Chinese fossil eggs show the decline of dinosaurs

Photo: Artist’s Drawing of Late Cretaceous Ophiraptorosaurs, Hadrosaurs, and Tyrannosauruses Living in Central China
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Credit: IVPP

Roughly 66 million years ago, a large asteroid hit Earth and contributed to the global extinction of the dinosaurs, leaving birds with their only living descendants.

Scientists know that a diverse group of dinosaurs lived around the world at the end of the Cretaceous period, just before their extinction. However, scientists have debated whether the dinosaurs were in their prime or already in decline before their demise. In other words, did the dinosaurs come out with a bang or a whine?

Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, along with their collaborators, now have an answer. They found evidence to support the hypothesis that dinosaurs were not very diverse before their extinction and generally declined during the latter part of the Cretaceous.

Their findings were published in PNAS On September 19.

Most of the scientific data on the last days of dinosaurs comes from North America. Although some published studies indicate that groups of dinosaurs were thriving well before the extinction, other, more detailed research has suggested that dinosaurs were retreating instead, paving the way for their eventual mass extinction.

By examining China’s dinosaur record, Chinese researchers hoped to determine if this downward trend extended to Asia as well.

Researchers studied more than 1,000 fossilized dinosaur eggs and eggshells from the Shanyang Basin in central China. These fossils came from rock sequences with a total thickness of approximately 150 metres. The researchers obtained detailed age estimates for the rock layers by analyzing and applying computer modeling to more than 5,500 geological samples. This allowed scientists to create a timeline of nearly two million years at the end of the Cretaceous period – to an accuracy of 100,000 years – representing the period immediately before the extinction. This timeline allows direct comparisons to be made with data from around the world.

Scientists have identified a decrease in dinosaur diversity based on data from the Shanyang Basin. For example, the fossils of 1,000 dinosaur eggs collected from the aquarium represent only three different species: Macroolithus yaotunensisAnd the Elongatoolithus elongatusAnd the Stromatoolithus pinglingensis. In addition, two of the three dinosaur eggs belong to a group of toothless dinosaurs known as oviraptors, while the other belong to the group of hadrosaurids (also known as duck-billed dinosaurs).

Some additional dinosaur bones from the area show that tyrannosaurus and sauropod also lived in the area between about 66.4 and 68.2 million years ago. This low diversity of dinosaur species persisted in central China for the past two million years before the mass extinction. The few dinosaurs in the Shanyang Basin and central China are far from the world shown in Jurassic Park.

These findings – in concert with data from North America – suggest that dinosaurs may have been in decline globally before they became extinct.

This long-term global decline in dinosaur diversity until the end of the Cretaceous period and the persistently low number of dinosaur lineages over the past few million may have been caused by known global climatic fluctuations and massive volcanic eruptions, i.e. from the Deccan Traps in India. These factors may have led to instability at the ecosystem level, making non-avian dinosaurs vulnerable to mass extinctions in conjunction with the asteroid impact.

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