The evolutionary origin of metamorphosis in tetrapods: clues from palatal osteohistology in temnospondyls

The aim of this project is to study the evolutionary origin of metamorphosis in the amphibian lineage. Lissamphibians (i.e., salamander, frogs, and caecilians) are the only extant tetrapods that metamorphose, but it is still not clear whether metamorphosis is unique of this clade or if it evolved during the Palaeozoic in their putative ancestors, the dissorophoid temnospondyls. To date, metamorphosis cannot be inferred unequivocally for this fossil group because of the lack of complete ontogenetic series. In this project, we propose to infer metamorphosis in temnospondyls in a novel way: testing whether their palatal bones show histological signals of remodelling, a process that occurs in pterygoids and vomers during metamorphosis in salamanders. To do so, we propose to study using synchrotron x-ray imaging the ontogenetic changes in cranial bones at the histological level in a range of salamander taxa with different life cycles and compare them to key temnospondyl species.

本项目旨在研究两栖类演化支中变态发育的演化起源。滑体两栖类（Lissamphibians，即蝾螈、蛙类与蚓螈）是现存唯一具备变态发育特征的四足动物类群，但目前学界尚未明确变态发育是否为该演化支所独有，亦或是其在古生代便已在其推测的祖先类群——离片椎类（Temnospondyls）中的Dissorophoid类群——中演化产生。迄今为止，由于缺乏完整的个体发育序列，该化石类群的变态发育特征仍无法被明确推断。本项目拟通过全新方法推断离片椎类的变态发育特征：检验其腭骨是否存在组织学重塑信号——该过程在蝾螈变态发育阶段的翼骨与犁骨中均有发生。为此，本项目拟采用同步辐射X射线成像（synchrotron x-ray imaging）技术，对多种具有不同生活史的蝾螈类群的颅骨骨片开展组织学水平的个体发育变化研究，并将研究结果与关键离片椎类物种进行对比。