HIGH CONCENTRATIONS OF TURTLE EGGSHELLS (TESTUDOOLITHIDAE) IN AN EOCENE DELTAIC PLAIN (SOBRARBE DELTAIC COMPLEX, NORTHERN SPAIN)

Abstract

Here we describe an unusually high concentration of eggshell fragments from the Eocene Lutetian Sobrarbe Formation, in the Ainsa Basin, Huesca, Northern Spain. The eggshells were collected by screen washing of sediments from the Castejón de Sobrarbe 41 site, a sirenian dominated bonebed which has also provided several crocodilian and turtle specimens, including a complete carapace of a pleurodiran turtle. Sedimentological, taphonomical and paleoecological evidence supports that the accumulation was produced by the infilling of a tributary channel in the deltaic plain of the Sobrarbe Deltaic Complex. The eggshells are single layered, and are formed by subcilindrical shell units with radial ultrastructure. The outer surfaces are slightly sculptured, with compactituberculated ornamentation. The inner surfaces are cratered in most of the eggshell fragments. In thin sections, the eggshells are highly recrystallized, with its original composition completely lost. This contrasts with the apparent fine preservation of the ultrastructure under secondary electrons. All features are consistent with fossil and modern turtle eggshells, suggesting a very slow recrystallization of the original aragonitic eggshell, which preserves the fine morphology of the eggshells but not its crystal structure. The eggshells are assigned to the oofamily Testudoolithidae, and represent the third record of Eocene turtle eggshells in the world. Nevertheless, this scarcity is probably due to lack of sampling and the instability of their aragonitic composition. The high numbers of eggshell fragments, all of them assignable to the same ootaxon, and the absence signs of transport, suggests that the accumulation occurred in the proximities of a nesting area. Recent Pleurodira turtles are known to nest in different lentic and lotic environments within modern deltaic systems. Further prospection of the more proximal facies of the Sobrarbe Deltaic Complex may thus yield more complete specimens, including nests and clutches.

Full text

XIV EAVP Meeting, 6-10 July, 2016, Haarlem, The Netherlands
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174!
HIGH CONCENTRATIONS OF TURTLE EGGSHELLS
(TESTUDOOLITHIDAE) IN AN EOCENE DELTAIC PLAIN
(SOBRARBE DELTAIC COMPLEX, NORTHERN SPAIN).
M. Moreno-Azanza1*, R. Silva2, E.D. Berenguer3, J.I. Canudo3, and A. Badiola4
1Universidade Nova de Lisboa, Geobiotec. Dpto. de Ciências da Terra. Faculdade de
Ciências e Tecnologia, FCT, 2829-526. Caparica, Portugal. Museu da Lourinhã
2Universidad de Zaragoza, Área de Estratigrafía, Dpto. de Ciencias de la Tierra, Facultad de
Ciencias, Calle Pedro Cerbuna, 12, 50009, Zaragoza, España
3Universidad de Zaragoza, Grupo Aragosaurus-IUCA, Dpto. de Ciencias de la Tierra,
Facultad de Ciencias, Calle Pedro Cerbuna, 12, 50009, Zaragoza, España
4Universidad del País Vasco (UPV/EHU), Dpto. Estratigrafía y Paleontología, Facultad de
Ciencia y Tecnología, Apartado 644, 48080, Bilbao, España
*mmazanza@fct.unl.pt
Keywords: vertebrate eggshells, taphonomy, Ainsa Basin, outhern Pyrenees, Lutetian
Here we describe an unusually high concentration of eggshell fragments from the
Eocene Lutetian Sobrarbe Formation, in the Ainsa Basin, Huesca, Northern Spain.
The eggshells were collected by screen washing of sediments from the Castejón de
Sobrarbe 41 site, a sirenian dominated bonebed which has also provided several
crocodilian and turtle specimens, including a complete carapace of a pleurodiran
turtle. Sedimentological, taphonomical and paleoecological evidence supports that
the accumulation was produced by the infilling of a tributary channel in the deltaic
plain of the Sobrarbe Deltaic Complex. The eggshells are single layered, and are
formed by subcilindrical shell units with radial ultrastructure. The outer surfaces are
slightly sculptured, with compactituberculated ornamentation. The inner surfaces are
cratered in most of the eggshell fragments. In thin sections, the eggshells are highly
recrystallized, with its original composition completely lost. This contrasts with the
apparent fine preservation of the ultrastructure under secondary electrons. All
features are consistent with fossil and modern turtle eggshells, suggesting a very
slow recrystallization of the original aragonitic eggshell, which preserves the fine
morphology of the eggshells but not its crystal structure. The eggshells are assigned
to the oofamily Testudoolithidae, and represent the third record of Eocene turtle
eggshells in the world. Nevertheless, this scarcity is probably due to lack of sampling
and the instability of their aragonitic composition. The high numbers of eggshell
fragments, all of them assignable to the same ootaxon, and the absence signs of
transport, suggests that the accumulation occurred in the proximities of a nesting
area. Recent Pleurodira turtles are known to nest in different lentic and lotic
environments within modern deltaic systems. Further prospection of the more
proximal facies of the Sobrarbe Deltaic Complex may thus yield more complete
specimens, including nests and clutches.