The Dinosaur Eggshell Fossil of the Late Cretaceous Period from Xixia Basin,Henan Province, China:Supperhigh Content of Strontium and Its Significance

Key words: 
• dinosaur egg /
• egg shell fossil /
• strontium /
• supperhigh anomaly /
• XiXia

Abstract: Xixia basin is situated at the southeast margin of Henan province,China.It is one of the most important localities of the dinosaur eggs fossil in the world.It is famous for its abundant egg-bearing horizons,the vast quantity,the varied categories and the well preserved original states.The dinosaur eggs fossil included 19 species,assigned to 10 genera and belonged to 7 families. Up to now,several ten thousand dinosaur eggs fossils have been excavated,and most of them are in single.They are associated with Hadrosaurs,Tyrannosaurs,Sauropods,Charophytes,sporopollen and trace fossils, etc..Red clastic sediments mixed sand,gravel,mud and lime-mud occured within the late Cretaceous Xixia basin.A detailed study on this basin has made it possible to establish the normal stratigraphical sequence.The subdivision of red beds includes the Gaogou,Majiacun and Sigou Formations from bottom to top.The sequence is based on the type of dinosaur fossil eggs and their distribution,lithology,sedimentary facies,microfossils,formation and evolution of the basin,and absolute age dating,etc.. It is found that a little organism,asphalt,remaining in the dinosaur eggshell fossils,appears yellowish-brown or light dark under the microscope,and gives off asphalt foul smell and cracking sound on the eggshells when being burned,and gives off pungent smell on reacting with thick hydrochloric acid.Rock-Eval analysis reveals that S 1+S 2 varies from 0.44% to 2.77%,indicating that a little organism remains in the dinosaur eggshell fossils. The thirteen dinosaur eggshell fossils of the Late Cretaceous Period from Xixia Bsain and some surrounding rocks have been investigated by ICP-AES.In the elemental composition of the dinosaur eggshell fossils from Xixia Basin,the main element is Ca,next is Al,Mg,Fe,Na,K and Ti,and the microelement is mainly Sr,Mn,Ba,La,Zn etc.. By analyzing the thirteen dinosaur eggshell fossils,it is found that the content of strontium varies from 1 962 ×10^-6 to 11 010×10^-6,averaging 5 532×10^-6,and is one to several times higher than that in the shells of ancient brachiopods or modern ones,being a quantity level higher than that of the earth's crust.Obviously,the dinosaur eggshell fossils show the clear strontium superhigh anomaly. The strontium is a kind of widely-distributed microelement.Its ionic radium is close to that of Caion and Kion.As a result,it easily displaces Ca²⁺ and K⁺ so as to enter the carbonate minerals.According to the result of research at present,the biomineral is deposited in the impure solution.Other ions in solution enter the main crystal,resulting in the crystal growth and change of the from and chemical property of the crystal.When the main crystal ion is replaced in isomorph by ion with the same size and similar electric charge,the main crystal composition is worse-distributed because of the different relative density of ions deposition in different period.At the same time, different ions prefer different main crystal lattices,for example,Mg²⁺ prefers the calcite lattice,Sr²⁺ prefers the aragonite lattice.Thus it can be seen that the aragonite is rich in Sr,the content of which is much higher than that of the other carbonate minerals.So we can infer that a quantity of aragonite once existed in the dinosaur eggshell fossils.Due to the little stability of the aragonite,it could be changed into calcite gradually during its evolution.So there is no aragonite in the eggshells at present, however,the strontium was remained.It is indicated that the eggshells of terrestrial reptile are similar to the shells of marine organisms,in which there are aragonite and calcite at the same time.