ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 15: 1–6 (2016)© by the Fukui Prefectural Dinosaur Museum

FIRST CONFIRMED FOSSIL TURTLE EGGSHELLS (OOGENUS TESTUDOOLITHUS)
FROM THE LOWER CRETACEOUS OF THAILAND
Takuya IMAI1,2, Pratueng JINTASAKUL3, Yoichi AZUMA1,4, Yoshikazu NODA1
and Duangsuda CHOKCHALOEMWONG3
Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, Japan
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan
3
Northeastern Research Institute of Petriﬁed Wood and Mineral Resources, Nakhon Ratchasima Rajabhat University,
184 Moo 7, Suranaree Subdistrict, Mueang, Nakhon Ratchasima 30000, Thailand
4
Institute of Dinosaur Research, Fukui Prefectural University, 4-1-1 Matsuoka Kenjojima, Eiheiji, Yoshida, Fukui 910-1142, Japan1

2

ABSTRACTIn contrast to the nearly global occurrence of fossil turtle eggs and eggshells, Southeast Asia had long been void ofsuch fossil record. Consequently, the study of the turtle reproductive paleobiology was difficult in the region, despitethat it helps to understand the evolutionary success of turtles through the Mesozoic. Here, we describe the first fossil
turtle eggshells from the Lower Cretaceous of Thailand, and discuss their paleobiogeographical and paleoenvironmental
implications. The specimens include two eggshell fragments (BHK-D585 and BHK-D586) of different types, which arefrom the Berriasian to the lower Barremian Sao Khua Formation, the Khorat Group exposed in Chaiyaphum Province,
Thailand. The Sao Khua Formation represents a ﬂoodplain and yields numerous terrestrial vertebrate fossils. BHK-D585
and BHK-D586 are contained in the reddish-brown conglomerate. These eggshells share a smooth external surface andtightly-interlocking acicular crystal. BHK-D585 and BHK-D586 have the shell thicknesses of 0.62 – 0.64 mm and 0.55 –0.68 mm, and 1.5:1 and 2:1 height-to-width ratio of an eggshell unit, respectively. A non-branching, sinuous pore is present
in BHK-D586. These microstructural characters are consistent with those of the oogenus Testudoolithus, and suggest bothof the fragments belong to non-marine turtles. Compared to other known turtle ootaxa with rigid eggshell, BHK-D585 and
BHK-D586 have unique combinations of the shell thicknesses and height-to-width ratio of an eggshell unit. We conclude
that BHK-D585 and BHK-D586 in the Sao Khua Formation are the ﬁrst reproductive evidence of non-marine turtles inSoutheast Asia. They increase the morphological diversity of known turtle ootaxa and imply the presence of a suitablepaleoenvironment for preservation of their eggshells in northeastern Thailand during the Early Cretaceous.
Key words : turtle, fossil egg, fossil eggshell, vertebrate paleontology, the Sao Khua Formation, Thailand, Early Cretaceous
今井拓哉・Pratueng JINTASAKUL・東 洋一・野田芳和・Duangsuda CHOKCHALOEMWONG（2016）
タイの下部白亜系から初めて見つかったカメ類卵殻化石（卵属Testudoolithus ）について．福井県立恐竜博
物館紀要 15：1–6．
これまで東南アジアからは，化石カメ類の卵や卵殻化石の記録はなかった．本論では，タイの下部白
亜系から初めて発見されたカメ類卵殻化石を記載する．標本は 2 点の卵殻（BHK-D585，BHK-D586）で，
タイ北東部に露出する河川成のサオ・クア層（ベリアシアン～下部バレミアン）から産出した．両標本
は，凹凸のない表面と隙間なく連なる針状結晶という共通の特徴を持つ．卵殻の厚さはそれぞれ 0.62‒0.64
mm，0.55‒0.68 mmで，卵殻単位の偏平比はそれぞれ 1.5：1 と 2：1 である．これらの微細構造は卵属
Testudoolithus のものと一致し，非海生カメ類のものと考えられる．既知のカメ類卵タクサと比較すると，
BHK-D585 とBHK-D586 はそれぞれ特徴的な厚さと卵殻単位比の組み合わせを持つ．このことから，両標本
は東南アジアからの初めての非海生カメ類繁殖の証拠であり，前期白亜紀のタイ北東部には，卵殻化石が保存されうる環境があったことを意味する．

