A key trend in the 210‐million‐year‐old history of modern turtles was the evolution of shell kinesis, that is, shell movement during neck and limb retraction. Kinesis is hypothesized to enhance predator defense in small terrestrial and semiaquatic turtles and has evolved multiple times since the early Cretaceous. This complex phenotype is nonfunctional and far from fully differentiated following embryogenesis. Instead, kinesis develops slowly in juveniles, providing a unique opportunity to illustrate the postembryonic origins of an adaptive trait. To this end, we examined ventral shell (plastral) kinesis in emydine box turtles and found that hatchling plastron shape differs from that of akinetic‐shelled relatives, particularly where the hinge that enables kinesis differentiates. We also demonstrated shape changes relative to plastron size in juveniles, coinciding with a shift in the carapace‐plastron structural connection, rearrangement of ectodermal plates, and bone repatterning. Furthermore, because the shell grows larger relative to the head, complete concealment of the head and extremities is only achieved after relative shell proportions increase. Structural alterations that facilitate the box turtle's transformation are probably prepatterned in embryos but require function‐induced changes to differentiate in juveniles. This mode of delayed trait differentiation is essential to phenotypic diversification in turtles and perhaps other tetrapods.

中文翻译：

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Emydine盒龟壳的胚后转化。

在2.1亿年前的现代乌龟历史中，一个关键趋势是壳运动学的发展，即脖子和四肢缩回期间的壳运动。有人认为运动学可以增强小型陆生和半水生乌龟的捕食者防御能力，并且自白垩纪早期以来已经演化了多次。这种复杂的表型是无功能的，并且在胚胎发生后还没有完全分化。相反，运动学在青少年中发展缓慢，提供了独特的机会来说明适应性状的胚胎后起源。为此，我们检查了艾美迪德盒龟的腹壳（质体）运动，发现孵化的plast体形状与无弹壳的亲戚的形状不同，特别是在能够运动的铰链上有所区别的地方。我们还展示了幼体中相对于to骨大小的形状变化，这与甲壳－ron体结构连接的移位，外胚层板的重排以及骨再吸收相吻合。此外，由于外壳相对于头部变得更大，因此只有在相对外壳比例增加之后才能完全隐藏头部和四肢。有助于盒龟转化的结构改变可能是在胚胎中预先设定的，但需要功能诱导的改变才能在幼鱼中分化。这种延迟性状分化的模式对于海龟和其他四足动物的表型多样化至关重要。因为外壳相对于头部会变大，所以只有在相对外壳比例增加后才能完全隐藏头部和四肢。有助于盒龟转化的结构改变可能是在胚胎中预先设定的，但需要功能诱导的改变才能在幼鱼中分化。这种延迟性状分化的模式对于海龟和其他四足动物的表型多样化至关重要。因为外壳相对于头部会变大，所以只有在相对外壳比例增加后才能完全隐藏头部和四肢。有助于盒龟转化的结构改变可能是在胚胎中预先设定的，但需要功能诱导的改变才能在幼鱼中分化。这种延迟性状分化的模式对于海龟和其他四足动物的表型多样化至关重要。