Skeletal remodelling suggests the turtle's shell is not an evolutionary straitjacket.

Recent efforts to decipher the enigma of the turtle's shell revealed that distantly related turtle species deploy diverse processes during shell development. Even so, extant species share in common a shoulder blade (scapula) that is encapsulated within the shell. Thus, evolutionary change in the correlated development of the shell and scapula probably underpins the evolution of highly derived shell morphologies. To address this expectation, we conducted one of the most phylogenetically comprehensive surveys of turtle development, focusing on scapula growth and differentiation in embryos, hatchlings and adults of 13 species. We report, to our knowledge, the first description of secondary differentiation owing to skeletal remodelling of the tetrapod scapula in turtles with the most structurally derived shell phenotypes. Remodelling and secondary differentiation late in embryogenesis of box turtles (Emys and Terrapene) yielded a novel skeletal segment (i.e. the suprascapula) of high functional value to their complex shell-closing system. Remarkably, our analyses suggest that, in soft-shelled turtles (Trionychidae) with extremely flattened shells, a similar transformation is linked to truncated scapula growth. Skeletal remodelling, as a form of developmental plasticity, might enable the seemingly constrained turtle body plan to diversify, suggesting the shell is not an evolutionary straitjacket.

Re-emergence of the Painted Turtle (Chrysemys picta) as a reference species for evo-devo.

Evolutionary developmental biology requires integration of knowledge from multiple levels of organization. Information at the genome level is quickly outpacing crucial descriptive information on developmental processes at the cell, tissue, and organ levels. Unprecedented progress in genomics has enabled the rise of so-called non-traditional reference species. However, a substantial, though overlooked, amount of phenotypic information has long been available for species such as the Painted Turtle (Chrysemys picta), which recently became the first turtle to feature a fully sequenced and annotated nuclear genome. In this commentary, I highlight research on the comparative embryology of Chrysemys during the twentieth century and briefly discuss future directions for research on this re-emerging species for evo-devo.