and axis regarding
the rest of the cervical vertebrae, especially those located in the
middle of the cervical spine (i.e., C4eC5). Fifth, as expected, removing
the effect of size resulted in a reduction of the degree of craniocervical integration in all vertebrae except in the atlas. Indeed, all the cervical vertebrae except the atlas yielded significant shape variation explained
by cranial size.
"In sum, although the pattern of integration is relatively constant
in mammals, we observed in our results subtle differences in the
craniocervical pattern and magnitude of integration between
humans and gorillas and chimpanzees. These differences are
mainly circumscribed to cranial and cervical characters implicated
in maintaining head balance. However, whether these relative
differences are related to postural and/or locomotor factors is
something that requires more specific analyses to be answered.
Finally, we also found that the module comprising the atlas and axis
reflects a certain degree of lower magnitude of shape change during
evolution with the rest of the craniocervical complex."
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