新发现的长吻型离龙(双孔亚纲:离龙目)幼年个体--袖珍蒙山龙,兼论新离龙类的个体发育

A juvenile skull of the longirostrine choristodere(Diapsida:Choristodera),Mengshanosaurus minimus gen.et sp.nov.,with comments on neochoristodere ontogeny

离龙是一类生活在中侏罗世至中新世的半水生掠食性动物。白垩纪早期,部分离龙类演化为与现生鳄鱼形态类似的大型长吻爬行动物,称为新离龙类。报道了来自山东省下白垩统蒙阴组的一长吻型离龙新种--袖珍蒙山龙(Mengshanosaurus minimus),正型标本头骨全长仅35 mm,是迄今为止发现的最小新离龙类个体。根据未完全骨化的颅腔和额骨-顶骨间尚存未愈合的圆孔,推测蒙山龙正型标本为一幼年个体。系统发育分析显示,蒙山龙属于新离龙类,具有单个外鼻孔,单一鼻骨和下颞孔开放等新离龙类的共有衍征。在新离龙类内部,蒙山龙分类位置较为基干,为伊克昭龙属(Ikechosaurus)、车尔龙属(Tchoiria)、西莫多龙属(Simoedosaurus)和鳄龙属(Champsosaurus)组成的支系的姐妹群。蒙山龙区别于其他新离龙类的特征包括泪孔位于前额骨与泪骨之间,以及增大的腭面齿(宽度超过上颌齿的1/3)。蒙山龙具有长吻和紧密排列的尖利牙齿,推测离龙类幼体与现生鳄类幼体食性类似,以水生昆虫和无脊椎动物为食。现生鳄类在发育过程中,吻部的形态常发生改变,牙齿也会从尖细的形状转向圆钝;而蒙山龙的吻部和牙齿的形态与已知大体型的成体新离龙类没有明显区别,这说明相较于现生鳄类,新离龙类在发育过程中生态位的变化并不显著。

Choristoderes were an important clade of semi-aquatic predators that occupied Laurasian freshwater ecosystems from the Middle Jurassic to the Miocene.During the Early Cretaceous,the neochoristodere lineage evolved large size and long snouts,converging on the body plan of modern crocodilians.Here,we describe a new longirostrine choristodere,Mengshanosaurus minimus gen.et sp.nov.from the Lower Cretaceous Mengyin Formation of Shandong Province,China.The holotype is the smallest reported neochoristodere individual,with a skull length of only 35 mm.The poorly ossified braincase,along with retention of a fontanel at the frontal-parietal suture,indicates this individual was a juvenile.Phylogenetic analyses recovered Mengshanosaurus as a neochoristodere,a placement supported by the presence of a single narial opening,fusion of the nasals,and expansion of the temporal fenestrae.In the Neochoristodera,Mengshanosaurus is sister to a clade consisting of Ikechosaurus,Tchoiria,Simoedosaurus,and Champsosaurus.It differs from other neochoristoderes in having the lacrimal foramen between the prefrontal and lacrimal,in addition to having large vomerine teeth(exceeding one-third the width of corresponding maxillary teeth).The closely arranged marginal teeth and large vomerine teeth suggest juvenile choristoderes may have fed on invertebrates and insects,similar to juveniles of modern crocodilians.However,the observation that very young neochoristoderes had similar skull proportions and marginal tooth shapes to adults,along with features suggesting a more fully aquatic ecology,suggest that neochoristoderes exhibited less pronounced ontogenetic niche shifts than modern crocodilians.