Abstract
Morphological integration and modularity are concepts that refer to the covariation level between the components of a structure. Morphological modules are independent subsets of highly correlated traits. The horse skull has been studied as a whole functional structure for decades, but the integrative approach towards quantitative examination of modules is scarce. We report here the first evaluation of cranial modularity in the horse at basal level. For this, we studied the modularity hypothesis for splanchnocranium and basicranium modules in the horse, two phenotipic regions under local influence by soft-tissue-hard-tissue interfaces. Using geometric morphometrics to capture the shape and location, we examined both modules in a sample of 23 dry skulls belonging to Pyrenean Horse Breed using 57 two-dimensional cranial landmarks. Modules were compared through partial least squares analyses and Escoufier (RV) coefficient. We tested whether the integration (measured by Escoufier RV coefficient) of splanchnocranium and basicranium strength modules and their covariation pattern (as analysed by partial least squares analysis) subordinate and express similar integration results. A clear modularity was observed. The lack of disproportions in the skulls of domestic horse breeds (compared to dog and cat breeds, for instance) might be an expression of the lack of single modules to evolve. On the other side, integration might have a positive impact on survival as long as the selection pressure is along the trajectory of integrated variation.
Highlights
Skull is integrated functionally as a whole, but its morphological integration is not uniform throughout
Using geometric morphometrics to capture the shape and location, we examined both modules in a sample of 23 dry skulls belonging to Pyrenean Horse Breed using 57 two-dimensional cranial landmarks
We tested whether the integration of splanchnocranium and basicranium strength modules and their covariation pattern subordinate and express similar integration results
Summary
Skull is integrated functionally as a whole, but its morphological integration is not uniform throughout. It is composed of multiple parts that are more or less distinct from each other on the basis of genetics, development or function (Curth et al, 2017). This coordination into subunits has long been known as morphological integration (Püschel, 2014). Some authors have suggested the rostrum and the braincase as two different modules of the dog cranium, as they are units whose parts are strongly integrated internally but are weakly integrated between them (Drake & Klingenberg, 2010) (Curth et al, 2017), the modules of the skull can never be fully independent from one another (Curth et al, 2017). Skull modules can constrain or promote the potential of the skull to evolve into new shapes probably in the course of horse evolution in general
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