What hatched from these 200-million-year-old dinosaur eggs was almost like seeing dinosaurs emerge from extinction.
Maybe actual dinosaurs didn’t break their way out like that infamous scene in Jurassic Park, but close. The Massospondylus embryos inside were so impeccably preserved that they were (with some help from modern technology) able to tell their secrets across many millennia. Postdoc Kimberley Chapelle of the Evolutionary Studies Institute at Wits University in South Africa, along with colleague Jonah Choiniere's research team, found out some surprising parallels to modern dino relatives in the embryonic development of this dinosaur.
“Dinosaurs are an excellent natural laboratory for understanding postural transitions because they demonstrate at least four instances of quadrupedality evolving from bipedality,” Chapelle and her team said in a study recently published in Paleontology.
Massospondylus was an omnivorous sauropodomorph — one of a group of long-necked herbivores that eventually evolved from being bipedal to quadrupedal. Sauropodomorphs evolved their neverending necks to be able to reach leaves that were high up in prehistoric trees. The more food fueled it, the more energy an individual of this species had to grow larger. The opposite would happen when food was scarce. It has been rendered on both two and four legs before, but digital reconstructions of those embryo skeletons revealed that at this phase in its evolution, it stood on two.
To digitally re-articulate the Massospondylus embryos, which are some of the oldest ever unearthed, Chapelle and Choiniere had the fossils imaged by the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, which transmits powerful X-rays from its immense ring of electrons whizzing at the speed of light. ESRF isn’t just any CT scanner. These X-rays were able to CT scan the fossils at such a high resolution that they made even single bone cells visible. It was the scans from the ESRF that enabled the research team to bring the skull of an embryo back to life on a computer screen.
Until that skull appeared onscreen, the team thought the embryos had perished right before hatching. It turned out that comparing the embryonic development of Massospondylus to that of distant dinosaur relatives like crocodilians, chickens, turtles, and lizards revealed that the embryos had only made it through 60% of their incubation period. This means that the skull of such creatures has been developing in the egg the same way throughout reptile evolution — over 250 million years.
There was, however, something much stranger that this extinct lizard had in common with creatures crawling around now. Chapelle was surprised to find that the embryos developed with two sets of teeth, one of which is thought to have been resorbed in the body much like what happens in geckos and crocodiles. But why?
“One reason could be that it is a shared feature from a common ancestor,” Chapelle tells SYFY WIRE. “Another hypothesis is that the process of having a first set of teeth and replacing it with a functional set of teeth is important to kickstart the dental replacement pattern that the dinosaur goes through its entire life. Massospondylus, like crocodiles for example, constantly replaced their teeth during their life. They are not limited to two sets like we are.”
Not everything has clawed its way out of the egg yet. Now that the skull has spoken to us after hundreds of millions of years of being buried, the rest of the skeleton could reveal more about how these dinosaurs grew in the egg, and more similarities (and possibly differences) between them and their relatives which still roam the earth.
“I guess we expect the rest of the skeleton to form in a similar way as well,” Chapelle says. “I would be surprised if it didn’t.”
(via Wits University)