Mesozoic avian eggs are rare, especially from the mid-Cretaceous basins of Zhejiang Province, China. Here we report an avian egg from the Lower Cretaceous (Albian) Liangtoutang Formation. The specimen (JYM F0033) measures 50 mm × 32 mm and the 166-µm-thick eggshell consists of three structural layers of calcite. The mammillary layer (ML) and overlying continuous layer (CL) each measure approximately 46 µm, whereas the outermost, external layer (EL) measures 74 µm. Ratios of these layers are ML:CL:EL = 1:1:1.6. An external layer that exceeds the thickness of the continuous layer represents an autapomorphy of this new egg type, which we herein establish as Pachycorioolithus jinyunensis oogen. et oosp. nov. within Pachycorioolithidae oofam. nov. Documentation of eggshell features and their first occurrence in the fossil record provides phylogenetically important information that may potentially aid in clarifying the evolution of avian reproduction and biology.
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Vol. 36 • No. 3