Received June 28, 2016. Accepted November 24, 2016.
Corresponding author ― Takuya IMAI
E-mail: t-imai＊dinosaur.pref.fukui.jp
1

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T. IMAI, P. JINTASAKUL, Y. AZUMA, Y. NODA AND D. CHOKCHALOEMWONG

INTRODUCTION
Fossilized hard-shelled eggs and eggshells of turtles (order
Testudines) occur in every continent except Antarctica, and inas old as Early Jurassic sediments (Lawver and Jackson, 2014).
This observation indicates that turtles which lay hard-shelled
eggs were present at least in the Early Jurassic, and gained
world-wide distributions through the Mesozoic. Knowledge ofthe evolution of reproduction in turtles during the Mesozoic is
important because this era is most likely the time where turtles
first appeared, dispersed, and evolved substantially toward themodern forms (Li et al., 2008; Schoch and Sues, 2015). Thus,
among oviparous amniotes, the processes through which turtles
acquired their reproductive strategies, and the contribution ofthese strategies to their evolutionary success are key to better
understand the evolutionary history of turtles. In addition, turtle
eggshell is characteristically composed of aragonite, in contrast
to calcitic eggshells of other amniotes (Hirsch, 1983). Therefore,
the fossil record of turtle eggs and eggshells helps to address suchquestions as: when turtles acquired hard-shelled eggs, in whatways turtles with hard-shelled eggs radiated, and what selective
pressure may have caused the evolution of unique aragonite
eggshell only among turtles.
The Mesozoic fossil record of turtle eggs and eggshells
is biased, which limits our understanding of the evolution of
turtle reproduction through the time. According to Lawver andJackson (2014), while the fossil record of Mesozoic turtle eggs
and eggshells is ubiquitous in Europe, East Asia, North America,
other parts of the world often lack such a fossil record. The lackof a record in some regions may be due to a lack of fossil-bearing
Mesozoic strata or paleontological expertise. In other areas where
fossil turtle skeletons are present, it is more likely that theirabsence is biological (e.g., reproduction with soft-shelled eggs)
or taphonomic (e.g., sediment ideal for preservation of skeletal
elements but not eggshells).
Southeast Asia is one of such regions where there is no fossil
record of turtle eggs and eggshells. Particularly in Thailand,
numerous skeletal remains of turtles had been discovered, while
there has been an apparent lack of their eggs and eggshells in
Mesozoic strata, without plausible explanation.
Here, we report turtle eggshells from the Berriasian to the lower
Barremian Sao Khua Formation of the Khorat Group, cropping out
in Chaiyaphum Province, northeastern Thailand. These eggshells
add to the known morphological diversity of the Mesozoic nonmarine turtle eggshell. In addition, as the first Mesozoic turtle
eggshells from Thailand, they ﬁll the paleobiogeographical gap of
the Mesozoic turtle eggshells of Southeast Asia, and suggest thepossible presence of the better-preserved Mesozoic turtle eggs andnests in Thailand, aiding our understanding of turtle reproduction
during the Mesozoic in the region.
GEOLOGY
The Khorat Group is mostly composed of the Mesozoic
sediments. It is widely exposed in the northeastern part of

FIGURE 1. Maps and a photo of the locality. A, location map ofChaiyaphum Province, scale bar = 100 km; B, location map of Ban Huai
Takhaeng (square in A), scale bar = 10 km; C, overview of the locality.

