In a new study, researchers scanned two Massospondylus carinatus dinosaur embryos first found in 1976 in South Africa’s Golden Gate Highlands National Park. They used synchrotron scanning, which is a kind of 3D X-ray scan.
What they discovered was completely unexpected. The dinosaurs are far more similar to modern reptiles than scientists had anticipated.
The reasons why rest on two key findings:
1. The seemingly full skeletons were actually only about 60 percent developed.
2. The dinosaur embryos have two different sets of teeth.
Deceptive dinosaurs — The embryos “look like the skeleton of an animal ready to hatch,” study co-author Vincent Fernandez, a researcher at the Natural History Museum in London, tells Inverse.
But the bones in the tiny dinos' skulls, which had remained obscured in the rock, suggest otherwise.
"They are a lot younger that we expected,” Fernandez says.
“With the scan we discovered that these bones were there, but poorly ossified," he says. "Comparing with modern reptiles, we estimated that the dinosaur embryos were about 60 percent of incubation."
In modern reptiles and birds, “the pattern in which bones start ossifying is quite similar.” In these creatures, the skull bones start to harden at the tip of the snout. The upper and back part of the skull, also known as the "braincase," is the last piece of the puzzle to ossify, along with some jaw bones.
The findings indicate that these prehistoric herbivores were already using an embryo-development strategy common to modern crocodiles, chickens, turtles, and lizards.
The dinosaur specimens also had two different types of teeth. They range in size from 0.4 to 0.7 millimeters — with one set of tiny, triangular, first-generation teeth.
Crocodiles and certain other modern reptiles have similar first-generation teeth, that they resorb or shed before they even hatch. Now, researchers know that ancient dinosaurs did, too.
The idea that both modern reptiles and ancient dinosaurs might develop similarly is not new, but this is some of the first concrete evidence that modern reptile development is actually prehistoric.
“This development strategy has been around for 190 million years, which is quite surprising," Fernandez says.
A paper detailing the new findings was published Thursday in the journal Scientific Reports.
"Beautiful" embryos held big mysteries — Since their discovery some five decades ago, researchers believed these Massospondylus carinatus embryos had perished just before their eggs hatched.
“These embryos are beautiful, but information that could get obtained from the prepared surface was quite limited,” Fernandez said. To discover whether that was indeed the case, the researchers needed to look below the surface of the fossils' rock.
That’s where the synchrotron scanning came in.
They used high-powered X-ray beams to scan the bones down to the cellular level. That enabled them to recreate the embryos’ 2-centimeter-long skulls as a 3D model, or "virtually extract them from the rock," as Fernandez puts it.
“This is a very special moment, knowing you are the first one to see such a beautiful specimen,” he says. It is a slow process, however. “When you have access to a synchrotron, you have to use the machine day and night for days on.”
The team took turns to do the research, eat, and sleep, Fernandez says. The results are a just reward for their travails. Using the novel technology, the team got the first glimpse inside the eggs, and even identified a third embryo inside the samples.
“Virtual extraction of the bone is a painstaking process,” Fernandez says. Researchers had to extract each bone, one at a time.
The work is testament to how technology can reveal new, granular details of old fossils, Fernandez says. in the future, synchotron scanning could allow researchers to estimate the incubation time of dinosaur embryos, revealing yet more about their early development and how it stacks up to modern reptiles.
The results of such high-level scans are also just visually stunning, Fernandez says.
“I keep being impressed by the level of details we can achieve: We can see these tiny teeth, smaller than the tip of a toothpick,” he says.
“I particularly love when the bones are put back in 3D, and we see the big eye socket of the dinosaur, it really starts looking like a baby.”
Abstract: Dinosaur embryos are among the rarest of fossils, yet they provide a unique window into the palaeobiology of these animals. Estimating the developmental stage of dinosaur embryos is hindered by the lack of a quantitative method for age determination, by the scarcity of material, and by the difficulty in visualizing that material. Here we present the results of a broad inquiry, using 3D reconstructions from X-ray computed tomography data, into cranial ossification sequences in extant saurian taxa and in well-preserved embryos of the early branching sauropodomorph dinosaur Massospondylus carinatus. Our findings support deep-time conservation of cranial ossification sequences in saurians including dinosaurs, allowing us to develop a new method for estimating the relative developmental percentage of embryos from that clade. We also observe null-generation teeth in the Massospondylus carinatus embryos which get resorbed or shed before hatching, similar to those of geckos. These lines of evidence allow us to confidently estimate that the Massospondylus carinatus embryos are only approximately 60% through their incubation period, much younger than previously hypothesized. The overall consistency of our results with those of living saurians indicates that they can be generalized to other extinct members of that lineage, and therefore our method provides an independent means of assessing the developmental stage of extinct, in-ovo saurians.