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Unusual Dinosaur Eggs Are Shaped Like Easter Eggs

Paleontologists have found hen-sized, hen-shaped dinosaur eggs sticking out of ancient sandstone in Spain’s Pyrenees mountains.

The species that laid the 70-million-year-old eggs remains a mystery, but it was probably a small meat-eating creature that straddled the increasingly blurry divide between birds and dinosaurs.

“The size and shape of these eggs … are roughly similar to modern hen eggs, which is unusual in the Cretaceous fossil egg record,” wrote the authors of a study published Mar. 13 in Paleontology.

‘When I first saw images of the shell, it was hard for me to tell they were fossilized.’

The oldest-known dinosaur eggs date back 190 million years, but most fossilized shells come from rock formed in the Cretaceous. That period that lasted from about 145 million years ago to 65 million years ago and was teeming with land-dwelling dinos.

Paleontologists Nieves Lopez-Martinez of Universidad Complutense de Madrid (who passed away in December 2010) and Enric Vicens of Universitat Autònoma de Barcelona discovered the two new egg nests in the Pyrenees. Poking out from scattered fossil eggshell fragments stood a mostly complete 2.75-inch-long egg, roughly the size of a jumbo chicken egg. The authors named its mysterious parent Sankofa pyrenaica, which means “learning from the past” in the West African language of the Ashanti (a nod to the close geologic relationship shared by Africa and the Pyrenees of northeastern Spain).

Based on the composition of the sandstone rock the eggs were plucked from, Lopez-Martinez and Vicens think the nests existed on the beach of a barrier island lagoon. Until some cataclysmic event buried them, that is.

Dark gray eggs (left) are those of dinosaurs, including the new egg, Sankofa. Light-gray eggs (right) are those of modern birds. (Click to enlarge.)

“When I first saw images of the shell, it was hard for me to tell they were fossilized. The preservation is just exceptional,” said paleontologist Darla Zelenitsky of the University of Calgary, who was not involved in the study. “It’s possible an overflowing river deposited very fine sand or sediment on top of the egg nests.”

That ultrafine preservation of the egg allowed the research team to see how the eggs differed from modern bird eggs. Microscope images (below) revealed two thin, hard layers of shell, called the mammillary and continuous layers. They lacked a third external layer which is common to all modern bird eggshells.

To find out more precisely how similar or dissimilar the shells were from other eggs, the researchers adopted a model created for modern bird eggs in 1997, which uses four different measures of egg volume and shape to generate a score.

They discovered that small, meat-eating dinosaur eggs neatly drifted to one side of the chart while modern bird eggs fell into a group on the other side. Seated squarely in the middle? The new Sankofa eggs.

“Their analysis, which hasn’t been attempted before, makes these new fossil eggs found in northern Spain sort of transitional between modern birds and small meat-eating dinosaurs,” Zelenitsky said. “The eggs between these groups were thought to be different. Based on shape, this study suggests otherwise.”

Paleontologist Mark Purnell of the University of Leicester said the egg shape analysis was clever, especially given the absence of any fossilized parent dinosaurs sitting on or near the nest.

“It’s a nice approach to dealing with problematic fossils, and I think other researchers will find it useful in the future,” he said.

Sankofa fossilized eggshells under an optical microscope (top) and scanning electron microscope (bottom).

Images: Nieves Lopez-Martinez/Enric Vicens/Paleontology

Citation: “A new peculiar dinosaur egg, Sankofa pyrenaica oogen. nov. oosp. nov. from the Upper Cretaceous coastal deposits of the Aren Formation, south-central Pyrenees, Lleida, Catalonia, Spain.” By Nieves Lopez-Martinez and Enric Vicens. Paleontology, published online Mar. 13, 2012. DOI: 10.1111/j.1475-4983.2011.01114.x