Thailand, namely the Khorat Plateau. Some of the formations
belonging to the Khorat Group, including the Sao Khua
Formation, have yielded a variety of the Mesozoic terrestrial
vertebrates (Department of Mineral Resources, 2014).
The Sao Khua Formation represents the middle part of the
Khorat Group, conformably underlying the Phra Wihan Formationand overlaying the Phu Phan Formation (Department of Mineral
Resources, 2014). It consists of reddish-brown, calcrete-bearing
sedimentary rocks, ranging from claystone to conglomerates(Racey, 2009), likely representing a meandering fluvial system
under semi-arid paleoclimatic conditions (Meesook, 2000).
A palynological analysis suggests that the sediments were
deposited in the Berriassian to the early Barremian (Racey andGoodall, 2009). The formation is known to produce abundantvertebrate remains (Department of Mineral Resources, 2014).
Notably, Buffetaut et al. (2005) described a small fossil amniote
egg containing embryonic skeletons, reporting that the embryo
belongs to Aves. However, a more detailed observation utilizing
the micro-computed-tomography technique by Fernandez et al.
(2015) identiﬁes the embryo as Anguimorpha.
MATERIALS AND METHODSStudied materials include two eggshell fragments, BHK-D585
and BHK-D586, which are currently housed in the Northeastern
Research Institute of Petrified Wood and Mineral Resources,
Nakhon Ratchasima Rajabhat University, Thailand. The locality issituated in a community forest of Ban Huai Takhaeng, Khok Sung
Subdistrict, Mueang Chaiyaphum District, Chaiyaphum Province,
Thailand (Fig. 1), where the Sao Khua Formation predominantly
crops out. The matrices of BHK-D585 and BHK-D586 arecomposed of reddish-brown conglomerates consistent with thegeneral lithology of the Sao Khua Formation.
Each eggshell fragment was broken into half, and each half
fragment was prepared for a standard 30 µm-thick petrographic

EARLY CRETACEOUS TURTLE EGGSHELL FROM THAILAND

3

FIGURE 2. External surface and a radial thin-section of BHK-D585. A, smooth external surface, scale bar = 1 mm; B–C, radial-thin section under apetrographic microscope in unpolarized light. Note radiating acicular crystals typical of turtle eggshells, and a possible nucleation center (blue arrow in C),
scale bars = 0.2 mm.

FIGURE 3. External surface and a radial thin-section of BHK-D586. A, smooth external surface, note a round pore opening (red arrow), scale bar = 1 mm;
B–C, radial-thin section under a petrographic microscope in unpolarized light. Note radiating acicular crystals typical of turtle eggshells, and a pore canal (bluearrow in C), scale bars = 0.2 mm.

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T. IMAI, P. JINTASAKUL, Y. AZUMA, Y. NODA AND D. CHOKCHALOEMWONG

thin-section and examined under a Nikon E600 POL polarized
light microscope. Although microstructural observation on goldor platinum-coated specimens by a scanning electron microscope(SEM) is desirable in the studies of fossil eggshells, eachfragment in this study was too small and we chose not to performthe analysis by SEM to preserve unaltered materials. The macroand micro-structural characters are compared with other Mesozoic
eggshells.
SYSTEMATIC PALEONTOLOGY
Oofamily TESTUDOOLITHIDAE Hirsch, 1996
Type oogenus.—Testudoolithus Hirsch, 1996.
Diagnosis.—Eggshell of the oofamily Testudoolithidae isdiagnosed by the following unique combination of characters:
testudoid basic type, the spherurigidis morphotype, a calcareous
layer with interlocking shell units, simple and widely spaced pores
between shell units, and spheroidal to ellipsoidal egg shape.
TESTUDOOLITHUS Hirsch, 1996
(Figs. 2 and 3)
Type oospecies.—Testudoolithus rigidus Hirsch, 1996.
Diagnosis.—Eggshell of the oogenus Testudoolithus is diagnosedby the following unique combination of characters: spherical egg
shape, shell unit height greater than width, and shell thickness of
0.2 – 1.0 mm.
Oospecies TESTUDOOLITHUS oospp.
Referred specimens.—BHK-D585 and BHK-D586, eggshellfragments of different types.
Distribution.—Ban Huai Takhaeng, Khok Sung Subdistrict,
Mueang Chaiyaphum District, Chaiyaphum Province, Thailand; theSao Khua Formation (Berriasian-early Barremian) (Figs. 2 and 3).
Descriptions.—BHK-D585 has smooth external surface andhas a thickness of 0.62 – 0.64 mm. In the thin-section, it exhibitsa shell microstructure of a single layer of tightly interlocking,radiating acicular crystals. In one of the mammillary cones
appears a colorless circular structure, likely a nucleation center.
Individual shell units have approximately 1.5:1 height-to-width
ratio (Fig. 2C).
BHK-D586 has smooth external surface and its thickness
ranges 0.55 – 0.68 mm. A circular pore opening, which isapproximately 0.05 mm in diameter, is present on the externalsurface. In the thin-section, a non-branching, sinuous pore canalruns from the inner surface to the external surface of the shell,
narrowing toward the external side (Fig.3C). The lower half of theshell is characterized by tightly-interlocking, radiating acicular
crystals, whereas the upper half exhibits a blocky texture with an
extinction pattern different from the lower half under polarized
light. A possible nucleation center is observable in one of themammillary cones. A shell unit has approximately 2:1 height-towidth ratio (Fig. 3C).

