Evrim Yazgin
Cosmos science journalist
About 66 million years ago, the history of Earth changed forever.
A 10km-wide asteroid struck Earth, wiping out 75% of all life. This event marked the end of the Cretaceous period and the “Age of Dinosaurs.”
With the extinction of the non-avian dinosaurs, our mammal ancestors were able to come out of the shadows and dominate on land. In the skies, the dinosaurs that did survive – birds – diversified.
Some palaeontologists have suggested that dinosaurs were doomed even before this cataclysmic event. For example, a 2022 study of Chinese dinosaur egg fragments suggested that the 2 million years before the asteroid struck saw a decline in dinosaur species abundance.
But a new study has challenged the idea that dinosaurs were on the decline before their extinction. This theory may actually be the result of scant fossil evidence, the authors of the new paper published in Current Biology argue.
“It’s been a subject of debate for more than 30 years,” says lead author Chris Dean from University College London (UCL) in the UK.
A downward slope for T. rex and co.
The researchers focused on the fossil record from North America during the last 18 million years of the Cretaceous subdivided into the Campanian, 84 to 72 million years ago (mya), and the Maastrichtian, 72 to 66 mya.
More than 8,000 dinosaur fossils have been found from this time in North America. It appears that the number of dinosaur species peaked about 75 mya, before declining.
Dean’s team analysed the fossils of 4 dinosaur groups, or clades: Ankylosauridae (armoured herbivores like the club-tailed Ankylosaurus), Ceratopsidae (horned plant eaters like Triceratops), Hadrosauridae (duck-billed herbivores such as the Edmontosaurus), and Tyrannosauridae (carnivores such as Tyrannosaurus rex).
Ceratopsians were the most likely of the 4 clades to be found later in the Cretaceous. These horned dinosaurs favoured green plains which are the main habitat preserved in late Cretaceous rocks.
“The late Cretaceous of North America was home to a whole bunch of different families of dinosaurs,” Dean told Cosmos in an email. Some other dinosaur groups not included in the study are small meat-eaters and long-necked sauropods.
Dean notes that the Campanian and Maastrichtian were different to earlier periods. Sauropods were less common than in the Jurassic period (201 to 145 mya).
“Also, the stegosaurs had died out by this point and were already fossilised. So, all these dinosaurs would have been walking around on top of stegosaur fossils!”
Absence of evidence is not evidence of absence
Dean’s team tested the fossil record in the spirit of the quote attributed to American astronomer Carl Sagan: “Absence of evidence is not evidence of absence.”
They modelled the habitat and ecosystem of the 4 clades to map out how likely a species was to inhabit particular areas.
The model suggests the land occupied by these dinosaurs remained constant over the last 18 million years of the Cretaceous. This means their habitats were stable and their risk of extinction (barring a Mt Everest-sized rock hurtling at them from space) was low.
They also assessed how likely it is to find fossils from this time period. This is affected by the amount of exposed rock and factors like accessibility, for example if land is covered by dense vegetation.
“If we take the fossil record at face value, we might conclude dinosaurs were already experiencing a decline before their final extinction,” says co-author Alessandro Chiarenza, also from UCL. “In this study, we show that this apparent decline is more likely a result of a reduced sampling window.”
Chiarenza says geological changes in the rock layers from the end of the Cretaceous haven’t been as well preserved. These changes could include “tectonics, mountain uplift and sea-level retreat”.
“Dinosaurs were probably not inevitably doomed to extinction at the end of the Mesozoic,” Chiarenza adds. “If it weren’t for that asteroid, they might still share this planet with mammals, lizards, and their surviving descendants: birds.”
Were other dinosaurs so prosperous?
“North America contains 50% of the total global record of dinosaurs during the last 18 million years of their existence,” Dean explains.
He says the UCL study could not be replicated outside North America yet.
“Unfortunately, the fossil record of other parts of the world at this time is not quite good enough at the moment to apply this method elsewhere. But new dinosaurs are being found all the time, and other researchers are doing excellent work in expanding our knowledge in places like Mongolia and China, so hopefully there’ll be an opportunity to do so in the future.”
Australia is one of those parts of the world where dinosaur remains from just before the end of the Cretaceous is very limited.
“We do have some end-Cretaceous fossils in Australia,” Melbourne Museum palaeontology PhD candidate Jake Kotevski says. “There’s a place called the Miria Marl in Western Australia that has some Maastrichtian fossils. But they’re fragments of fragments, so they’re not very informative.”
Kotevksi says youngest dinosaur fossils which help piece together an ancient Australian ecosystem come from the Winton Formation in Queensland where there are lots of sauropod fossils, as well as remains of ankylosaurs and smaller herbivores.
“North America is just somewhere super rare in that they have stuff from right at the end of the Cretaceous and it’s pretty well studied. The most famous dinosaurs come from there right at the end of the Cretaceous.
“It’s just a time period that doesn’t play well around the rest of the world. The fossil record and the geologic record, unfortunately, are a mix between chance, tectonics and time.”
An animal needs to have died under the right conditions to be fossilised. It needs to have been quickly covered in sediment. And that sediment needs to be relatively undisturbed. That’s why ancient lake and riverbeds are a good source of fossils.
He says there are places in Australia where such end-Cretaceous fossils may be found.
“There’s some Cretaceous rock in Victoria that outcrops. We can see it at the coastlines. It’s extensive, but it’s kilometres underground so you just can’t access it unless someone wanted to pump millions of dollars into digging material and getting down there,” Kotevski laughs.
He explains that other parts of the world that once belonged to the supercontinent Gondwana, along with Australia, have similar issues.
“New Zealand doesn’t really have a great dinosaur record. … Antarctica is sort of like New Zealand and us. It’s not much beyond evidence that there were dinosaurs here at the end of the Cretaceous.
“South America is better. We’re seeing diverse dinosaurs towards the end of the Cretaceous, particularly sauropods and some theropod groups.”
Kotevski is hesitant to say for sure that dinosaurs were or weren’t on the decline before the asteroid impact.
“It may well be that this is an accurate representation of dinosaur diversity, and they were on a decline. It may be that, as in this paper, there is no evidence that there was a decline because it’s a restricted environment that’s preserving them.”
The only way to get closer to the answer is to find more fossils. “There’s a saying that a lot of people use from Tom Rich, curator at Museums Victoria, and that’s the best fossil is the one you have,” Kotevski says.
UCL’s Chris Dean says similar gaps in the fossil record may account for difficulties in painting more complete pictures of the ancient world.
“We know that these biases in the fossil record are commonplace and are found all throughout Earth’s history. So it’s likely that there are other times and places where we have similar issues influencing our understanding of past life.”
