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

Microbiota mediated plasticity promotes thermal adaptation in the sea anemone Nematostella vectensis

Laura Baldassarre, Hua Ying, Adam M. Reitzel, Sören Franzenburg, Sebastian Fraune

Summarized by Blair Stuhlmuller

What data were used? Researchers used cloned Nematostella vectensis, a sea anemone found in estuaries and brackish water environments of the US and UK. N. vectensis hosts many helpful small friends, or symbiotic microbiota. In other words, microscopic organisms that live on the host anemone and help it deal with environmental stressors like temperature changes. These symbionts can be passed onto the offspring from the parent anemone or be acquired from the environment during development. The symbiote assemblage can also change during an anemone’s lifetime in response to changing environmental conditions. The researchers looked at the composition of the microbial communities, the genetics of the host anemone and mortality rates at different temperatures.

Methods: First, in order to control for genetic diversity between individuals, the researchers created clones from a single female polyp (anemone). These individuals were divided into different test groups based on temperature–low (15℃) temperature, medium (20℃)  temperature and high (25℃) temperature–that were studied over the course of three years. Each test group had 5 cultures of 50 cloned anemones.

Results:  After 40 weeks and after 132 weeks, the polyps were exposed to high heat stress (6 hours at 40 ℃) and mortality was measured. In both tests, all of the polyps in the low temperature group died. The high temperature group had the highest survival rate after 132 weeks. Polyps in the high temperature group experienced a lower mortality rate overall, but were also 3 times smaller, and asexually reproduced 7 times more rapidly than those in the low temperature group. These results show that long-term temperature differences have a great impact on heat tolerance, organism size, and reproduction rates.

Next, changes in the microbial symbiont communities were measured through 16S rRNA sequencing (or the process of reading the small section of ribosomal RNA molecules that is in charge of turning the genetic code into actual functioning cell parts) at the 40, 84 and 132 week intervals. The results showed that both the temperature and exposure duration to said temperature had a significant effect on the microbial community composition. Three distinct microbial communities were found for each temperature test group and these communities stabilized within the first two years. 

 

Third, all the active genes (or genes that are making mRNA) were analyzed in order to see if any changes occurred. One polyp from each culture was selected. The polyp’s mRNA was extracted and sequenced or read. Gene expression, or what genes are actively determining an organism’s features and functions, can be influenced by outside factors and can cause changes to an organism’s phenotypes, or physical characteristics, within its lifetime. While the actual DNA sequence is not changed, certain genes can be turned on or off that can then help or hurt the organism. In this study, polyps in the high temperature group experienced a significantly increased expression of genes involved with immunity, metabolism, outer skin cell production and other positive changes. 

Lastly, researchers wanted to determine if the microbial community and thus changes in gene expression were transferable and could increase the heat tolerance of new individuals and future generations of anemones. Thus they transplanted the temperature adapted microbial communities/symbionts to new, non temperature adapted polyps which were cloned from the same female as the experiment population. Then the heat tolerance of the new polyps were tested. Survival rates of the polyps with transplanted microbial communities depended on the source of the transplanted microbial community. Polyps with microbes from the high temperature group had an 80% survival rate, a significantly higher rate compared to the 33% of the polyps with the low temperature microbes. This shows that microbial transplants could prove to be a quick and effective way to help certain organisms cope with environmental changes. 

The researchers also tested if both the gene expression and microbial communities could be naturally transferred from one generation to the next. rRNA sequencing revealed that large parts of the parent microbial community were successfully transplanted to the offspring. The offspring were then subjected to high heat stress. The offspring from the high temperature group showed a significantly higher survival rate compared to the offspring from both the low and medium temperature groups.

Why is this study important? Members of the Cnidarian phylum like corals and sea anemones are under threat due to rapid climate change. Warming water temperatures are causing coral bleaching and other harmful effects. Since coral and many anemones are mostly sessile, or non moving, when mature, they only have two options–adapt or die. And with climate changing so quickly in recent decades, one might expect extinction to be the more likely option for many species. Adaptation is typically limited by random mutations and natural selection, neither of which happens overnight. However, this study shows how adaptation can happen within just one generation. 

Sessile animals that host a range of symbiotic microbiota exposed to high water temperatures can adapt and become more heat stress resistant. Microbes tend to have much faster generation times and can thus evolve more quickly than their hosts. These microbes can then influence the gene expression of their host by turning on or off certain genes further helping the host to survive and adapt during its lifetime. Most excitingly, these changes in gene expression and microbial communities can be passed to the next generation. This study also helps pave the way towards assisted evolution and potentially huge successes in coral conservation. Heat tolerant microbial communities could potentially be selected for in the lab and then transplanted to wild populations. This would allow scientists and conservation groups to improve the fitness of wild populations quickly and effectively help counter the effects of climate change. 

The big picture: Climate change is a looming threat especially for those living in the oceans. As ocean temperatures rise, many marine species will likely migrate towards the poles in order to remain in their desired temperature ranges. However, sessile or mostly non-moving marine organisms like sponges, coral and sea anemones will have a harder time doing that as many are only mobile during their planktonic larvae stages. This study gives a glimpse of hope that these animals will be better able to adapt and survive than previously expected. This study specifically shows that animals exposed to high temperatures, like N. vectensis, can quickly become more heat stress resistant as the symbiotic microbiota shift and adapt. Most importantly these high heat tolerances can be passed to other organisms and future generations. These lab results are mirrored by long term observational studies that show wild populations becoming less heat sensitive than past generations. Overall, this has huge positive conservation implications for coral reefs and other sessile marine communities as climate rapidly changes.

Citation: Baldassarre, L., Ying, H., Reitzel, A.M. et al. Microbiota mediated plasticity promotes thermal adaptation in the sea anemone Nematostella vectensis. Nature Communications 13, 3804 (2022). https://doi.org/10.1038/s41467-022-31350-z