Sciency Thoughts: Turtle eggs from the Late Cretaceous of Madagascar.

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.

Pages

Monday, 26 October 2015

Turtle eggs from the Late Cretaceous of Madagascar.