The youngest egg of avian affinities from the Cretaceous of Patagonia

We report the youngest fossil egg of avian affinities from the Cretaceous (Allen Formation, Campanian-Maastrichtian) of Patagonia, Argentina. The fossil consists in most of the internal cast of a single egg, preserving some eggshell fragments on one of its poles. Its axes are 4.5 cm and 3.06 cm. The eggshell surface is smooth, and its thickness average is 400 µm. The eggshell has two layers, with an ornithoid basic type and a ratite morphotype. The well-developed mammillary layer is 154 µm thick, with petal-shaped mammillae, the continuous layer is 251 µm thick. Shell microstructure is consistent with Laevisoolithidae, an ootaxon traditionally associated with enantiornithid birds. The shape of fibers of the testaceous membrane (TM) is preserved; these are flattened, with a thickness of 1.1 µm and a width of 4.8 µm each. The arrangement of the fibers of the TM is random, similar to that observed in extant reptiles. EDAX analysis reported that fibers are enriched in traces of Mg, Al, Si, P, F, and K, elements that are also present in both extant and fossil shell membrane. Cladistic analysis performed is consistent with the avian affinities established on the basis of macro, micro and ultrastructural characteristics.

本研究报道了产自阿根廷巴塔哥尼亚白垩纪（坎潘期-马斯特里赫特期，艾伦组（Allen Formation））的迄今最年轻的鸟类亲缘类群（avian affinities）化石蛋。该化石主体为单枚蛋的大部分内腔铸型，其一极区保存有部分蛋壳碎片。该蛋的长、短轴分别为4.5厘米与3.06厘米。蛋壳表面光滑，平均厚度为400微米（µm）。蛋壳具有两层结构，归属于鸟蛋型基本构造类型与平胸类形态型。发育良好的乳突层厚度为154微米，具花瓣状乳突；连续层厚度为251微米。蛋壳微结构与光滑蛋科（Laevisoolithidae）相符，该蛋分类群（ootaxon）传统上被认为与反鸟类（enantiornithid birds）具有演化关联。蛋壳膜（testaceous membrane, TM）的纤维形态得以保存；纤维呈扁平状，单根纤维厚度为1.1微米，宽度为4.8微米。该壳膜纤维的排布呈随机状态，与现生爬行动物中观察到的壳膜纤维排布模式相似。能量色散X射线能谱（EDAX）分析显示，壳膜纤维中富集镁（Mg）、铝（Al）、硅（Si）、磷（P）、氟（F）与钾（K）等痕量元素，这些元素同样存在于现生与化石蛋壳膜中。本次开展的分支系统学分析（cladistic analysis）结果，与基于宏观、微观及超微结构特征所确立的鸟类亲缘关系结论相符。