The Dinosaur Decline Started Long Before the Asteroid Hit
by Moises A. Hernandez, age 16
About 66 million years ago, an asteroid struck the Yucatán Peninsula and caused apocalyptic ecological havoc, including tsunamis, an overheated atmosphere, darkened skies, and a cold wave. It is estimated that this event wiped away seventy-five percent of known life on Earth, including dinosaurs. But, is this the whole story?
Paleontologists are studying life of past geological periods as shown from fossil remains. This disaster, which marks the shift from the Cretaceous geological time period to the Paleogene geological time period, has been named by scientists as the Cretaceous–Paleogene extinction event (K/Pg). To have a better understanding of this event, researchers have begun to examine the patterns of life before and after the impact, as opposed to only at the moment.
“In order to get an idea of what happened in the wake of the asteroid impact, we need solid baseline data on what rates of background extinction were like before the K/Pg took place,” said Paul Barrett, a paleontologist at the Natural History Museum in London, England. Having this baseline data would help scientists determine whether the asteroid strike was the primary cause of the extinction of so much of life on Earth or was simply the coup de gre of destructive events already underway.
Researchers have determined that they must study not solely life after the moment of the asteroid impact, but also the patterns of life both before and after the event. The ecosystem that the dinosaurs lived in, the types and sizes of other animals living at the time, the ways in which habitat manifested, and a number of other factors that could have influenced this extinction are still unknown.
Dinosaurs were not the only living creatures around during the time and they existed in a constantly-changing world. Towards the end of the Cretaceous period, sea levels began to drop, the climate began to cool down, and the Deccan Traps--part of old-world India--began to bubble with significant volcanic activity.
Not much is known about the effect of the K/Pg event in different locations around the globe, but some of the best evidence is located in western North America, where a continuous sequence of sedimentary layers recording the end of the Cretaceous period through the beginning of the Paleogene period are found. The exposure of these rock layers during periods both before and after the asteroid strike has allowed Emily Bamforth, a paleontologist at the Royal Saskatchewan Museum in Regina, Saskatchewan, Canada, to investigate what was happening during the 300,000 year time period before the end of the Cretaceous period.
Bamforth said that looking at the geologic record of southwest Saskatchewan, Canada, when determining patterns of ancient biodiversity, the local conditions--like the commonality of forest fires and characteristics of a particular habitat--were as important as what was happening on a global scale. “I think this is an important message to keep in mind when thinking of causes of the extinction,” she said. “Each different ecosystem could have had its own smaller scale biodiversity drivers that were in operation before the extinction, which underlay the big, global factors.” Something that was good in one place for living creatures -- turtles, amphibians, and plants -- could have been bad in another location. “Ecosystems are complicated things, and I think that is worth keeping in mind when considering the cause and duration of the mass extinction,” Bamforth said.
Describing the ecological community in Saskatchewan during the time leading up to the extinction as a big game of Jenga, she said, “the tower remains standing, but factors like climate change are slowly pulling blocks out from it, weakening the system and making it vulnerable.” The constantly changing ecological stability made it so that any major event--such as an asteroid strike at the wrong place and wrong time--would be very destructive. The idea of changing ecosystems complicates the analysis of the K/Pg event.
It’s been difficult for scientists to figure out why some nonvolant creatures like dinosaurs died while others had the ability to pass through to the next geological time period. Most of the species that survived were small, semi-aquatic or made burrows, and were able to live off of a variety of foods, but there is some incongruity. There were some small terrestrial dinosaurs that had these advantages but still went extinct, while many reptiles, birds, and mammals died despite belonging to bigger groups that survived. In general, smaller dinosaurs and animals should have had a better chance to survive compared to larger animals, but that didn’t always happen.
“This is something I struggle to explain,” Barrett said.
Pat Holroyd, Senior Museum Scientist at the University of California Museum of Paleontology in Berkeley, California compared the investigations into species survival to investigations of airline accidents. “They go in and they gather all the data and they try to figure out, ‘Well, ok, why did the people in the tail section survive, and the people in the other parts of the plane didn’t make it?’” she said. Even though some of these tragedies may be singular events with their own causes, it’s still possible to look at multiple occurrences together to evaluate the impact of patterns and information on the possibilities of a singular event.
For the K/Pg event, patterns are still being found with Holroyd estimating for a lot of the relevant research on surviving species being published or uploaded to the Paleobiology Database in the last decade.
Turnover is the amount of time species spent on land and in associated freshwater habitats, and the new information has allowed Holroyd and her colleagues to study these patterns from long before and after the impact of the asteroid.
“People have been observing this pattern since at least the 50’s, and probably before,” Holroyd said. Some of the patterns found were familiar; fish, turtles, amphibians, and crocodilians all survived better, in general, than terrestrial organisms.
Holroyd found that the difference between the survivors and the extinction of the K/Pg event followed a pattern that has been true for tens of millions of years before and after the event. Terrestrial organisms, mainly large ones, tend not to survive as long as those living in freshwater environments and often go extinct at a faster rate than those in aquatic environments, even without a large event to kill them. By contrast, species that lived in and around freshwater habitats seem to have survived longer, even without a catastrophe, and when the extinction event happened, these organisms had an advantage over their terrestrial relatives.
Holroyd noted Cretaceous turtles, for example, lost fifty percent of their diversity around the globe, emphasizing how important it is to understand local in addition to global patterns. Lineages, like that of Marsupial mammals, which as a group survived the mass extinction but had their diversity and abundance greatly reduced, can be considered ‘survivors’ but still suffered losses and may not have bounced back to their former glory.
The way local ecosystems were affected by these changes must also be understood. Holroyd pointed out the “three-horned face” Triceratops as an example; a dinosaur common across much of western North America at the end of the Cretaceous period and a big part of its ecosystem. There is no doubt that the extinction of Triceratops greatly impacted ecosystems after the K/Pg event as plants that may have depended on Triceratops to spread seeds would have suffered and other plants that were destroyed by the dinosaurs might have grown freely.
“The western interior of North America gives us our only detailed window on what happened to life on land during the K/Pg extinction, but it’s totally unclear if this was typical,” Barrett said. “We don’t know much about how the intensity of the extinction varied around the world,” especially in locations that were distant from the asteroid strike. “It seems unlikely that a one-size-fits-all model would be responsible for cutting down organisms as different from each other as Edmontosaurus on land and coil-shelled ammonites in the seas, among so many other species lost to the Cretaceous.”
Research today is conducted across Europe, South America, Asia and Australia, creating a nexus for a much searched global picture for the most famous extinction event in history.
As Bamforth puts it, “It’s like one gigantic jigsaw puzzle that we’ve started to turn up more of the pieces to.”
[Source: Smithsonian Magazine]