Macroevolutionary patterns in Rhynchocephalia: is the tuatara (Sphenodon punctatus) a living fossil?

The tuatara, Sphenodon punctatus, known from 32 small islands around New Zealand, has often been noted as a classic ‘living fossil’ because of its apparently close resemblance to its Mesozoic forebears and because of a long, low-diversity history. This designation has been disputed because of the wide diversity of Mesozoic forms and because of derived adaptations in living Sphenodon. We provide a testable definition for ‘living fossils’ based on a slow rate of lineage evolution and a morphology close to the centroid of clade morphospace. We show that through their history since the Triassic, rhynchocephalians had heterogeneous rates of morphological evolution and occupied wide morphospaces during the Triassic and Jurassic, and these then declined in the Cretaceous. In particular, we demonstrate that the extant tuatara underwent unusually slow lineage evolution, and is morphologically conservative, being located near the centre of the morphospace for all Rhynchocephalia.

喙头蜥（tuatara）即斑点楔齿蜥（Sphenodon punctatus），分布于新西兰周边32座小型岛屿，因其形态与中生代祖先高度相似，且演化历史长期呈现低多样性特征，常被视作典型的‘活化石（living fossil）’。但该称谓曾引发争议：一方面中生代类群具有丰富的形态多样性，另一方面现存楔齿蜥属（Sphenodon）类群已演化出衍生适应性特征。本研究基于支系演化速率缓慢、形态接近类群形态空间（morphospace）质心的标准，提出了‘活化石’的可检验定义。研究显示，自三叠纪（Triassic）以来的演化历程中，喙头目（Rhynchocephalia）类群的形态演化速率存在异质性，且在三叠纪与侏罗纪时期占据了广阔的形态空间，随后在白垩纪（Cretaceous）时期形态空间范围出现缩减。具体而言，本研究证实现存喙头蜥的支系演化速率异乎寻常地缓慢，且形态高度保守，其形态位置处于整个喙头目类群形态空间的中心附近。