Macroolithus sp.

Yang, Tzu-Ruei, Wiemann, Jasmina, Xu, Li, Cheng, Yen-Nien, Wu, Xiao-Chun & Sander, P. Martin, 2019, Reconstruction of oviraptorid clutches illuminates their unique nesting biology, Acta Palaeontologica Polonica 64 (3), pp. 581-596 : 587-588

Macroolithus sp.

Figs. 1, 3 .

Material.— Five egg clutches (HGM-41H-V0074, DM-2014-P0154; PFMM-0014002972, PFMM-0014004392, PFMM-0014003019) from the Late Cretaceous , Nanxiong Group of Dayu County, Ganzhou, Jiangxi, China

Description.—All eggs in the five clutches (HGM-41HV0074, DM-2014-P0154; PFMM-0014002972, PFMM-0014004392, PFMM-0014003019) display elongate egg shape and linearituberculate surfacial ornamentaions ( Fig. 2 ). Eggs are arranged in three or four rings ( Fig. 2 ). While the length and width of the eggs in the inner rings are not measurable, the eggs in the outermost ring of the five studied clutches present similar gross mophology (length: 152–197 mm and width: 58–86 mm). The eggshell thickness of the studied egg clutches varies from 1.0– 1.5 mm ( Fig. 3 ). In some eggs, a layer of secondary calcite is observed on the calcitic egshell layer (see Fig. 3B, D–F ). The secondary calcite layer is distinguishable based on its different extinction pattern from the eggshell layer under the polarzied microscope, thus indicating a lack of diagenetic replacement in the eggshell layer that may lead to overestimation of shell thickness. The ML thickness varies from 0.2–0.3 mm and the CL thickness ranges from 0.8–1.0 mm; therefore, the CL:ML ratio is from 2:1–3.4:1. The ML/CL boundary is apprant and wavy.

Remarks.—The elongate egg shape presented by the studied egg indicates the assignment to Elongatoolithidae ( Zhao 1975) , which is laid by oviraptorosaurians as evidenced by several reports of oviraptorosaurian embryo-containing eggs ( Cheng et al. 2008; Wang et al. 2016; Pu et al. 2017). The oofamily Elongatoolithidae contains Elongatoolithus , Macroolithus , Macroelongatoolithus , Nanhsiungoolithus, Heishannoolithus , Paraelongatoolithus , Undulatoolithus , and Megafusoolithus . Based on the egg length (152–197 mm; Fig. 2 ), Macroelongatoolithus and Megafusoolithus (usually more than 400 mm; Table3; Zhao et al. 2015) are excluded.The eggshell thickness of the studied egg (1.3–1.5 mm; Fig. 3 ) is significantly greater than that of Elongatoolithus (usually less than 1.2 mm; Table 3; Wang et al. 2016), Nanhsiungoolithus ( Zhao, 1975) , Paraelongatoolithus (0.5–0.8 mm; Table 3; Wang et al. 2010) and Undulatoolithus (0.75–1.46 mm; Table 3; Wang et al. 2013a). Although Heishanoolithus eggs present similar eggshell thickness (1.2–1.5 mm; Table 3; Zhao and Zhao 1999), the studied eggs display distinctive eggshell surface ornamentation (linearituberculate), thus further constraining the studied egg clutches to Macroolithus . While the wavy ML/CL boundary present in the studied egg clutches is indicative of Macroolithus yaotunensis ( Fig. 3 ), we do not assign the clutches to a specific ootaxon because of ootaxonomic ambiguity. Moreover, despite the report of an Heyuannia huangi embryo inside a Macroolithus yaotunensis egg ( Cheng et al. 2008), Macroolithus eggs could possibly pertain to any similar sized oviraptorid dinosaur because many elongatoolithid eggs are microstructurally similar to each other. We therefore refer to all studied eggs simply as “oviraptorid eggs.”

Figures

Fig. 3. Radial petrographic thin sections of eggshells from an adult-associated clutch (A) from the Late Cretaceous of Bayan Mandahu, Inner Mongolia, China (Dong and Currie 1996) and the studied clutches (B–F) from the Late Cretaceous Nanxiong Formation of a construction site in Dayu County, Ganzhou City, Jiangxi Province, China. A. HGM-41H-V0074. B. DM-2014-P0154. C. PFMM-0014002972. D. PFMM-0014004392. E. PFMM-0014003019. F. IVPP V9608. Dashed line, the ML/CL boundary; arrow, the calcite layer. CL, continuous layer; ML, mammillary layer.

Fig. 2. A schematic illustration of an oviraptorid clutch affected by different degrees of compaction. A. Original configuration. B. The same configuration affected by a compaction factor of 25%. C. The same configuration affected by a compaction factor of 50%. Note the change of inclination angle and shape of the eggs.