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Around 66 million years ago, an asteroid wiped out 75% of the species on Earth. This event caused the extinction of all dinosaurs except birds, referred to as non-avian dinosaurs. However, an open question in paleontology is whether the dinosaurs were already declining before the asteroid hit. Many scientists think that around 5 to 10 million years before the extinction event, dinosaurs became less biologically diverse across America, making them more prone to extinction. 

To investigate this hypothesis, scientists recently revisited fossils in a rock layer from the end of the dinosaur era that previous researchers had incorrectly dated. The bulk of the fossils from this period had previously been collected from a different section of rocks called the Hell Creek Formation in Montana. These researchers studied a rock layer in New Mexico that is exposed to the surface, known as the Naashoibito Member. It’s approximately 30 meters (about 100 feet) thick and formed from ancient river sediments. 

To determine when the Naashoibito Member formed, the scientists measured the decay of radioactive elements within its mineral grains over time, a technique called radiometric dating. The scientist reasoned that these mineral grains must have crystallized before being transported and incorporated into the Naashoibito Member, so their youngest ages should provide a maximum depositional age. 

They determined the ages of 1,046 crystals of the mineral feldspar from 2 dinosaur-bearing rock layers in the Naashoibito Member. Based on these ages, the researchers found that a sandstone within 5 meters (about 16 feet) of the lowest boundary of the Naashoibito Member must be younger than 66.9 million years old. They found that the other layer, which is around 3.5 meters (11 feet) from the base of the lowest boundary, must be younger than 66.4 million years old. These age ranges indicated that the sandstone contains dinosaur fossils formed 400,000 years before their extinction.

Next, the researchers measured the direction of magnetic minerals inside the rocks, using a practice called magnetostratigraphy. The direction of these minerals indicates where Earth’s magnetic field was pointing at that time. Scientists have shown that the Earth’s magnetic field has flipped multiple times over Earth’s history, so they can use these magnetic signals to create a timescale for when the sediments formed. 

The researchers wanted to determine whether the magnetic minerals were lined up in a direction like today, called normal polarity, or the opposite, called reverse polarity. The researchers found one rock layer with normal polarity at the base of the Naashoibito Member, and one with reverse polarity in the upper part of the Naashoibito Member. These magnetic field directions lined up with 2 known polarity intervals over a timeline that matched the feldspar dates. Together, these data demonstrated that the Naashoibito Member formed at the end of the dinosaur era, around the same time as the Hell Creek Formation.

Next, the researchers wanted to test whether non-avian dinosaurs maintained their diversity across different regions of Western North America during this time period. They compared fossil patterns from locations across North America using statistical grouping methods that determine how similar or different their environments were. They found that temperature, rather than geographical location alone, was the best predictor of diversity at the extinction event. 

The team concluded that a diverse population of Naashoibito dinosaurs lived at around the same time as the Hell Creek dinosaurs. Earlier scientists surmised that dinosaurs were already on the decline because they studied the colder northern regions, where diversity was lower. However, these results suggest that warmer conditions in the south still supported diverse dinosaur populations. Together, this evidence suggests that dinosaurs were still diverse and thriving 66 million years ago, and that their extinction was indeed a result of the asteroid impact.

 

The post Were dinosaurs doomed before the asteroid hit? appeared first on Sciworthy.

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As our world is rapidly reshaped by climate change, glaciers have retreated and revealed new swaths of bare land. Throughout the coming decades to centuries, this rocky ground will become dappled with lichens and scrub, eventually giving rise to newborn forests in a process known as ecological succession. 

Ecologists have mapped out the stages of ecological succession for plant communities, observing which species are the first to colonize land and how these pioneer species promote secondary growth. Yet before any plants sink their roots into the soil, it is populated by thriving communities of single-celled microbes that prepare it for further habitation. Scientists study how these microbial communities arise to understand how healthy ecosystems develop.

Newly exposed land is nutrient-poor and subject to large temperature fluctuations, so the first species to settle there must overcome these challenges. Pioneer plant species are often habitat generalists, meaning they can thrive under a variety of environmental conditions. However, all plants convert water and sunlight into carbon and energy. Microbes can use many different energy sources, and often contain genes for multiple metabolisms in their genomes. Therefore, researchers wondered whether pioneer microbes could also be defined by their metabolic flexibility. 

A team of scientists based at Monash University in Australia tested this hypothesis by studying land left in the wake of 2 retreating glaciers: one on an island off the coast of Antarctica and one in the Swiss Alps. The researchers sampled soils along a path from the tip of the glacier outward, which had been exposed to the air for different amounts of time. By comparing the microbial communities within these soils, the researchers could track the different stages of ecological succession after glacial retreat.

The researchers extracted DNA from the soils and analyzed it using 2 sequencing methods. First, they sequenced a gene called 16S rRNA, which is like a microbial fingerprint for the different species present. They used this method to understand the diversity of these communities and track their species overlap, helping them to identify habitat generalists that thrived under different soil conditions. 

To understand the metabolic flexibility of these microbes, the researchers used a second method, known as metagenomics, which sequences all of the DNA in a sample rather than just a single gene. They used these data to reconstruct the entire genomes of microbes present in the soils, providing information on what kinds of metabolic activities they were capable of. The team also measured chemicals like ammonium and sulfide in the soils, as well as atmospheric gases like methane and carbon monoxide, to assess how the microbes used these compounds for growth.

The researchers found microbes inhabiting even the youngest soils, demonstrating the speed at which life takes hold in new environments. The abundance of microbes increased by approximately 8-fold in older soils, alongside a similar rise in species diversity, indicating that complex communities persisted over time. They also found the metabolic capabilities of microbes inhabiting Antarctic and Swiss glacial soils to be remarkably similar, suggesting common selective pressures drove the establishment of these new ecosystems.

They were surprised to discover that the most abundant microbes in younger soils were habitat specialists that were rare in older soils. These pioneer microbes, while still metabolically flexible, had adapted to utilize meager energy sources like atmospheric trace gases, including hydrogen, methane, and carbon monoxide. Many of these microbes could also derive energy from chemicals that dissolved out of rocks, like inorganic sulfur compounds. The researchers proposed that pioneer microbes could quickly take advantage of newly opened ecological niches, like soils exposed during a glacier’s retreat, because of their efficiency at using scant resources. 

They found that habitat generalists, on the other hand, tended to dominate older soils.  This result suggested that the habitat specialists were eventually outcompeted by the slow and steady growth of the habitat generalists in a real-life turtle-and-hare race.

The team concluded that using multiple growth strategies helps microbes adapt to new environments. However, they acknowledged that ecological succession could occur differently in other landscapes, such as in the aftermath of volcanic eruptions, meteorite impacts, or forest fires. They suggested that future researchers work to uncover how microbial communities lay the groundwork across these diverse ecosystems. 

The post When glaciers retreat, microbes take over appeared first on Sciworthy.

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