The Backbone of Regionalisation: Refining Axial Regionalisation Through 3D Morphometric Sampling Strategies on Elapid...

Area of Science:

• Evolutionary Biology
• Comparative Anatomy
• Vertebrate Morphology

Background:

• Axial elongation is a key evolutionary trait in vertebrates.
• Snakes, with their elongated bodies, provide a model for studying body plan evolution.
• Previous studies suggest snakes retain ancestral vertebral regionalization, with heart position near a regional boundary, but methodological impacts are unclear.

Purpose of the Study:

• To investigate vertebral regionalization in elapid snakes using 3D geometric morphometrics.
• To assess the impact of sampling density and landmark dimensionality on detecting regional boundaries.
• To explore interspecific shape differences in snake vertebral columns.

Main Methods:

• Employed three-dimensional geometric morphometrics (3D GM) and segmented linear regressions on complete vertebral columns of 12 elapid snakes.
• Compared subsampling strategies (2%-5% vertebral density) and various landmarking schemes to evaluate resolution effects.
• Analyzed regionalization patterns and interspecific shape variations.

Main Results:

• A 4- or 5-region model was most suitable for elapid snakes, featuring short cervical (2%-4%) and lumbar (final 5%) regions.
• A significant morphological shift occurred around 20% of the vertebral column, correlating with heart position.
• Fine-scale vertebral sampling (down to 2.5% density) improved detection of smaller regions; coarser sampling reduced accuracy.

Conclusions:

• Snakes maintain short cervical and lumbar regions, with most elongation in the thoracic region, which exhibits axial repatterning into three modules.
• Vertebral regionalization is conserved despite significant axial elongation in snakes.
• Methodological choices, particularly sampling density, significantly influence the detection of regional boundaries in vertebral column analyses.