Expert paleontologists at the University of Calgary and the Royal Tyrrell Museum are lucky to have discovered the fossilized remains of a 75-million-year-old pregnant turtle and nest of 26 eggs.
The findings are said to be the first Adocus specimens of a pregnant female turtle and nest site in the world and were discovered in the badlands of southeastern Alberta.
According to Tyrrell Museum dinosaur palaeoecology curator François Therrien, the fossil is the second pregnant, egg-containing fossil of any prehistoric animal recorded. A similar discovery was studied by the U of C dinosaur paleontology assistant professor Dr. Darla Zelenitsky and her colleagues, who examined two eggs safely preserved inside a dinosaur’s body cavity.
“Adocus is a really rare turtle in Alberta,” said Therrien, who is in the process of comparing the egg laying and reproductive traits of Adocus to modern turtles. “It’s really common in the United States in terms of fossils. But in Alberta, it’s relatively rare, probably because there were differences in environment and climate.”
Therrien added that its even more remarkable how the skull of the pregnant Adocus fossil was still attached to the body. In most circumstances, skulls are eaten by predators or washed away by the sea after the turtle dies.
Both specimens are important because they give researchers insight about the evolution of turtles and tortoises, specifically the reproductive function and egg laying strategies of Adocus turtles.
The details of the carapace, or the hard, upper layer of shell, is one factor that has helped paleontologists identify the remains as belonging to the Adocus genus.
“Adocus is really peculiar because the carapace of turtles usually has ornamentations,” said Therrien. “So it has features– bumps, grooves, holes and depressions– on the shell. Some of the pattern that’s imprinted on the carapace is diagnostic for many turtles.”
Zelenitsky and her colleagues determined that the pregnant turtle was about to lay the eggs before it died. Researchers found five crushed eggs and computerised tomography, or CT scans, detected a few more eggs under the turtle shell’s surface.
The nest of eggs, which belonged to a different female, revealed that the Adocus produced 19 to 26 eggs near large masses of water. Each egg is approximately four centimetres in diameter with a hard, off-white shell. The eggshell is almost a millimetre thick, which allowed the embryos to stay well protected from dinosaur predators and desiccation in dry climates.
“Based on what we know of modern turtles, they just made a little dug out in the ground to lay their eggs,” said Zelenitsky. “Often the eggs are buried in the sand or dirt. It was probably the same situation for Adocus.”
The turtle was approximately 40 centimetres in length and based on its long, flattened shell it was highly aquatic. Researchers compared its general body form to modern sliders and cooters which are small aquatic turtles found in pet stores. In terms of appearance, the Adocus closely resembles the Mesoamerican river turtle, but without any direct family relation.
The Tyrrell staff unearthed the pregnant Adocus fossil with eggs in 1999 while Zelenitsky found the nest of eggs with the help of her field assistant in 2005. The discovery of the eggs was published last month in the British journal Biology Letter. The pregnant fossil was found about 50 kilometres away from the nest site.
“We hope to learn more about the evolution of modern turtles, but also the evolution of [Zelenitsky’s] main field research, which is reproductive biology of extinct animals, specifically dinosaurs,” said Therrien, who would like to see more research done on the discovery. “We know what they looked like when they were adults, but we don’t know much about how they reproduce and how they grew from the egg to full adulthood.”
The eggs from the nest will be sent to Austin, Texas in September for a complete CT scan.
“The next step will be the CT scanning of the eggs from the nest themselves to see if there are any embryos or babies inside them,” said Zelenitsky. “Hopefully, if there are, then we can say something about the early development of Adocus.”
The fossilized pregnant turtle is currently being showcased in the Discovery Hall at the Tyrrell Museum, where it has become one of the most popular attractions on display.