Fossil turtle eggs from the Upper Cretaceous Gaogou Formation, Xiaguan-Gaoqiu Basin, Neixiang County, Henan Province, China: Interpretation of the transformation from aragonite to calcite in fossil turtle eggshell

In 2007, a clutch of approximately 30 fossil eggs reported from the Upper Cretaceous Gaogou Formation, Xiaguan-Gaoqiu Basin, Neixiang County, Henan Province, China was assigned to the oofamily “Crocodiloolithidae”. After 11 years, another clutch of 15 eggs of the same type associated with Mosaiceratops azumai and from the same county was identified as turtle eggs (Testudoolithus), based on the needle-like crystals interpreted as aragonite from their eggshells. Our detailed study of the clutch of 15 eggs and six clutches of the same ootaxon using transmitted/polarized light microscope, followed by scanning electron microscope observations combined with electron backscatter diffraction analysis showed that their eggshells are dominated by secondary calcite; with a few pristine aragonite crystals that unequivocally support the turtle affinity of the eggs. Furthermore, superimposed cone-shaped structural units with radial ultrastructures, combined with the extremely thick eggshell challenges the assignment to the oogenus Testudoolithus. Hence, we suggest that this ootaxon could represent a new oogenus of Testudoolithidae. Although additional eggshell units were reported in the abnormal fossil turtle egg with multilayered eggshell from the Judith River Formation in Montana, U.S.A, the relatively constant eggshell thickness indicates that the eggs from Neixiang are not pathological. Therefore, this is the first report of superimposed structural units inside normal turtle eggshells, revealing the unique eggshell formation mechanism of the extinct turtle from Neixiang. Moreover, the aragonite discovered in the eggs may extend the temporal record of confirmed aragonite in turtle eggs. The crystallographic comparison between the eggs from Neixiang and the fossil turtle eggshells from Zhejiang Province and Montana revealed the transformation process from aragonite to calcite in fossil turtle eggshell.