辽西早白垩世离龙类地理地史分布及其演化

离龙类是一在形态和生态适应上高度特化的水生—半水生爬行动物双孔亚纲的灭绝支系。离龙目的地理分布局限于北半球,地史分布从侏罗纪一直延续到中新世。辽西是中国最为重要的离龙类化石产地,化石点主要分布于凌源、义县、朝阳和建昌4个地区,尤其以义县地区化石点的分布最为密集。目前辽西已经发现的离龙类包括3科4属5种,其中潜龙和满洲鳄2属的化石仅发现于义县组,而伊克昭龙与戏水龙2属的化石则仅见于九佛堂组。化石证据表明,白垩纪离龙类在辽西范围内的延展分布呈现一个自西向东、由南向北的地理地史演化过程。化石证据也表明辽西的离龙类在较短的时期和较窄的地域范围内经历了一个快速分异演化的过程。长颈潜龙类和短吻满洲鳄类在辽西同时首现于凌源义县组大新房子层,并先后灭绝于义县组的大康堡层和金刚山层。潜龙类最后出现的大康堡层的火山凝灰岩层密集保存了数以千计的潜龙类个体,可能代表该类群于早白垩世发生了与火山喷发活动紧密相关的集群灭绝事件。而后,短吻戏水龙和长吻伊克昭龙同时首现于九佛堂组第2段,并于该组第3段沉积时期内灭绝。整个辽西地域内的离龙类的分布从126～120Ma,在经历了大约6Ma的快速分异演化之后,均于早白垩世阿普第期灭绝。

Early Cretaceous evolution of choristoderes in western Liaoningbased on geographic and stratigraphic evidence

The Choristodera,an extinct and specialized clade of aquatic-semiaquatic diapsid reptiles,had an evolutionary history in close association with the continental ecosystems of Jurassic through Miocene on northern hemisphere. During the Early Cretaceous in western Liaoning Province of China,choristoderes went through a rapid evolution with diverse ecologic adaptations,as evidenced by their extensive fossil record from the Yixian and Jiufotang Formations. Choristoderan fossils have been found from localities within the Lingyuan,Yixian,Chaoyang and Jianchang areas,with a significant concentration of Hyphalosaurus localited at the Yixian area. The found Choristoderan fossies can be divided into five species of four genera in three families. In terms of geological range,fossils of Hyphalosaurus and Monjurosuchus are simply restricted to the Yixian Formation,while those of Ikechosaurus and Philydrosaurus are confined to the Jiufotang Formation. The extended distribution of the Early Cretaceous choristoderes in western Liaoning indicates a dispersal pattern of the group from west to east and from south to north. And last occurrence of Hyphalosaurus choristoderes in western Liaoning is recorded with thousands of fossil specimens preserved in tuffaceous deposits of the Dakangpu beds of the Yixian Formation. Such an unusual concentration of fossils indicates a possible mass extinction event of this clade during the Early Cretaceous Aptian Age. All choristoderes were wiped out from the western Liaoning during the Early Cretaceous Aptian Age.