110-Million-Year-Old Turtle Eggs Unearthed in South Korea

Paleontologists have found a clutch of fossilized turtle eggs in the Lower Cretaceous Hasandong Formation in South Korea.

Paleoenvironmental restoration of the fossil locality in the Hasandong Formation and the Testudoolithus aff. curiosa egg clutch. Image credit: Hyunjoo Shin.

“Ever since the first recognition of dinosaur eggs from Mongolia, East Asia has produced an enormous record of fossil eggs,” said Dr. Seung Choi from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and colleagues.

“However, fossil eggs from clades other than non-avialan dinosaurs are rare.”

“Turtle eggs are no exception and there are only a few reliable reports from East Asia, including Cretaceous eggs from Mongolia, China, and Japan.”

“There is still a large knowledge gap in the fossil record of East Asian turtle eggs and new findings could shed light on the reproductive history of turtles.”

The newly-discovered turtle egg clutch dates back 110 million years (Early Cretaceous period).

It contains at least 20 spherical to ellipsoidal eggs, around 17-21 mm in diameter; shell thickness is 0.27-0.31 mm.

The eggs belong to the oospecies Testudoolithus aff. curiosa with the smallest recorded egg size within its genus.

The specimens were recovered from a rock fragment of the Uppermost Hasandong Formation in the northwest outskirts of Jinju City, South Gyeongsang Province, South Korea.

“The Hasandong Formation is the lowest egg-fossil-bearing formation in the Korean Peninsula,” the paleontologists said.

“Thus, the find represents the oldest Korean fossil eggs along with theropod eggs.”

Clutch and eggshells of Testudoolithus aff. curiosa. Scale bars – 1 cm in (A, B), 100 μm in (C, E, G, H), 20 μm in (D, F)

The researchers also detached two eggs from the clutch and cut them to expose radial sections of the eggshells.

The two exposed sections were used for polarized light microscopy (PLM) observation, EBSD, and Raman spectroscopy analysis.

During the detachment, seven eggshell fragments were obtained, and they were used for scanning electron microscopy (SEM) observations.

“Unlike all other amniotic eggs, which are composed of calcite, originally aragonitic turtle eggs provide an opportunity to test the preservation of aragonite in vertebrate paleontological materials,” the scientists explained.

“In the fossil record, aragonite is a metastable mineral, and is easily converted into a more stable calcite.”

The relict aragonite detected by the team in the Hasandong clutch represents one of the oldest recorded presences of aragonite among turtle fossil eggs.

“The Raman spectra of the thermally altered organic matter inside the eggshells indicated that the eggs experienced a maximum temperature of almost 260 degrees Celsius during their taphonomic history,” the authors said.

“This implies that aragonite can be preserved even under hostile thermal conditions and earlier reports of ‘calcite-only’ turtle fossil eggs may preserve undetected relict aragonite, which is only detectable via careful investigation using advanced microscopic techniques.”

“The combined use of mineralogical and spectroscopic approaches adopted in the study may also be useful to invertebrate paleontology and archeology to further understand the relationship between the preservation of aragonite and the maximum paleotemperature that the materials experienced.”

The findings were published in March 2023 in the Journal of Vertebrate Paleontology.