Shell constraints on evolutionary body size-limb size allometry can explain morphological conservatism in the turtle body plan

Turtles are a small clade of vertebrates despite having existed since the Late Triassic. Turtles have a conservative body plan relative to other amniotes, characterized by the presence of a shell and quadrupedality. This morphology is even retained in strong ecological specialists, such as sea turtles, which are secondarily adapted to marine locomotion by strong allometric scaling in their hands. It is possible that the body plan of turtles is strongly influenced by the presence of the shell, acting as a constraint to achieving greater diversity of body forms. Here, we explore the evolutionary allometric relationships of fore- and hindlimb stylopodia (i.e., humerus and femur) with one another as well as their relationship with shell size (carapace length) to assess evidence of constraint. All turtles, including Triassic shelled stem turtles, have near-isometric relationships that do not vary strongly between clades, and evolve at slow evolutionary rates. This indeed indicates that body proportions of turtles are constrained to a narrow range of possibilities. Minor allometric deviations are seen in highly aquatic sea turtles and softshell turtles, which modified their shells by bone losses. Our allometric regressions allow accurate body size estimations for fossils. Several independent sea turtle lineages converged on maximum sizes of 2.2 m of shell length, which may be a biological maximum for the group.

尽管龟类（Turtles）所属的脊椎动物演化支（clade）规模相对较小，但其演化历史可追溯至晚三叠世。相较于其他羊膜动物（amniotes），龟类的躯体构型极为保守，以拥有背甲与四足行走为典型特征。这一形态特征甚至在高度特化的生态类群中得以保留——例如海龟，它们通过前肢的显著异速生长缩放（allometric scaling），次生适应了海洋运动环境。有观点认为，龟类的躯体构型深受背甲存在的制约，难以演化出更多样的躯体形态。本研究聚焦龟类前后肢近段肢节（stylopodia，即肱骨与股骨）间的演化异速生长关系，以及其与背甲尺寸（carapace length）的关联，以此评估躯体构型受制约的相关证据。包括带壳的三叠世基干龟类在内的所有龟类，其前后肢近段肢节与背甲尺寸间均呈现近乎等速生长关系（isometric relationships），且不同演化支间的该关系差异极小，整体演化速率缓慢。这一结果确证，龟类的躯体比例被限制在极窄的形态范围之内。仅在高度水生的海龟与软壳龟（softshell turtles）中存在轻微的异速生长偏离，这类类群通过骨骼缺失改造了自身的背甲结构。我们构建的异速生长回归（allometric regressions）模型可实现对化石物种体型的精准估算。多个独立的海龟演化支均趋同演化出了2.2米背甲长的最大体型，这或许是该类群的生物学体型上限。