Three-toed fossil footprints dating back more than 210 million years were pressed into soft mud by bipedal reptiles with feet like a bird's, a new analysis of the tracks has revealed.
The footprints, found at several locations in southern Africa, have recently been identified as the oldest bird-like tracks ever found, predating the earliest known skeletal fossils of birds by about 60 million years.
"Given their age, they were probably created by dinosaurs," says Dr. Miengah Abrahams, lecturer in geological sciences at the University of Cape Town in South Africa. Abrahams is lead author of the new study describing the tracks, published Tuesday in the journal PLOS One.
Theropods, including Tyrannosaurus rex, were a diverse group of bipedal carnivores with three-toed feet. But among these newly examined dinosaur tracks, there were some that differed from typical theropod prints. The outliers had a shorter extension of the central finger, a much wider spread and "significantly narrower toes," making them look more like bird footprints, Abrahams told CNN in an email.
However, because the animals that made the tracks are unknown, their relationship to birds is unclear. The prints could represent a missing clue about bird evolution, or they could belong to reptiles not close to bird lineages but independently developed bird-like feet, the researchers reported.
Fossils without bones
The footprints were discovered in the mid-20th century and were given the scientific name Trisauropodiscus by French paleontologist Paul Ellenberger. The name is an ichnogenus, meaning it describes a genus based on trace fossils, or fossilized impressions left by an animal, rather than fossils of its body.
There are thought to be seven ichnospecies associated with Trisauropodiscus tracks, and for decades paleontologists debated the group's avian affinity. Some called the tracks bird-like, but others weren't so sure. Ellenberger may have muddied the waters by assigning many differently shaped tracks to the ichnogenus, "and they're not all bird-like," Abrahams said.
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Additionally, the shape of a footprint can vary greatly depending on the type of material the animal has stepped on. This can make it difficult to determine the physical characteristics of extinct animals, when fossilized tracks are the only clues they left behind, said Dr. Julia Clarke, professor of vertebrate paleontology at the University of Texas at Austin, who was not involved the research was involved.
"Footprints is a truly unique record," Clarke told CNN. "But that zone of uncertainty will always remain there, just in the nature of the data we have."
At the same time that Trisauropodiscus' tracks were imprinted in the mud, evolutionary adaptations were flourishing in archosaurs - the ancient reptilian group that includes dinosaurs, pterosaurs and crocodiles - so it's intriguing to find evidence of bird-like feet in an unknown member of this group, she added. .
"The footprints do not directly correspond to fossil animals known from this region and period. They could belong to other reptiles or cousins of dinosaurs that evolved bird-like feet," Clarke said. "It contributes to our understanding of morphological diversification during this very important period in archosauria."
Follow footsteps
The researchers' investigation began in 2016: the UCT team "followed in the footsteps of Paul Ellenberger and documented his sites using modern ichnological standards," Abrahams said.
During a trip to Maphutseng, a fossil site in Lesotho, the team found a number of bird tracks from the Triassic. "It took us a minute to realize we were looking at Trisauropodiscus," she said. "Our first impression was that these tracks did indeed look very similar to birds and we knew we had to investigate them further." That involved visits to fossil sites; analysis of archival photographs, sketches and casts; and creating digital 3D models of the footprints.
The scientists assessed 163 numbers and divided them into two categories, or morphotypes, based on their shapes. Tracks categorized as Morphotype I were tagged as non-avian. These prints were slightly longer than wide, with rounder, more robust toes that were closely spaced. "They also have a distinct 'heel'" made by the pads of the third and fourth digits, Abrahams said.
In comparison, the Morphotype II spores were smaller. They were wider than long, with more slender toes. In their shape and in the wide distribution of their digits, this second group of tracks closely resembled those of a bird from the Cretaceous period (145 million to 66 million years ago): the wading bird Gruipeda, another ichnogenus known only from footprints. And overall, the tracks of Morphotype II closely resembled modern bird prints, the scientists reported.
The oldest fossil evidence for paravians - the dinosaur group to which the earliest birds and their closest relatives belong - appears around the mid-Jurassic period (201.3 million to 145 million years ago); the Morphotype II Trisauropodiscus tracks, which are at least 210 million years old, indicate that bird-like feet are even older.
Mindy Weisberger is a science writer and media producer whose work has appeared in the magazines LiveScience, Scientific American, and How It Works."Trisauropodiscus shows that bird-like foot morphology is much older, a common feature between modern birds and other late Mesozoic archosaurs," Abrahams said. "This research contributes to our collective, ongoing understanding of dinosaur and bird evolution."
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