Our knowledge of the reproductive biology of dinosaurs covers a range of aspects, from brooding behaviour to nesting style and the timing of sexual maturity. Yet, the basic anatomy and function of the cloaca in non-avialan dinosaurs remains unknown. Here, we describe the outer morphology of the only known non-avialan dinosaur cloaca, preserved in an exceptional specimen of the early-diverging ceratopsian dinosaur Psittacosaurus. We clarify the position of the cloaca with respect to the ischia and caudal vertebrae and document the scales immediately adjacent to the abdomen and tail. We find that the cloaca is from a near-sexually mature subadult individual and is most similar to the cloaca of crocodylians, to the exclusion of lepidosaurians and birds. However, the sex of SMF R 4970 could not be determined as the cloaca and the rest of the specimen does not yield any sexually dimorphic information. This study highlights the ongoing role of exceptional specimens in providing rare soft tissues that help to bridge longstanding gaps in our knowledge of the basic biology of dinosaurs and other extinct reptiles.

中文翻译：

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非鸟类恐龙中的鳄鱼式泄殖腔

我们对恐龙生殖生物学的了解涵盖了一系列方面，从育雏行为到筑巢方式和性成熟的时间。然而，非鸟类恐龙泄殖腔的基本解剖结构和功能仍然未知。在这里，我们描述了唯一已知的非鸟类恐龙泄殖腔的外部形态，保存在早期分化的角龙类恐龙鹦鹉嘴龙的特殊标本中. 我们澄清泄殖腔相对于坐骨和尾椎的位置，并记录紧邻腹部和尾部的鳞片。我们发现泄殖腔来自接近性成熟的亚成年个体，与鳄鱼的泄殖腔最相似，不包括鳞龙类和鸟类。然而，SMF R 4970 的性别无法确定，因为泄殖腔和标本的其余部分没有产生任何性别二态信息。这项研究强调了特殊标本在提供稀有软组织方面的持续作用，这些软组织有助于弥合我们对恐龙和其他已灭绝爬行动物的基本生物学知识的长期认识。