Dinosaurs' apparent decline prior to asteroid may be due to poor fossil record

The study, published in Current Biology, analysed the fossil record of North America in the 18 million years up to the asteroid impact at the end of the Cretaceous period (between 66 and 84 million years ago).

Taken at face value, these fossils -- more than 8,000 of them -- suggest the number of dinosaur species peaked about 75 million years ago and then declined in the nine million years leading up to the asteroid impact.

But the research team found this trend was due to fossils from that time being less likely to be discovered, primarily because of fewer locations with exposed and accessible rock from the very latest Cretaceous.

Lead author Dr Chris Dean (UCL Earth Sciences) said: "It's been a subject of debate for more than 30 years -- were dinosaurs doomed and already on their way out before the asteroid hit?

"We analysed the fossil record and found that the quality of the record of four groups of dinosaur (clades) gets worse during the final 6 million years prior to the asteroid. The probability of finding dinosaur fossils decreases, while the likelihood of dinosaurs having lived in these areas at the time is stable. This shows we can't take the fossil record at face value.

"Half the fossils we have from this time were found in North America. Our findings hint that, in this region at least, dinosaurs may have been doing better than previously suggested in the lead-up to the asteroid impact, potentially with a higher diversity of species than we see in the raw rock record."

For the study, the research team looked at the dinosaur clades Ankylosauridae (armoured herbivores such as club-tailed Ankylosaurus), Ceratopsidae (large three-horned herbivores including Triceratops), Hadrosauridae (duck-billed herbivores such as the Edmontosaurus), and Tyrannosauridae (carnivores such as Tyrannosaurus Rex).

They adopted a technique, occupancy modelling, previously used in ecology and biodiversity studies to estimate how likely a species is to inhabit a particular area.

They divided North America into a grid and, based on the geology, geography and climate of the time, estimated how many of these grid cells the four dinosaur types likely occupied at four different times during the last 18 million years of the Cretaceous.

They found that, during this time, the proportion of land the four dinosaur clades likely occupied remained constant overall, suggesting their potential habitat area remained stable and risk of extinction stayed low.

At the same time, they estimated the likelihood of the four dinosaur types being detected in each area, based on factors such as how much land is accessible to researchers (i.e., if it is covered in vegetation), how much relevant rock is exposed, and how many times researchers had attempted to find fossils from that area.

The team found that the likelihood of detection declined over the four time periods, with the most influential factor being how much relevant rock was exposed and accessible.

The researchers also found that, in contrast to the other three clades, Ceratopsian dinosaurs (such as Triceratops) were more likely to be detected later on in this period, as well as occupying more areas.

They suggested this was due to Ceratopsians favouring green plains away from rivers at a time when this kind of habitat became the main type of environment being preserved. This was due to the retreat of a large inland sea that split the continent in two and river systems feeding this sea drying up.

Co-author Dr Alessandro Chiarenza (UCL Earth Sciences), who has previously published on end-Cretaceous dinosaur diversity and extinction, said: "If we take the fossil record at face value, we might conclude dinosaurs were already experiencing a decline before their final extinction.

"In this study, we show that this apparent decline is more likely a result of a reduced sampling window, caused by geological changes in these terminal Mesozoic fossil-bearing layers -- driven by processes such as tectonics, mountain uplift, and sea-level retreat -- rather than genuine fluctuations in biodiversity.

"Dinosaurs were probably not inevitably doomed to extinction at the end of the Mesozoic. If it weren't for that asteroid, they might still share this planet with mammals, lizards, and their surviving descendants: birds."