Amongst Mesozoic marine reptiles, metriorhynchid crocodylomorphs were unique in evolving into pelagically adapted forms with little-to-no posterodorsal retraction of the external nares. Narial retraction is a common adaptation seen in sustained swimmers, notably occurring during cetacean evolution. Mesosaurids and the basalmost known members of ichthyosauriforms, thalattosaurians, saurosphargids, sauropterygians, pleurosaurids and mosasauroids had the external nares divided by an ossified bar, bound by multiple cranial bones and were positioned back from the tip of the rostrum. However, metriorhynchids evolved from taxa with a single external naris bound solely by the premaxilla, and positioned near the tip of an elongate rostrum. We posit that metriorhynchids were uniquely disadvantaged in evolving into sustained swimmers. Herein we describe three Late Jurassic metriorhynchid cranial rostra that display differing degrees of narial retraction. In our new phylogenetic analyses, the backwards migration of the narial fossa posterior margin occurred independently at least four times in Metriorhynchidae, whereas the backwards migration of the anterior margin only occurred twice. Although Rhacheosaurini share the backwards migration of the anterior and posterior narial margins, posterodorsal retraction occurred differently along three lineages. This culminated in the Early Cretaceous, where a rhacheosaurin evolved nares bound by the premaxilla and maxilla, and significantly posterodorsally retracted.

中文翻译：

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上层鳄形外鼻孔后发性收缩的收敛演变和可能的限制

在中生代海生爬行动物中，腹汉沟鳄形态独特，演变成上层适应形式，外鼻孔几乎没有后气味。缩回是持续性游泳者常见的适应症，尤其是在鲸类进化过程中。中龙和鱼龙科，丘脑龙科，蜥龙类，蜥翅类龙眼，胸膜龙类和胸骨龙类的最基础的已知成员的外鼻孔被骨化的骨块分开，被多个颅骨所束缚，并位于讲台顶端。然而，子宫单侧鼻软骨是由类群进化而来，单个外部鼻孔仅由前颌骨束缚，并位于伸长的讲台尖端附近。我们认为，在发展成持续的游泳者中，胸腹肌是独特的劣势。在这里，我们描述了三个晚侏罗纪的子宫斜方肌鼻咽癌，它们显示出不同程度的鼻缩。在我们的新的系统发育分析中，斜纹龙科的鼻窝后缘向后迁移至少发生了四次，而前缘向后迁移仅发生了两次。尽管Rhacheosaurini共享前，后房边缘的向后迁移，但后嗅回缩在三个谱系中发生的方式有所不同。这在白垩纪早期达到了高潮，在那里，水苏素在前上颌骨和上颌骨的束缚下逐渐形成鼻孔，并在后嗅处明显回缩。斜纹龙科的鼻窝后缘向后移动至少发生了四次，而前缘向后移动仅发生了两次。尽管Rhacheosaurini共享前，后房边缘的向后迁移，但后嗅回缩在三个谱系中发生的方式有所不同。这在白垩纪早期达到了高潮，在那里，水苏素在前上颌骨和上颌骨的束缚下逐渐形成鼻孔，并在后嗅处明显回缩。斜纹龙科的鼻窝后缘向后移动至少发生了四次，而前缘向后移动仅发生了两次。尽管Rhacheosaurini共享前，后房边缘的向后迁移，但后嗅回缩在三个谱系中发生的方式有所不同。这在白垩纪早期达到了高潮，在那里，水苏素在前上颌骨和上颌骨的束缚下逐渐形成鼻孔，并在后嗅处明显回缩。