According to another study, dinosaurs were already on their way out before the asteroid struck

The extinction of dinosaurs has long occupied paleontologists. Their mass extinction, after a fiery meteorite struck Earth some 66 million years ago as volcanoes erupted and global temperatures rose and fell, marked a tumultuous end to the reign of these once-dominant beasts.

But now another study suggests dinosaurs were already on their way millions of years before the fateful meteorite struck, according to an analysis of over 1,000 fossilized eggshells unearthed in central China.

“Dinosaurs died out gradually over millions of years, rather than dying out in sudden catastrophes,” said study author Qiang Wang of the Chinese Academy of Sciences South China tomorrow post.

However, expect these conclusions to be tested. The study strains a long and back-and-forth debate among paleontologists about whether non-avian dinosaurs met an abrupt end or whether they were already on the brink of extinction before a 10-kilometer-wide asteroid sealed their fate.

New research by a team of geologists and paleontologists working in China suggests that dinosaur biodiversity faded at least two million years before the dinosaurs became extinct at the end of the Cretaceous period, leaving birds as their only living descendants.

His conclusions are based on a collection of egg fossils, including several complete and incomplete dinosaur eggs preserved in 150-meter-thick layers of rock deposited 68.2 to 66.4 million years ago, just before the curtain fell on dinosaurs.

The Shanyang Basin, where the egg fossils were found, is home to one of the richest dinosaur finds from the late Cretaceous period. And yet the researchers found only three dinosaur taxa in the fossilized egg shells — a significant decline in species diversity compared to older fossil finds.

This drop in variation, the researchers suggest, may have weakened the dinosaurs’ collective ability to recover from the impact of the asteroid Chicxulub, which struck modern-day Mexico, or to adapt to the turbulent environments of the time. With fewer options in their evolutionary playbook, they became snookered.

“Our results support a long-term decline in global dinosaur biodiversity 66 million years ago that likely set the stage for the mass extinction of non-avian dinosaurs at the end of the Cretaceous,” write Fei Han and colleagues from the China University of Geosciences in their published paper.

Despite their claims, the team rightly notes that dinosaur extinction remains controversial for a variety of reasons, including “sampling error in the fossil record, differences in analytical approaches used, and the rarity of highly accurate geochronological dating of dinosaur fossils.”

In their work, Han and colleagues used a variety of techniques to layer thousands of rock samples that enclose the fossilized egg shells, estimate the ages of those samples, and create a timeline of those data that had a resolution of 100,000 years.

Two of the three “oospecies” dinosaur eggs (a species category for dinosaurs if you only have eggs) identified in the mix were from a group of toothless, parrot-like dinosaurs called oviraptors, with the third group of egg-laying dinosaurs identified as herbivorous hadrosaurs with duckbill.

The low biodiversity the researchers found is consistent with skeletal remains of ancient dinosaurs also discovered in the Shanyang Basin, and is comparable to other fossil deposits in southern and eastern China and some bone beds in North America, which also indicate declining dinosaur diversity In the same period.

All of this “suggests reduced diversity and an overall decline among dinosaurs on a global scale,” Han and colleagues argue, suggesting that the decline may be due to global climate variability and volcanic eruptions.

Other studies that also suggest dinosaurs are in danger of extinction suggest that their diversity may have started to dwindle as early as 10 million years before the meteorite hit Earth.

However, previous studies have found otherwise. A recent, wide-ranging analysis that simulated dinosaur speciation — the rate at which new species emerge — found that less than 20 percent of dinosaurs before the asteroid impact were in ultimate decline, while other species thrived.

The only way to resolve these conflicting interpretations, Han and colleagues conclude, is to find, sample, and analyze more fossils and integrate existing data from fossil sites around the world to better understand extinction patterns.

Certainly not an easy task, but one that could help finally clarify what happened in the fading light of dinosaur rule.

“Our study in Asia of abundant and geochronologically dated fossils is an important step in this direction,” the researchers write.

The study was published in PNAS.