Handy prints: Artist's impression of the formation of the tracks by the carnivorous dinosaur Dilophosaurus.
Credit: H. Ky Luterman
SYDNEY: Rare fossilised handprints of a carnivorous dinosaur have helped answer questions about the orientation of limbs which later evolved into wings in birds.
"Other supposed theropod hand prints had been reported in the past, but they were all either shapeless blobs or made by animals with downward-facing palms," said Martin Lockley of the University of Colorado in Denver, a dinosaur tracks expert and co-author of a paper detailing the find in the open access journal PloS One.
Zombie pose all wrong
"These [earlier handprints] clearly cannot have been made by theropods that had restricted arm mobility, and must have been made by other kinds of animals – maybe more primitive dinosaurs. This [new] discovery helps us rewrite this piece of our understanding of dinosaur evolution," said Lockley.
Theropod dinosaurs include Velociraptor, Allosaurus and Dilophosaurus and are the group that evolved into birds. They were bipedal species leaving their forelimbs free for other functions.
Many early reconstructions of theropod dinosaurs, such as Tyrannosaurus rex, suggested that their front limbs hung forward, palms down, something like a sleepwalker or a zombie. Experts now know that this was not possible and that the palms must have faced inwards, towards each other, in the same way as birds wings.
This is the first time the orientation has been confirmed in track prints, though, and helps date one of the traits that allowed flight to later develop in birds.
Wing orientation
"Theropod arms couldn't pivot to make the palms face up or down the way a human's hands can," said co-author and palaeontologist Jerry Harris of Dixie State College in St George Utah. But "the wrist could pivot up and down, allowing the outside of the hand to move toward the side of the arm… the same motion birds have that allows them to fold their wings."
"These tracks show that this ability evolved long before feathery wings did, and much earlier than this posture is known from theropod skeletons," he said.
Dated at 198 million years old, the tracks were discovered in 2004 when grounds for a new museum were dug in St. George. The handprints were likely made when an unknown theropod sat down and extended its arms so that they made marks in sediment, which were then preserved as stone, said the authors.
The same site has yielded over 1,000 track prints of a variety of dinosaur and early relatives of crocodiles and alligators.
"The discovery of theropod dinosaur tracks are common, but the vast majority of tracks are marks made by the feet, which is understandable given that these were bipedal animals," commented Darren Naish a palaeontologist at Portsmouth University in England.
"Resting traces - made when the animals squatted down and allowed other parts of their bodies to make contact with the ground - are comparatively rare, and they're always exciting because they can provide us with additional details on anatomy," he said
"Dinosaur workers have been saying for a while now that theropods palms faced inwards. Developing hypotheses based on anatomy is fine, but when data is provided by actual, once-living animals – well, that's the icing on the cake and it shows that our anatomical inferences were right on the money," added Naish.
