Create a list of articles to read later. You will be able to access your list from any article in Discover.
You don't have any saved articles.
The Morrison Formation is one of the most well-studied sites for dinosaur fossils in the world. © Zack Frank/Shutterstock.
Dinosaur populations appeared to be thriving at the end of the Jurassic before disappearing by the early Cretaceous.
Scientists are trying to unravel the mysteries of this extinction, but evidence suggests it may have been an event that was local to North America.
During the Late Jurassic, iconic dinosaurs like Stegosaurus and Diplodocus roamed vast areas of the western United States. But by the beginning of the following Cretaceous Period, many of these dinosaurs had vanished from the landscape and were replaced by other species.
What caused this species changeover and the scale of this event is still debated among scientists. However, a recent study sheds light on how populations changed in North America leading up to the end of the Jurassic.
Dr Susannah Maidment, a palaeontologist at the Museum was the author of the research. She looked at remains found in the Morrison Formation, a renowned rock formation in North America famous for their richness in dinosaur fossils.
The study, published in the Journal of Vertebrate Palaeontology, showed that many species of animals appeared to be thriving in the Morrison Formation at the end of the Jurassic, suggesting that the extinction was a relatively sudden event rather than a gradual dwindling of species over a more extended period.
“To understand how biodiversity has changed over time, we need to focus on regions where we can divide time into fine slices,” says Susie.
“The Morrison Formation is a great place to do this because it has been intensely studied for almost 150 years. It is probably the best known Late Jurassic ecosystem on earth, so it is a good place to do these kinds of studies on biodiversity.”
The Morrison Formation covers an are of 1.2 million square kilometres across North America © Chris Curtis/Shutterstock.
The Morrison Formation was deposited between 154 and 145 million years ago during the last nine million years of the Jurassic Period. At this time, the landscape was dominated by dinosaurs like stegosaurs, sauropods, megalosauroids and allosauroids.
By the following Cretaceous Period, the stegosaurs had become extinct and sauropods were a much more minor part of the fauna. Instead, the herbivorous niche was dominated by iguanodontians and ankylosaurs.
But the Cretaceous rocks in this part of North America are between five and 20 million years younger than the Morrison Formation, which means there is a gap in the fossil record. What happened to the Morrison dinosaurs during this period is debated among scientists.
Some dinosaurs that went extinct in North America were still seen in other parts of the world into the Cretaceous period, which suggests that whatever happened to the Morrison dinosaurs was likely caused by a local event rather than a global phenomenon.
“Why this change in fauna happened isn’t clear,” says Susie. “Some think it is just a faunal turnover where the climate might have changed, and it just didn’t favour those dinosaurs anymore. In Europe and Asia, in rocks of the same age, stegosaurs seemed to live on until the early Cretaceous, which could point to this extinction being a restricted North American event.”
“We know that a mountain-building event at the end of the Jurassic and into the Early Cretaceous caused uplift of the Morrison basin, which would have changed the climate and environments. I think that is likely what caused the extinction of the Morrison dinosaurs. But to say more about this, we need to understand more about the climate during this time.”
Stegosaurus went extinct in the Morrison Formation at the end of the Jurassic, but fossil evidence suggests they lived on in other parts of the world. © The Trustees of The Natural History Museum, London
The Morrison Formation covers an area of 1.2 million square kilometres and extends across the United States from Montana in the north to New Mexico in the south.
During the Late Jurassic, the area was predominantly a floodplain with a mountain range building to the west. But this environment changed considerably during the formation’s nine-million-year history.
Initially, extensive river systems flowed from the mountains into the basin, before becoming a vast lake surrounded by marshy environments.
Until now, it was assumed that many of these Late Jurassic dinosaurs would have ranged across this entire area. However, this latest study takes a more in-depth look at how different dinosaurs may have been dispersed across the landscape.
Although some animals, like Stegosaurus and Diplodocus, do appear to have occurred across the whole basin, others seemed restricted to particular areas.
For example, fossils of the predatory dinosaur Allosaurus show that different species occupied different parts of the basin. Allosaurus fragilis is mainly found in the southern part of the basin, while Allosaurus jimmadseni is more to the north, with a slight overlap in the middle.
Similar results were found for two species of Camarasaurus. Remains of Camarasaurus grandis have been found across southern Colorado and New Mexico, but Camarasaurus lentus appears to be found in northern Utah with no apparent overlap in their ranges.
“It is really not clear why the species were divided in the way,” Susie says. “There is no evidence of any geographic barriers within the Morrison formation.”
“There was almost certainly climatic variation from the north to south, similar to today, although with much less of a temperature gradient. So, it is possibly related to some climatic variation. But the problem is that we need super high-resolution climate models or very detailed climate proxies, and we don’t yet have that on this level of resolution.”
Once researchers are able to get this detailed information on the climate of the region during the Late Jurassic, they will hopefully be able to answer more questions about how dinosaur populations spread out and why there was such dramatic turnover in diversity in the Morrison Formation.