Easter eggs from the most South African dinosaur

Deep in the distant past, a hunched-over dinosaur with a long neck and longer tail returned to her nesting site in what was not yet Southern Africa.

There she would lay her eggs after each mating season, and brood until her clutch hatched — until one year, when cataclysm struck: the heavens opened, and the waters rose and conspired with the earth to seal her eggs away, never to hatch.

Until nearly 200-million years later, that is, when they would be picked up at the side of a new road, brushed up, pored and puzzled over and, finally, stuck in a particle accelerator.

The eggs were first discovered in 1976, when legendary fossil hunter James Kitching was scouting out a road-excavation site in the Golden Gate National Highlands Park. Lots of hills in the area, so building a new road meant carving through the countryside, slicing through the ter-

rain and exposing geological strata in all their cross-sectioned glory.

As unfunded paleontological excavations go, this was about as rough and indiscriminately destructive as it gets — but pragmatic, and effective: the fossils practically tumbled out.

Among them, our clutch of eggs, laid by perhaps the most South African of all the dinosaurs, Massospondylus carinatus.

“It has similar cousins elsewhere around the world, but Massospondylus has only been found in South Africa, Lesotho and Zimbabwe – and it’s the most common dinosaur we have in South Africa,” says Dr Kimberley Chapelle, of the Evolutionary Studies Institute at the University of the Witwatersrand. “So as fossils go we’ve got hundreds and hundreds of them.”

Including the eggs.

Kitching would first describe them in a paper published in 1979, but getting to see what was inside the eggs would prove to be a tricky business; any fossilised embryos they contained were very small and extremely fragile. In the mid-2000s a specialist in fossil preparation painstakingly brushed away sandstone and shell to expose two of the embryos, allowing Canadian paleontologist Robert Reisz to describe the embryonic anatomy he observed in papers published in 2005, 2011 and 2012.

But the eggs still had secrets to share. In 2015, then-PhD-student Chapelle packed them into a secure travelling case and travelled to the European Synchrotron Radiation Facility in Grenoble, France with Professor Jonah Choiniere, her supervisor.

The synchrotron particle accelerator there can function as a gigantic CT scanner, producing a massive ring of electrons that emit high-powered X-ray beams, which can scan objects

like, let’s say, fossilised dinosaur eggs, with an unprecedented level of detail — allowing resolution down to a cellular level. Eventually. After a lot of computer crunching. But finally, after years of data processing at the Wits laboratory, Chapelle was able to construct a 3D model of a baby Massospondylus.

Fully grown, Massospondylus had the sort of long neck you might associate with Brontosaurus or Diplodocus, but it was much smaller, though not exactly small, measuring about 5m from tip to tail. And it walked on two feet, carrying its long tail behind it as ballast against its forward-leaning slouch.

Its head was relatively small in relation to its body, and so, compared with modern-day birds and reptiles, it cut a strange figure. But inside its head, its skull would prove to be quite familiar to herpetologists.

It was the formation of these cranial bones that Chapelle focused on — specifically their ossification sequence, or the order in which the growing embryo’s bone tissue formed and hardened inside the egg.

Her initial findings appear in this week’s edition of the Nature journal Scientific Reports. In a paper co-authored with Choiniere and synchrotron-imaging specialist Vincent Fernandez, Chapelle describes the embryonic cranial development of Massospondylus, comparing it with

that of birds, lizards, turtles and crocodiles living today. They have found that, even though separated by hundreds of millions of years of evolution, the way in which the lives of dinosaurs and their living reptilian relatives start out has remained remarkably unchanged.

Taking this understanding one step further, and with a map of the newly charted saurian ossification sequence in hand, Chapelle goes on to present a novel method for deter-

mining how far along in its development a fossilised embryo may be said to be.

“Until now, it was believed that the embryos in those eggs had died just before hatching,” says Chapelle. “However, they are actually much younger than previously thought — only 60% through their incubation period.”

“Hopefully, now we can apply it to other eggs of different dinosaurs and

determine where they are.”

Though, as Chapelle notes, there are more secrets to unlock from this clutch. “The rest of the embryo’s skeleton awaits, and then I have a larger project to determine incubation period for Massospondylus embryos. We now have a better idea of how they developed; the next step is to see how long they took to hatch.”

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