Complex Life Cycles and Postcranial Skeletal Morphology: Morphometric and Biomechanical Analysis of the Eastern Newt (Notophthalmus viridescens)

Eastern newts (Notophthalmus viridescens) possess a complex, polyphenic life cycle in which discrete phenotypes (terrestrial juveniles, aquatic juveniles, paedomorphic adults, and semi-aquatic adults) occupy different habitats through distinct developmental pathways. This intraspecific variation provides a powerful system for examining how environmental and developmental factors shape skeletal morphology and mechanical performance. We used micro-computed tomography (µCT), three-dimensional geometric morphometrics (GMM), and finite element analysis (FEA) to investigate whether terrestrial loading constrains postcranial morphology across the eastern newt life cycle and whether external morphological similarity is sufficient to predict internal mechanical performance. Vertebral shape was quantified across four axial regions (cervical, trunk, sacral, caudal) and limb bone shape across six appendicular elements (humerus, radius, ulna, femur, tibia, fibula) in four life stages. Across both skeletal systems, terrestrial-experiencing life stages (terrestrial juveniles and semi-aquatic adults) shared allometric trajectories and were indistinguishable in size-adjusted shape, while fully aquatic stages (aquatic juveniles and paedomorphic adults) exhibited greater morphological disparity. Life stage effects were greatest in regions experiencing the highest locomotor demands: posteriorly along the vertebral column and from forelimb to hindlimb. Critically, aquatic juveniles and paedomorphic adults diverged despite sharing an aquatic habitat, indicating that developmental pathways shape morphology beyond the effects of habitat alone. All shape differences among life stages were mediated through allometric scaling rather than size-independent shape divergence, consistent with canalization of shape–size relationships during the terrestrial juvenile phase. Exploratory finite element analysis comparing a single representative femur from each terrestrial-experiencing life stage provided preliminary evidence that external morphological similarity may not extend to internal mechanical performance, with stress distributions diverging in a load-case-dependent pattern and cross-sectional architecture differing between specimens. Together, these findings demonstrate that terrestrial constraint operates broadly across the postcranial skeleton through allometric canalization, with the degree of constraint regionalized by locomotor demand, and that external shape conservation alone may be insufficient to predict internal mechanical equivalence.