Turtles are
unique among living Amniotes (Vertebrates that can lay eggs out of
water, or that give birth to live young and are thought to be
descended from ancestors which could lay eggs out of water, i.e.
Reptiles, Birds and Mammals) in that they produce eggs with shells of
aragonite rather than calcite. As aragonite is less chemically stable
than calcite Turtle eggs have lower preservational potential than
other Amniote eggs and are therefore less abundant in the fossil
record (this lower chemical stability is not likely to be an issue
over the lifetime of the egg). Nevertheless fossil Turtle eggs have
been found on every continent except Antarctica, where the absence of
known fossils is quite likely to be due to the low levels of
collecting rather than an absence of specimens
In a paper
published in the Journal of Vertebrate Paleontology on 7 August 2015,
Daniel Lawver of the Department of Earth Sciences at Montana StateUniversity, Armand Rasoamiaramanana of the Départementde Paléontologie at the
Université d’Antananarivo
and Ingmar Werneburg of the PaláontologischesInstitut und Museum der Universitát Zürich, the Museum fürNaturkunde at the Leibniz Institut fürEvolutions und Biodiversitátsforschung
and the Institut fürBiologie at the Humboldt-Universitãtzu Berlin describe a clutch of Turtle eggs from the Late Cretaceous
Morondava Basin of western Madagascar.
The clutch
comprises three spherical eggs, one of which retains traces of
eggshell. The eggs lacking shells measure 35.5 and 34.8 mm in
diameter, the egg with a shell also measures 35.5 mm in diameter.
None of the eggs contains any trace of an embryo, suggesting that
they were either infertile or did not develop to the point where
biomineralization of the skeleton began to occur. The preserved shell
is 440 µm
thick and is comprised of tightly packed mineral elements indicative
of a rigid shell. In areas where the shell has broken away the
underlying surface shows numerous small circular features 259-300 µm
in diameter made up of crystals radiating from a central point. Such
features are typical of Turtle eggs, where aragonite crystals are
formed radiating around an organic nucleation point.
Fossil
Turtle egg. (A) Whole egg with eggshell preserved on approximately
half of the specimen; (B) close-up of underlying sediment with
impression of shell units. Scale bars equal 1 cm (A) and 1 mm (B).
Lawver et al. (2015).
The
nature of the Turtle that laid the Malagasy eggs is unclear. The eggs
were found in deposits interpreted as having been deposited close to
the shore, raising the possibility they could have been laid by a
Marine Turtle; however the origin of true Marine Turtles remains
controversial, and it is not universally accepted that they existed
when the eggs were buried. Fossil Turtles are known from the End
Cretaceous of Madagascar, but these are younger than the eggs, and
all known specimens are Cryptodires (Side-necked Turtles) a
non-Marine group known only from the Southern Henisphere. Most modern
Turtles, including all Sea Turtles, lay semi-rigid, spherical eggs,
while one group, the Kinosternids (Musk Turtles and Mud Turtles) lay
rigid, elongate eggs. The Malagasy eggs were clearly rigid-shelled
and spherical in shape, a pattern not found in any modern group. This
could be seen as a potential ancestral state for Turtle eggs, but
semi-rigid spherical Turtle eggs have been found from the Middle
Jurassic of England, suggesting that this form had already appeared
long before the Malagasy eggs were laid. This suggests that the eggs
were laid by a member of an extinct Turtle lineage, though how
closely related this animal was to modern Turtles is obscure.
See
also...
Pappochelys rosinae: A Proto-Turtle from the Middle Triassic of Badem Württemberg, Germany. Turtles
have a fairly good fossil record, as would...
Desmatochelys padillai: A Protostegid Turtle from the Early Cretaceous of Colombia. The
Protostegid Turtles were a group of Turtles known from the Cretaceous
that are thought to have been members of the Chelonioidea, the group
that includes the two living Marine Turtle groups, the Chelonidoidea
(Sea Turtles) and Dermochelyidae (Leatherback...
Calcified Lizard eggs with preserved embryos from the Early Cretaceous of Thailand. Among living Vertebrate groups,
Lizards show the most diverse range of reproductive strategies, with species
known that reproduce sexually and parthanogenically (check spelling – a form of asexual reproduction in which
the female fertilizes her own eggs, rather...
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