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Cranial anatomy of Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications
PeerJ ( IF 2.7 ) Pub Date : 2021-05-06 , DOI: 10.7717/peerj.11179
Gabriele Bindellini 1 , Andrzej S Wolniewicz 2 , Feiko Miedema 3, 4 , Torsten M Scheyer 4 , Cristiano Dal Sasso 5
Affiliation  

Besanosaurus leptorhynchus Dal Sasso & Pinna, 1996 was described on the basis of a single fossil excavated near Besano (Italy) nearly three decades ago. Here, we re-examine its cranial osteology and assign five additional specimens to B. leptorhynchus, four of which were so far undescribed. All of the referred specimens were collected from the Middle Triassic outcrops of the Monte San Giorgio area (Italy/Switzerland) and are housed in various museum collections in Europe. The revised diagnosis of the taxon includes the following combination of cranial characters: extreme longirostry; an elongate frontal not participating in the supratemporal fenestra; a prominent ‘triangular process’ of the quadrate; a caudoventral exposure of the postorbital on the skull roof; a prominent coronoid (preglenoid) process of the surangular; tiny conical teeth with coarsely-striated crown surfaces and deeply-grooved roots; mesial maxillary teeth set in sockets; distal maxillary teeth set in a short groove. All these characters are shared with the holotype of Mikadocephalus gracilirostris Maisch & Matzke, 1997, which we consider as a junior synonym of B. leptorhynchus. An updated phylogenetic analysis, which includes revised scores for B. leptorhynchus and several other shastasaurids, recovers B. leptorhynchus as a basal merriamosaurian, but it is unclear if Shastasauridae form a clade, or represent a paraphyletic group. The inferred body length of the examined specimens ranges from 1 m to about 8 m. The extreme longirostry suggests that B. leptorhynchus primarily fed on small and elusive prey, feeding lower in the food web than an apex predator: a novel ecological specialisation never reported before the Anisian in a large diapsid. This specialization might have triggered an increase of body size and helped to maintain low competition among the diverse ichthyosaur fauna of the Besano Formation.

中文翻译:

贝萨诺龙 leptorhynchus Dal Sasso & Pinna 的颅骨解剖学,1996 年(爬虫类:鱼龙目),来自意大利/瑞士圣乔治山中三叠世贝萨诺组:分类学和古生物学意义

贝萨诺龙 leptorhynchus Dal Sasso & Pinna,1996 年是根据近三十年前在贝萨诺(意大利)附近挖掘的单一化石进行描述的。在这里,我们重新检查了它的颅骨学,并将另外五个标本分配给了轻喙龙,其中四个标本迄今为止尚未被描述。所有提到的标本都是从圣乔治山地区(意大利/瑞士)的中三叠世露头采集的,并收藏在欧洲的各个博物馆中。修订后的分类单元诊断包括以下颅骨特征组合:极端长喙;额叶细长,不参与颞上窗;方形的突出“三角过程”;颅顶眶后的尾腹侧暴露;上角突出的冠突(前盂);微小的圆锥形牙齿,牙冠表面有粗纹,牙根有深沟;上颌近中牙镶嵌在牙槽窝中;上颌远端牙齿排列在短凹槽中。所有这些特征都与 Mikadocephalus gracilirostris Maisch & Matzke, 1997 的正模标本相同,我们认为它是 B. leptorhynchus 的初级同义词。一项更新的系统发育分析(包括对薄喙龙和其他几种沙斯塔龙科的修订分数)将薄喙龙恢复为基础美亚莫龙类,但尚不清楚沙斯塔龙科是否形成一个分支,或代表一个并系群。推断所检查标本的体长范围为 1 m 至约 8 m。极端的长喙表明,细吻喙嘴鱼主要以小型且难以捉摸的猎物为食,在食物网中的进食位置比顶端捕食者低:这是一种在安尼西亚人之前从未在大型双孔动物中报道过的新型生态特化。这种专业化可能引发了体型的增加,并有助于维持贝萨诺组不同鱼龙动物群之间的低竞争。
更新日期:2021-05-06
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