DISCUSSION
The microstructural characters of the specimens are
consistent with those seen in rigid eggshells of extant turtles,
suggesting their afﬁnity to extinct species of non-marine turtles.
Turtle taxa from the Sao Khua Formation include the adocid
Isanemys srisuki Tong, Buffetaut and Suteethorn, 2006, andthe carettochelyid Kizylkumemys sp., whose remains frequently
occur in the formation (Tong et al., 2009). It is unknown whether
BHK-D585 and BHK-D586 were laid by any of these species,
because they were not associated with skeletal remains of turtles.
Both adocids (Zelenitsky et al., 2008) and carettochelyids(Ewert, 1979) are known to have rigid eggshell with shell units
taller than wide.
BHK-D585 differs from other previously known valid ootaxa
of Testudoolithidae in terms of the thickness (Table 1). The
height-to-width ratio of a shell unit of BHK-D585 resembles
that of Haininchelys curiosa Schleich, Kästle and Groessens-van
Dyck, 1988, although H. curiosa is much thinner.
BHK-D586 most closely resembles Emydoolithus
laiyangensis Wang et al., 2013 in terms of the height-to-width
ratio of a shell unit and the shell thickness (Table 1). However,the shell thickness slightly exceeds that of E. laiyangensis.
While Testudoolithus rigidus and Haininchelys curiosa have theranges of the shell unit height-to-width ratio within which thatof BHK-D586 falls, it is much thinner than BHK-D586. The
blocky texture observed in the radial thin section of the upper
half of the shell is likely a result of recrystallization, based on
the extinction pattern under polarized light. Isaji et al. (2006)
reports similar pattern of recrystallization in the upper part of
the fossil turtle eggshell from the Lower Cretaceous of Japan.
BHK-D585 and BHK-D586 are the first record of the
Mesozoic turtle eggshells from Thailand (Lawver and Jackson,2014), and increases the morphological diversity of the
known turtle ootaxa. Their occurrence indicates that, during
the Berriasian to the early Barremian, one or more turtles
reproduced with hard-shelled eggs in the ﬂoodplain represented
as the Sao Khua Formation today.
It is paleobiogeographically and paleobiologically expectablethat turtle eggshells occur in the Sao Khua Formation, because
1) turtle eggshells are not uncommon in other Mesozoic
terrestrial strata of Asia and 2) well-preserved amniote eggs
were recovered from the formation previously (Buffetaut et al.,2005), suggesting that the Sao Khua Formation generally allows
the preservation of eggshells composed of calcium carbonate.
Nevertheless, the discovery of BHK-D585 and BHK-D586 fromthe region that had long been void of the record of fossil turtle
eggshells advocates that more rigorous search for such eggshells
are desired. The presence of BHK-D585 and BHK-D586 in the
Sao Khua Formation suggests there is possibly better preserved
turtle eggs and nests in the locality, as well as presently
unreported eggs and eggshells of other amniotes including
dinosaurs which were as thriving as turtles during the time.

