English: Hypothesis of progressive correlation between neck and head during turtle evolution. The origin of the turtle shell initiates the first selective regime (α) in which muscle rearrangements enable more flexible necks that overcome the lack of mobility of the shelled body, but also exert distinct compression and tension loads on the skull. This is compensated by an increasing stiffness (through palatoquadrate fusion, closure of basipterygoid region (1), secondary braincase lateral wall (2) formation). The new, stiffened skull architecture can withstand higher loads at the same time allowing material reduction in the temporal region, opening the path to a second selective regime (β). Longer and flatter skulls (6) evolve, together with posterior expansion of temporal crests (5) and the enlarged otic chamber (3), both features reducing available volume inside the adductor chamber and triggering the appearance of a trochlear mechanism (4). Broader insertion sites for neck musculature offered by expanded temporal crests (5) support a new round of neck modifications, resulting in longer necks and modern types of neck retraction (7). Temporal emarginations (8) evolve in response to new forces generated by more powerful neck musculature. Model of †Eubaena cephalica as in Supplementary Fig. 9.