According to two studies published in Nature this week, dinosaur eggs … mushy.
“The assumption has always been that the ancestor’s dinosaur egg was hard-shelled,” said Mark Norell, paleontologist at the American Museum of Natural History and lead author of one of the studies, in a press release. Earlier eggs in the fossil record date largely from the late period of the dinosaurs, which neared their final end about 66 million years ago. We have fossilized bones from creatures that are 240 million years old, but no eggs.
“We found thousands of skeletal remains of Ceratopsian dinosaurs, but almost none of their eggs. Why weren’t their eggs preserved?” Asks Norell.
Norell believes that his team answered this question in his study result. Despite the assumption that dinosaurs produce hard-shelled eggs, there is fossilized evidence to confirm some types of dinosaurs, namely Protoceratops and Mussaurusproduced soft-shelled eggs. The creatures buried and incubated these eggs in moist soil, as some reptiles do today. Soft-shelled eggs are not as exquisitely preserved as their hard-shelled counterparts – they crumble before they have a chance to petrify – and leave a gap in the fossil record.
Now this gap can be filled.
A second study provided further evidence for the investigation of a soft shell egg found in Antarctica, which is now dated to be around 66 million years old. It is the first petrified egg in Antarctica that was far less cold than it is today.
However, the scientists responsible for the discovery are not sure of the mother of the egg. Without embryonic remains in the egg, it is impossible to tell who it belongs to.
The team suspects that due to its estimated weight and its proximity to other discovered petrified eggs from non-bird dinosaurs, it was probably laid by a dinosaur. The egg itself is huge – the largest softshell ever found and the second largest ever. The suspicion is that it was something of a Mosasaurs, a giant marine dinosaur that patrolled the oceans.
Together, the studies reevaluate the theory that all dinosaurs have laid calcified, hard-shelled eggs – like their chicken offspring – and instead postulate that dinosaurs could have been more similar to reptiles.
There is little doubt that the eggshells have evolved over time to tend to be hard rather than soft shells. But because there are so few soft-shelled eggs in the fossil record, it is difficult to set a solid development schedule. Finding an answer depends on the time the egg was laid and the type of dinosaur in question.