EARLY CRETACEOUS TURTLE EGGSHELL FROM THAILAND

5TABLE 1. Comparison of BHK-D585 and BHK-D586 with known rigid fossil eggshells of the testudoid basic type. ＊Calculated by the authors.

ACKNOWLEDGMENTS
We thank Department of Mineral Resources, Ministry of
Natural Resources and Environment for permitting to study fossil
specimens from Thailand. We also thank P. Boonwan, District
Chief Officer of Muaeng Chaiyaphum and D. Phonmani, Chiefof Ban Huai Takhaeng for kindly permitting our fieldwork in
the area. The ﬁeldwork was not possible without the help of thestaffs of Northeastern Research Institute of Petrified Wood andMineral Resources and Fukui Prefectural Dinosaur Museum, as
well as the local workers and villagers of Ban Huai Takhaeng.
W. Naksri at Northeastern Research Institute of Petrified Woodand Mineral Resources, Nakhon Ratchasima Rajabhat University
and T. Sonoda at Fukui Prefectural Dinosaur Museum shared
their knowledge about the turtles from the Sao Khua Formation.
Finally, we thank R. Hirayama at Waseda University and D.
Zelenitsky at the University of Calgary for their constructive
comments and corrections.
REFERENCES
Bajpai, S., S. Srinivasan and A. Sahni. 1997. Fossil turtle eggshells
from infratrappean beds of Duddukuru, Andhra Pradesh. Journalof the Geological Society of India 49: 209 – 213.

Buffetaut, E., G. Grellet-tinner, V. Suteethorn, G. Cuny, H. Tong,
A. Kosir, L. Cavin, S. Chitsing, P. J. Grifﬁths, J. Tabouelle andJ. Le Loeuff. 2005. Minute theropod eggs and embryo from theLower Cretaceous of Thailand and the dinosaur-bird transition.
Naturwissenschaften 92: 477 – 482.
Department of Mineral Resources. 2014. Geology of Thailand.
Department of Mineral Resources, Ministry of Natural
Resources and Environment, Bangkok, Thailand, 508 pp.
Ewert, M. A. 1979. The embryo and its egg: Development andnatural history; pp. 333 – 413 in M. Harless and H. Morlock(eds.), Turtles: Perspectives and Research. John Wiley andSons, New York.
Fernandez, V., E. Buffetaut, V. Suteethorn, J. C. Rage, P. Tafforeau
and M. Kundrát. 2015. Evidence of egg diversity in squamate
evolution from cretaceous anguimorph embryos. PLoS ONE
10: 7 – 9.
Hirsch, K. F. 1983. Contemporary and fossil chelonian eggshells.
Copeia 1983: 382 – 397.
Hirsch, K. F. 1996. Parataxonomic classification of fossil
chelonian and gecko eggs. Journal of Vertebrate Paleontology
16: 752 – 762.
Isaji, S., A. Matsushita and R. Hirayama. 2006. Chelonian eggshells
from the Lower Cretaceous Kuwajima Formation of the Tetori
Group, Central Japan. Paleontological Research 10: 29 – 36.

