Birds set foot near South Pole in Early Cretaceous, Australian tracks show

Co-author 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 co-authors 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 co-authors 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.

Co-author 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.”

Story by Carol Clark.