Fish death, mammal extinction and tiny dino footprints

 

Paleontologists convened September 23–26 in Bristol, England, for the annual meeting of the Society of Vertebrate Paleontology. Science News writer Sid Perkins reports from the meeting on the latest findings on fish fossils in Wyoming, the loss of Australia’s megafauna and the smallest dinosaur tracks.

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Paleo-CSI: The real fish killers at Wyoming’s Fossil Lake

BRISTOL, ENGLAND—Researchers have narrowed the list of suspects in the massive fish kills that repeatedly swept an ancient North American lake about 50 million years ago. More and more, it looks like toxic algae may have been responsible, Jo Hellawell, a vertebrate paleontologist at Trinity College at the University of Dublin, reported September 23.

For more than a century, the Green River formation in southwestern Wyoming has been famed for its preserved fossils, most of them herring-sized fish. The thin-layered strata there were laid down as sediments in ancient lakes, and some layers in an aptly named area of the formation called Fossil Lake contain as many as 500 fish fossils per square meter (46 per square foot).

Scientists have identified more than 20 possible causes for modern-day fish kills worldwide, including disease, pollutants, or changes in climate or other environmental conditions. In an attempt to identify a culprit for Fossil Lake’s die-off, Hellawell and organic chemists at the University of Bristol in England analyzed samples of rocks from a six-meter-thick section of the formation.

The team found that some thin layers of volcanic ash coincided with fish kills, but most did not. Isotopic analyses of the rocks suggest that climate was stable throughout the period when fossils were accumulating. And although the remains of microbial mats in some layers hint that water at the lake bottom may have been low in oxygen or even toxic, that scenario doesn’t explain the fossilized remains of birds also found in the layers, Hellawell suggested.

Chemical analyses did, however, reveal the presence of 4-methylsteranes, biomarkers produced predominantly by a type of algae called dinoflagellates, Hellawell reported. Under certain environmental conditions, some modern-day species of dinoflagellates can proliferate and release massive amounts of toxic substances near the lake surface, she noted.

In future work, the researchers will see whether concentrations of biomarkers in layers throughout the formation correlate with the numbers of fossils in those layers. —Sid Perkins

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Pitter-patter of tiny dinosaur feet

BRISTOL, ENGLAND—Paleontologists have discovered the world’s tiniest dinosaur tracks — impressions so small that each would fit on a penny.

The wee, three-toed footprints appear in rocks between 100 million and 120 million years old along what is now the southern coast of South Korea, Jong-Deock Lim of the National Research Institute of Cultural Heritage in Daejeon, South Korea, reported September 25. Made in material laid down as fine-grained sediments along an ancient riverbank, the tracks include claw marks and impressions of the pads of a creature’s feet. Scientists haven’t yet found the remains of a dinosaur small enough to make these tracks, but characteristics of the tracks indicate that the creature was a theropod — a bipedal, typically meat-eating dinosaur.

Lim and his colleagues found about a dozen of the footprints, which range between 1.27 and 1.51 centimeters in length. Because paleontologists have previously found similar prints in this region that measure more than 6 centimeters long, the team suggests that the newly described tracks were made by a hatchling.

Some of the prints formed a set called a trackway, which allowed the researchers to measure the creature’s stride length. Using that data, they estimate that the dinosaur stood about 4 centimeters tall at the hip and about 10 centimeters tall overall. —Sid Perkins

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Changing climate factored into Australian extinctions 

BRISTOL, ENGLAND—Studies that have mostly blamed the arrival of humans for die-offs among Australia’s large mammals 50,000 years ago missed the role played by a changing climate, new research suggests.

Most assessments of Australian extinctions have used evidence gathered at sites that typically include fossils from only one narrow interval of time, Gilbert Price, a vertebrate paleontologist at the University of Queensland in St. Lucia, Australia, reported September 23. But he and his colleagues have analyzed fossils of creatures both large and small from Darling Downs, a site in eastern Australia with a fossil record that extends from about 120,000 to about 55,000 years ago. In all, the team has tallied about 70 species that lived nearby at some point during that interval.

The data don’t support a previously proposed human-only cause for Australian megafaunal extinctions, Price noted. From strata deposited about 120,000 years ago, the researchers recovered the remains of 15 species of large mammals. About 90,000 years ago, only eight species of large mammals lived there. By 55,000 years ago, still several millennia before humans arrived in the area, only four large mammal species remained.

That long-term drop in diversity also appeared among small creatures, and the types of species that disappeared suggest climate change played a role, Price said. Sediments deposited from 120,000 to 90,000 years ago contain the fossils of rodents, frogs and land snails as well as large mammals, suggesting that the surrounding area was a patchwork of woodlands, vine-choked thickets and open grasslands. By 55,000 years ago, however, many of the wet-loving and forest-adapted species had largely disappeared, signaling a transition to drier, more open conditions.

The new findings don’t pin the blame for Australia’s final spate of mammal extinctions on either climate change or human presence, Price cautioned. The long-term trend in species diversity at Darling Downs does hint, however, that climate change caused some species to die out. And the changes may have reduced the populations of other species enough that human arrival easily tipped them over the edge to extinction. —Sid Perkins