3月18日,《自然》Nature杂志在线发表了中科院古脊椎动物与古人类研究所周忠和院士及其团队成员与山东临沂大学郑晓廷、王孝理、王岩,以及南非比勒陀利亚大学Fritz Huchzermeyer合作完成的一项研究成果——“早白垩世鸟类卵泡的精美保存对研究恐龙繁殖行为的意义”(Exceptional preservation of ovarian follicles in Early Cretaceous birds hints at dinosaurian reproductive behaviour)。
该研究基于山东天宇自然博物馆保存的三件早期鸟类化石(一件为长尾的热河鸟,另外两件为未命名的反鸟类),对它们罕见保存的卵巢中的滤泡化石进行了研究。如此精美的鸟类软组织繁殖器官保存以前尚未见报道。该研究表明,尽管与鸟类关系最近的恐龙与鳄鱼一样,还使用两个卵巢和两条输卵管,但早期鸟类显然已经只保留一个有效的卵巢和一条输卵管(身体左侧)。他们据此推测,身体右测卵巢和输卵管的功能退失可能发生在恐龙向鸟类过渡的阶段,很可能与适应飞行需要的体重减少有关。
他们的研究还表明,尽管早期鸟类已经具有了一些现代鸟类的繁殖特征,但在卵泡的数量、卵泡大小的差异等方面还显示了原始的特征,表明了相对于现代鸟类较低的新陈代谢水平。在系统演化上,热河鸟代表了最原始的鸟类之一,而反鸟类代表了最基干的鸟类与进步鸟类之间的过渡类型。该研究也显示,热河鸟的繁殖行为相对更加原始,而反鸟类的繁殖行为较为进步,它们代表了爬行类与现生鸟类之间繁殖行为的不同演化过渡阶段。
该研究的另外一个结论是:与爬行类和恐龙相似,早期鸟类的性成熟早于身体(骨骼)的成熟,这不同于现生鸟类。此外,鸟类卵巢滤泡化石的发现还提供了该类标本确切的性别证据,将有助于进一步研究早期鸟类的性双型等科学问题。
该项研究得到了科技部973项目以及国家自然科学基金等项目的支持。(生物谷Bioon.com)
doi:10.1038/nature11985
PMC:
PMID:
Preservation of ovarian follicles reveals early evolution of avian reproductive behaviour
Xiaoting Zheng, Jingmai O’Connor, Fritz Huchzermeyer, Xiaoli Wang, Yan Wang, Min Wang & Zhonghe Zhou
The two groups of archosaurs, crocodilians and birds, form an extant phylogenetic bracket for understanding the reproductive behaviour of dinosaurs. This behaviour is inferred from preserved nests and eggs, and even gravid individuals1. Data indicate that many ‘avian’ traits were already present in Paraves—the clade that includes birds and their close relatives—and that the early evolution of the modern avian form of reproduction was already well on its way2, 3. Like living neornithine birds, non-avian maniraptorans had daily oviposition and asymmetrical eggs with complex shell microstructure, and were known to protect their clutches4, 5, 6. However, like crocodilians, non-avian maniraptorans had two active oviducts (one present in living birds), relatively smaller eggs, and may not have turned their eggs in the way that living birds do1, 6. Here we report on the first discovery of fossilized mature or nearly mature ovarian follicles, revealing a previously undocumented stage in dinosaur reproduction: reproductively active females near ovulation. Preserved in a specimen of the long bony-tailed Jeholornis and two enantiornithine birds from the Early Cretaceous period lacustrine Jehol Biota in northeastern China, these discoveries indicate that basal birds only had one functional ovary, but retained primitive morphologies as a result of their lower metabolic rate relative to living birds. They also indicate that basal birds reached sexual maturity before skeletal maturity, as in crocodiles and paravian dinosaurs. Differences in follicular morphology between Jeholornis and the enantiornithines are interpreted as forming an evolutionary gradient from the reproductive condition in paravian dinosaurs towards neornithine birds. Furthermore, differences between the two enantiornithines indicate that this lineage might also have evolved advanced reproductive traits in parallel to the neornithine lineage.