Abstract:

Ichthyosaurs, firstly appeared at the end of the Early Triassic. The previously reported Ichthyosaurs show very high level of adaptability to life in water, and their osteology shows structural characters that are fully adapted to life in water without any terrestrial characters that can relate them to their terrestrial ancestor. Two specimens are studied, one belonging to Cartorhynchus and the other to Chaohusaurus with embryos found from the Lower Triassic (Olenekian) of Chaohu, Anhui Province, South China. Based on studies of the number of presacral vertebra (i.e. 31), the ratio of snout length to skull length (i.e. 0.35), the ratio of forelimb length to presacral vertebra length (i.e. 0.45), and comparisons of these measurements and morphological characters with the derived Ichthyosaurs, Cartorhynchus is considered to be primitive in structural features and to have an amphibianlike habit. Combined with the analyses of the embryo-bearing specimens of the Early Triassic Chaohusaurus and the Jurassic Stenopterygius, Chaohusaurus appears more derives than Cartorhynchus in body structural characters and is fully adapted to life in water; however, Chaohusaurus retained a head-first birth posture when giving birth to offsprings. Therefore, during the evolution of Ichthyosaurs from terrestrial ancestors to marine types, the adaptive structural characters (such as flipper, and elongate snout) and the adaptive behavioral characters (such as the mode of reproduction) might have evolved in different tempo, and the former changed earlier.

Key words: Cartorhynchus, Chaohusaurus, Ichthyosaurs, Early Triassic, Anhui Province

摘要：

鱼龙类(Ichthyosaurs)起源于早三叠世晚期, 以往发现的鱼龙类已高度适应水生生活, 其身体结构无任何能将其与可能的陆地祖先相联系的陆生适应特征。针对近年在安徽巢湖下三叠统奥伦尼克阶采集的柔腕短吻龙(Cartorhynchus)和含胚胎巢湖龙两件鱼龙类标本, 通过对柔腕短吻龙的荐前椎数目(31节)、吻长与头骨长之比(0.35)、前肢长与荐前椎长之比(0.45)的研究以及与后期鱼龙类相关数据和骨骼形态学的对比, 认为柔腕短吻龙结构更为原始, 可两栖生活。结合对早三叠世含胚胎巢湖龙、侏罗纪含胚胎狭翼鱼龙幼体保存情况的对比分析, 认为巢湖龙虽比柔腕短吻龙身体结构更进化, 且完全适应水生生活, 但其生殖行为仍然保持陆生爬行动物头位胎生的特征。鱼龙类在其陆地祖先逐渐演化至适应水生生活的过程中, 结构的适应性特征(如肢体的鳍状化、吻部的增长等)和行为特征(如生产方式的变化)的变化不同步, 前者的改变更早。

关键词: 柔腕短吻龙, 巢湖龙, 鱼龙类, 早三叠世, 安徽