6

T. IMAI, P. JINTASAKUL, Y. AZUMA, Y. NODA AND D. CHOKCHALOEMWONG

Jackson, F. D., X. Jin, D. J. Varricchio, Y. Azuma and Y. Jiang.
2008. The first in situ turtle clutch from the Cretaceous
Tiantai Basin, Zhejiang Province, China. Journal of Vertebrate
Paleontology 28: 319 – 325.
Jackson, F., and J. Schmitt. 2008. Recognition of vertebrate egg
abnormalities in the Upper Cretaceous fossil record. Cretaceous
Research 29: 27 – 39.
Knell, M. J., F. D. Jackson, A. L. Titus and L. B. Albright. 2011.
A gravid fossil turtle from the Upper Cretaceous (Campanian)Kaiparowits Formation, southern Utah. Historical Biology 23:
57 – 62.
Kohring, R. R. 1998. Neue schlidkröten-eischalen aus
dem oberjura der grube guimarota (Portugal). Berliner
Geowissenschaftliche Abhandlungen 28: 113 – 117.
Kohring, R. R. 1999. Calcified shell membranes in fossil
vertebrate egg shell: evidence for preburial diagenesis. Journalof Vertebrate Paleontology 19: 723 – 727.
Lawver, D. R., and F. D. Jackson. 2014. A review of the fossil
record of turtle reproduction: eggs, embryos, nests andcopulating pairs. Bulletin of the Peabody Museum of Natural
History 55: 215 – 236.
Lawver, D. R., A. H. Rasoamiaramanana and I. Werneburg. 2015.
An occurrence of fossil eggs from the Mesozoic of Madagascar
and a detailed observation of eggshell microstructure. Journalof Vertebrate Paleontology 35: e973030.
Li, C., X. C. Wu, O. Rieppel, L. T. Wang and L. J. Zhao. 2008. Anancestral turtle from the Late Triassic of southwestern China.
Nature 456: 497 – 501.
Meesook, A. 2000. Cretaceous environment of northeastern
Thailand; pp. 207 – 223 in H. Okada and N. J. Mateer (eds.),
Cretaceous Environment of Asia. Elsevier, Tokyo.
Mikhailov, K. E., K. Sabath and S. Kurzanov. 1994. Eggs andnests from the Cretaceous of Mongolia; pp. 137 – 150 in K.
Carpenter, K. F. Hirsch and J. R. Horner (eds.), Dinosaur Eggs
and Babies. Cambridge University Press, New York.
Mohabey, D. M. 1998. Systematics of Indian Upper Cretaceous
dinosaur and chelonian eggshells. Journal of Vertebrate
Paleontology 18: 348 – 362.

Prasad, G. V. R., A. Sharma, O. Verma, A. Khosla, L. R. Singhand R. Priyadarshini. 2015. Testudoid and crocodiloid eggshells
from the Upper Cretaceous Deccan Intertrappean Beds ofCentral India. Comptes Rendus Palevol 14: 513 – 526.
Racey, A. 2009. Mesozoic red bed sequences from SE Asiaand the significance of the Khorat Group of NE Thailand.
Geological Society, London, Special Publications 315: 41 – 67.
Racey, A., and J. G. S. Goodall. 2009. Palynology and stratigraphyof the Mesozoic Khorat Group red bed sequences fromThailand. Geological Society, London, Special Publications
315: 69 – 83.
Schleich, H. H., W. Kästle and M. Groessens-van Dyck. 1988.
Paläogene Eischalenreste von Hainin (Belgien). Paläontologie
Zeitschrift 62: 133 – 146.
Schoch, R. R., and H. D. Sues. 2015. A Middle Triassic stemturtle and the evolution of the turtle body plan. Nature 523:
584 – 587.
Tanaka, K., D. K. Zelenitsky, T. Williamson, A. Weil and F.
Therrien. 2011. Fossil eggshells from the Upper Cretaceous(Campanian) Fruitland Formation, New Mexico. Historical
Biology 23: 41 – 55.
Tong, H., E. Buffetaut and V. Suteethorn. 2006. Isanemys, a newadocid turtle from the Sao Khua Formation (Early Cretaceous)of the Khorat Plateau, northeastern Thailand. Fossil Turtle
Research 1: 128 – 137.
Tong, H., J. Claude, V. Suteethorn, W. Naksri and E. Buffetaut.
2009. Turtle assemblages of the Khorat Group (Late
Jurassic-Early Cretaceous) of NE Thailand and theirpalaeobiogeographical significance. Geological Society,London, Special Publications 315: 141 – 152.
Wang, Q., X. Wang, Z. Zhao, J. Zhang and S. Jiang. 2013. New
turtle egg fossil from the Upper Cretaceous of the Laiyang
Basin, Shandong Province, China. Anais da Academia
Brasileira de Ciências 85: 103 – 111.
Zelenitsky, D. K., W. G. Joyce and D. B. Brinkman. 2008. First
fossil gravid turtle provides insight into the evolution of
reproductive traits in turtles. Biology Letters 4: 715 – 718.