Morphological Divergence in Marbled and Pygmy Newts: A Skull Shape Perspective

Abstract The vertebrate skull integrates vital functions such as feeding, brain protection and sensory perception, making it a key structure for studying morphological evolution. Using micro-computed tomography and geometric morphometrics, we examined skull shape variation in the salamander genus Triturus , focusing on five (sub)species within the recently revised T. marmoratus species group (marbled and pygmy newts). Results were compared with the sister T. cristatus species group (crested newts) which comprises eight species, including a recent diversification within T. carnifex documented by molecular data. The two groups, which diverged approximately 28 million years ago, take opposite positions over a shape gradient, from broad skulls with posteriorly positioned jaws to narrower, elongated skulls with more anteriorly positioned jaw articulations, respectively. Both groups exhibit comparable levels of morphological variation (disparity) in skull shape. In the T. cristatus species group, skull shape changes are paralleled by changes in axial morphology (with a vertebral count ranging from 13 to 17) whereas species of the T. marmoratus group have a uniform count of 12 trunk vertebrae. The subspecies Triturus m. marmoratus stands out from other taxa in its group in jaw articulation and pterygoid placement, and vomerine tooth row lengths – features that suggest functional differences in feeding mechanics or diet. Further research on the function morphology of feeding, feeding regimes and phenology of taxa may help to uncover drivers of morphological divergence.