鬣蜥类蜥蜴（爬行纲：有鳞目）全证据演化树与生物地理历史

Iguanians consist of a diverse group of lizards that are the focus of various phylogenetic and biogeographic studies. Isolated distribution of New World and Old World iguanians is a textbook example of ancient fauna vicariance of the two regions. Molecular studies suggest that vicariance of pleurodont (New World) and acrodont (Old World) iguanians dates back to 100 million years ago, a scenario established from mainly extant species with sparse fossil calibrations. In the past two decades, more than 1,000 specimens of fossil lizards have been discovered in the Mesozoic of Asia. New fossils provide accumulating evidences that pleurodont and acrodont iguanians co-existed in Asian lizard faunas until about 80 million years ago, a short time before the Cretaceous-Paleocene (K-Pg) extinction. Despite the significance of the Asian Mesozoic faunas, phylogenetic relationship of several iguanian members remain unresolved. This project aims to address the early evolution of iguanians using a total-evidence approach, which incorporates samples from modern and fossil species, data from morphology and molecules, and analytical methods including parsimony, concatenated maximum likelihood, and Bayesian optimality. The sampling includes more than 10 stem iguanians and new fossil materials from the Upper Cretaceous of China and Mongolia, which is based on extensive collections at the Institute of Vertebrate Paleontology and Paleoanthropology and the American Museum of Natural History. High-resolution X-ray CT will be used to develop new morphological characters. Data matrix for phylogenetic analyses includes more than 600 morphological characters and about 10,000 base pairs from nuclear DNA, mitochondrial DNA, and ribosomal RNA. Our goal is to provide a comprehensive phylogeny of Iguania and to reconstruct biogeographic history of the pleurodont and acrodont subclades. We will test the hypotheses that pleurodont and acrodont iguanians originated in Asia, and that stem pleurodont lizards dispersed from the Old World to the New Old. We expect to submit research results to international SCI journals, including PNAS (if warranted) and Journal of Vertebrate Paleontology (JVP).

鬣蜥亚目（爬行纲：有鳞目）的演化具有广泛的关注度。分子系统树显示鬣蜥亚目的两个主要支系（新大陆侧齿附目和旧大陆端齿附目）在早白垩世形成地理隔离；化石记录则显示一直到晚白垩世，侧齿附目和端齿附目仍共同出现在亚洲的蜥蜴生物群中。本项目将结合新化石材料建立鬣蜥亚目全证据演化树，讨论亚洲晚白垩世鬣蜥类的系统演化关系，恢复新、旧大陆鬣蜥地理隔离和扩散的演化历史。主要研究设计是检验侧齿附目和端齿附目是否都起源于旧大陆，而现生的侧齿附目是否由旧大陆扩散至新大陆的。研究方法包括融合化石种、现生种、形态学、分子数据进行全证据系统分析；用最简约（parsimony）、最大似然（Maximum likelihood）和贝叶斯（Bayes）方法重建演化树；定量推算侧齿附目和端齿附目的起源地区。实验手段包括X射线CT扫描分析化石形态特征。项目成果预计向PNAS，JVP等SCI刊物投稿。

鬣蜥的起源是蜥蜴类爬行动物演化研究中广受关注的课题之一。目前的记录显示，鬣蜥类起源的早期有很长的化石间断；从中侏罗世（最早的鬣蜥类）至晚白垩世，长达6千万年时间全球没有可靠的鬣蜥类化石记录。本项目着重研究了亚洲白垩纪时期的鬣蜥类化石，其中Dalinghosaurus longidigitus 产自中国辽西早白垩世热河生物群。通过微米级精度的X光CT扫描，项目组发现该化石种的脑颅部分有多个鬣蜥亚目的典型形态。对于D. longidigitus的系统分析采用了包含202个种一级分类单元，767个形态学特征的大数据矩阵，结果显示其为鬣蜥亚目最早的分支，与其它鬣蜥类形成姐妹群。这一发现填补了鬣蜥类在早白垩世的演化空白。..对Isodontosaurus gracilis的三维CT重建显示，其脑颅和下颌多个形态特征与端齿附目（Acrodonta）相同。对I. gracilis的系统分析使用了含49个鬣蜥种，767个形态学特征，5个基因片段共7005个碱基对的全证据矩阵。对于数据采用了最简约法、最大似然法和贝叶斯方法分析，结果显示I. gracilis为端齿附目的早期代表。基于全证据系统树的生物地理重建显示，美洲鬣蜥类起源于亚洲，最晚不晚于渐新世时期经欧洲扩散至美洲。..基于大量鬣蜥类头骨CT数据，发现鬣蜥类内耳平衡器官（半规管）的长度与头骨长度成正比，而生态适应对半规管长度影响不显著。尽管海生哺乳动物的内耳半规管明显小于同体重陆生种的内耳，但具潜水适应的海鬣蜥与同体型的陆生鬣蜥半规管长度没有明显差异。这说明爬行动物的水生适应没有伴随着半规管长度缩小，它们可能采取与哺乳动物不同的策略在水中保持平衡。