Arctic ice and the ecological rise of the dinosaurs
Paul Olsen, Jingeng Sha, Yanan Fang, Clara Chang, Jessica H. Whiteside, Sean Kinney, Hans-Dieter Sues, Dennis Kent, Morgan Schaller, Vivi Vajda
Summarized by Blair Stuhlmuller
What data were used? Researchers used three main sources of data. First, they looked at ancient lake sediments preserved in sedimentary rocks in the Junggar Basin, China. They then analyzed fossilized dinosaur footprints and other signs of “dinoturbation,” or the reworking or movement of soils and sediments by dinosaurs, in sedimentary rocks across the northern latitudes of China. The final set of data used was the phylogenetic tree of life for extinct and living dinosaurs, reptiles and mammals. A phylogenetic tree is a diagram showing lines of evolutionary descent of different organisms from a common ancestor. They used a preexisting phylogenetic tree but mapped preserved evidence of feather-like features and other key traits onto the extinct and living branches of organisms. This was done in order to make inferences about the presence of feathers and similar traits in extinct organisms where no fossil evidence exists yet to prove the presence of these features.
Methods: The researchers analyzed the grain size of the sedimentary rocks recovered from the Junggar Basin in China. These lake sediments were deposited millions of years ago in the Late Triassic (~210 million years ago) and Early Jurassic (~200 million years ago) and thus can reveal much about the climatic conditions during the End Triassic Extinction. The location of these sediments, the Junggar Basin, is also of particular importance. Using already established continental reconstructions for the Mesozoic (in other words where Pangea, our most recent supercontinent, was located), researchers determined that the Junggar Basin, currently located in the high latitudes of China (around 43°N latitude), would have been north of the Arctic Circle at about 71°N paleolatitude during the Triassic.
Lastly, the researchers used a generalized phylogenetic bracket analysis in order to infer certain traits (in this case the presence of some sort of feather-like feature or ‘protofeathers’) for which there is no current physical fossil evidence. This analysis revealed that feathers would be a primitive feature shared by many groups of dinosaurs.
Results: Grain size analysis revealed that most of these lake sediments were comprised of fine grained (~0.1 to 63 μm) mudstones with some larger grain exceptions. These smaller amounts of larger grains (small rock pieces upto 15mm in size) are indicative of ice-rafted debris. Ice acts as a raft that can pick up sediment and larger debris that comes in contact with it. This sediment is later deposited in the middle of a body of water like an ocean or lake. Thus ice-rafted debris (IRD) is any sediment that has been transported by floating ice. The origin of this particular ice-rafted debris is interpreted as seasonal ice coverage along the coastlines of ancient lakes. As the ice formed, it would grab larger grains and debris and then break off and drift out over the lake, slowly melting as the seasons changed. As the ice melts, it deposits the larger debris among the fine silts that typically accumulate at the bottom of lakes. This contradicts the long upheld mental image of dinosaurs stomping through a tropical warm climate throughout the Triassic and really the whole Mesozoic Era. The Late Triassic was one of the few times in Earth’s history that there is no evidence of ice sheets at the poles. However, these researchers claim that despite the high levels of carbon dioxide (CO2) in the atmosphere and the resulting greenhouse conditions during that time, there were freezing seasonal temperatures at high latitudes as supported by the ice-rafted debris they found.
Large plant eating dinosaurs during the Triassic were more commonly found in the forested higher latitudes as supported by the type of dinosaur footprints and ‘dinoturbation’ found in outcrops in modern day China. While actual fossil evidence of proto-feathers has not been found on fossils of these large herbivorous dinosaurs, the phylogenetic bracket analysis posits that they were in fact insulated by some sort of feather structure and thus were well suited to the seasonal winters. This enabled these animals to take advantage of the more abundant and stable plant life of the higher latitudes and potentially survive one of the worst mass extinctions in Earth’s history.
During the End Triassic Extinction, incredibly large volcanic eruptions, called the Central Atlantic Magmatic Province or CAMP, were going off. These eruptions would have contributed to global warming long term but on shorter decadal timescales, they would have caused volcanic winters. As the eruptions periodically belched out sulfur aerosols, light would have been blocked and the atmosphere would have cooled upwards of 10 ℃. Dinosaurs previously adapted (feathered and insulated) to the seasonal winters of the high latitudes survived and even spread out toward the now cooler tropics.
Why is this study important? This study contradicts the public’s perception of dinosaurs only thriving in a tropical climate and helps provide possibly the first empirical evidence for freezing temperatures and winter conditions in the Triassic rock record. It also provides a plausible explanation for why some dinosaurs went extinct at the end of the Triassic while others did not. Feathers were the key for survival in the volcanic winters that plagued the End Triassic Extinction. They offered life saving insulation that allowed some dinosaurs to survive the extinction and then reign supreme for the rest of the Mesozoic. That is, until the meteorite wiped out all non-avian dinosaurs 135 million years later.
The big picture: The distribution and type of life currently on our planet is in part due to what was able to survive the Triassic Extinction. Birds are the most biodiverse group of vertebrates (besides fish) and have over two times the number of species than mammals. Thus, feathers emerged as a life saving feature in the Triassic and they continue to reign supreme in modern times.
Citation: Olsen, P., Sha, J., Fang, Y., Chang, C., Whiteside, J. H., Kinney, S., Sues, H.-D., Kent, D., Schaller, M., & Vajda, V. (2022). Arctic ice and the ecological rise of the dinosaurs. Science Advances, 8(26). https://doi.org/10.1126/sciadv.abo6342