Ancient bird left tracks near South Pole during Cretaceous

The discovery of 27 avian footprints on the southern Australia coast—dating back to the Early Cretaceous when Australia was still connected to Antarctica—opens another window onto early avian evolution and possible migratory behavior.

PLOS ONE published the discovery of some of the oldest, positively identified bird tracks in the Southern Hemisphere, dated to between 120 million and 128 million years ago.

“Most of the bird tracks and body fossils dating as far back as the Early Cretaceous are from the Northern Hemisphere, particularly from Asia,” says Anthony Martin, first author of the study and a professor in Emory University’s department of environmental sciences. “Our discovery shows that there were many birds, and a variety of them, near the South Pole about 125 million years ago.”

Martin is a geologist and paleontologist focused primarily on ichnology—the study of traces of life such as tracks, burrows, nests, and tooth marks.

The 27 bird tracks vary in form and size and are among the largest known from the Early Cretaceous. They range from 7 to 14 centimeters (2.75 to 5.5 inches) wide, which is similar to tracks of modern-day shorebirds, such as small herons and oystercatchers.

The tracks were found in the Wonthaggi Formation south of Melbourne. The rocky coastal strata mark where the ancient supercontinent Gondwana began to break up around 100 million years ago when Australia separated from Antarctica.

The polar environment at that time was a rift valley with braided rivers. Although the mean annual air temperature was higher during the Cretaceous than today, during the polar winters the ecosystem experienced deep, freezing temperatures and months of darkness.

The Wonthaggi avian tracks occurred on multiple stratigraphic levels, indicating a recurrent presence of a variety of birds. It also suggests seasonal formation of the tracks during polar summers, perhaps on a migratory route.

“The birds would likely have been stepping on soft sand or mud,” Martin says. “Then the tracks may have been buried by a gentle river flow that deposited more sand or mud on top of them.”

The Wonthaggi Formation is famous for its variety of polar dinosaur bones, although bird-fossil finds are extremely rare. The Cretaceous strata of the formation has yielded only one tiny bird bone—a wishbone—and a few feathers.

“Birds have such thin and tiny bones,” Martin says. “Think of the likelihood of a sparrow being preserved in the geologic record as opposed to an elephant.”

Birds are also lightweight and don’t leave much of a foot impression, he adds.

Martin and colleagues discovered two 105-million-year-old bird tracks in Australia’s Eumeralla Formation in 2013, making them the oldest from Australia at the time.

Coauthor Melissa Lowery, a local volunteer fossil hunter for Monash University, first spotted some of the tracks in the current discovery in 2020. Dubbed “the doyenne of dinosaur discovery,” Lowery has found hundreds of bones and more than 100 dinosaur footprints.

“Melissa is incredibly skilled at finding fossil tracks,” Martin says. “Some of these tracks are subtle even for me, and I have lots of experience and training.”

Most of the tracks were only exposed at low tide and some of them were encrusted by marine life such as algae, barnacles, and mollusks.

Due to international travel restrictions in Australia during the COVID-19 pandemic, Martin had to wait until 2022 before he could travel to the site to lead the analyses of the tracks.

He was joined in the field by coauthors Patricia Vickers-Rich, professor of paleontology at Monash University, and Thomas Rich, curator of vertebrate paleontology at Museums Victoria Research Institute. The couple have led a major effort since the 1970s to uncover fossils in the Australian state of Victoria and to interpret the biota of Gondwana.

Also assisting in the field analyses were coauthors Mike Hall, a geologist at Monash University, and Peter Swinkels, a taxidermist at Museums Victoria Research Institute and an expert at preserving specimens through moldings and casts.

“At first I thought the tracks might have been made by young theropods,” Martin says of the footprints. “But I soon realized they were bird tracks.”

The fossil record indicates that birds evolved from theropods, a bipedal, carnivorous dinosaur clade. The theropod Tyrannosaursus rex, for instance, had a vestigial rear toe—evidence that T. rex shared a common ancestor with birds.

“In some dinosaur lineages, that rear toe got longer instead of shorter and made a great adaptation for perching up in trees,” Martin explains.

In 1998, scientists began uncovering fossils in China of small theropods sprouting hair-like filaments that appeared to be proto-feathers. Dubbed the Sinosauropteryx, or “Chinese dragon bird,” the small theropod lived in northeastern China during the Early Cretaceous.

The earliest-known fossil classified as a bird is Archaeopteryx lithographica. First found in 1861 in a Jurassic limestone from southern Germany, Archaeopteryx dates to about 150 million years ago and appears to be a transitional form between theropods and the birds we are familiar with today. Archaeopteryx had full-fledged feathers, a long bony tail, hands with fingers and a full set of teeth.

In 2021, paleontologists working in Brazil uncovered the fossil of a bird that lived in the Early Cretaceous about 115 million years ago. Dubbed Kaririavis mater, it combined some primitive and modern avian characteristics.

A nine-mile-wide asteroid slammed into the Earth 66 million years ago, striking the northern edge of the Yucatán peninsula at a site known as Chicxulub. The impact spewed massive amounts of debris into the air sparking a mass extinction.

“The only dinosaurs to survive the meteor impact were birds,” Martin says. “We don’t know why. My hypothesis is that there were some birds that nested underground and that behavior may have protected them.”

Many other mysteries remain about the evolution of birds and their behaviors.

The discovery of the Australian bird tracks raises questions about where the birds originated and whether the polar environment was part of a migratory route.

The thinness of the toes relative to the track lengths, the wide angles between the toes and the thin, sharp claws and rear toes on some of the tracks helped Martin to verify their avian identity.

Coauthor Claudia Serrano-Brañas, a paleontologist at the Benemérita Normal School of Coahuila and the National Museum of Natural History, Smithsonian Institution, verified similarities between the Australian bird footprints and ancient bird footprints from other parts of the world.

Swinkels created resin casts of the Australian tracks that brought into greater relief some of the nuances of the impressions. The casts provide a tool for further study. They also serve to preserve the finds. The silty, sandstone beds containing the footprints are rapidly eroding under the coastal tides and waves.

“Seven of the tracks that Melissa found in 2020 are no longer there,” Martin says. “Some fossils, including tracks, are exposed only for a brief amount of time after being buried for millions of years. We humans have to rush in and document them before they disappear again.”

Source: Emory University

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