The world's first pterosaur tracks from Japan, documented in a new study, suggest these Dinosaur-Age flying reptiles not only coexisted with birds, but that the two groups also hung out together when they weren't soaring the Cretaceous skies.
A lone siltstone slab contains the fossilized footprints, made by pterosaurs, birds and amphibians. It provides a literal slice of what prehistoric life was like in Japan around 127 million years ago.
"I think that a group of small pterosaurs was feeding together near a pond or near a lake," lead author Yuong-Nam Lee told Discovery News, adding "there are lots of feeding beak marks."
Lee, a researcher at the Korea Institute of Geoscience and Mineral Resources, thinks "there were probably abundant food (sources) in the sediments" of what is now the Kitadani Dinosaur Quarry at Fukui Prefecture, Japan.
The quarry is well named, as the remains of several dinosaurs, such as Fukuiraptor kitadaniensis and Fukuisaurus tetoriensis, have been found at the site. A new, as-of-yet unnamed dromaeosaurid and a new sauropod were also recently excavated at the quarry, which is on the Sugiyama River within the city limits of Katsuyama. Remains of now-extinct fishes, turtles and relatives of crocodiles were discovered there too.
For the latest study, accepted for publication in the journal Cretaceous Research, Lee and his colleagues focused on the pterosaur tracks. The scientists identified a total of 64 imprints made by five to six individuals that "show a clear quadrupedal gait pattern" with feet bearing curved "hook-like sharp" claws.
"The high density of the tracks suggest gregarious behavior, but the random orientation of the trackways does not show that they were moving in the same direction as a herd," Lee said.
He and his team instead think the pterosaurs and birds randomly gathered to feed. The eating marks consist of "small round depressions on the slab," possibly where the animals repeatedly pecked away for food.
Since the tracks don't match up with any other known pterosaur prints, the researchers believe they were made by a new species, called Pteraichnus nipponensis. The only other evidence for pterosaurs in Japan is an incomplete spinal column bone and a single print set, not yet fully documented, from another location.
Nevertheless, this evidence and the siltstone slab prints suggest that multiple tiny pterosaurs called Japan home from at least 113 to 127 million years ago. Evidence for small pterosaurs at that time has also been found in Spain and Korea.
David Unwin, a senior researcher in paleobiology at the University of Leicester, told Discovery News that the slab "provides a fascinating and important insight into life in the mid Lower Cretaceous of eastern Asia."
Since the tracks are similar, yet appear to have been made by different sized individuals, Unwin said it is possible "that pterosaurs of widely differing growth stages visited/walked around in the same small area."
Unwin was also struck by the presence of the bird tracks. He indicated that scientists have long puzzled over the relationship between birds and pterosaurs, wondering if they enjoyed "a long peaceful coexistence, or protracted competition." The former now appears to be the case.
Pterosaur and bird tracks, along with dinosaur prints, were also recently found together at Hakou Formation in Gansu Province, northwest China. Those tracks also date to the Lower Cretaceous.