Bird-like anatomy, posture, and behavior revealed by an early jurassic theropod dinosaur resting trace.
BACKGROUND
Fossil tracks made by non-avian theropod dinosaurs commonly reflect the habitual bipedal stance retained in living birds. Only rarely-captured behaviors, such as crouching, might create impressions made by the hands. Such tracks provide valuable information concerning the often poorly understood functional morphology of the early theropod forelimb.


RESULTS
Here we describe a well-preserved theropod trackway in a Lower Jurassic ( approximately 198 million-year-old) lacustrine beach sandstone in the Whitmore Point Member of the Moenave Formation in southwestern Utah. The trackway consists of prints of typical morphology, intermittent tail drags and, unusually, traces made by the animal resting on the substrate in a posture very similar to modern birds. The resting trace includes symmetrical pes impressions and well-defined impressions made by both hands, the tail, and the ischial callosity.


CONCLUSIONS
The manus impressions corroborate that early theropods, like later birds, held their palms facing medially, in contrast to manus prints previously attributed to theropods that have forward-pointing digits. Both the symmetrical resting posture and the medially-facing palms therefore evolved by the Early Jurassic, much earlier in the theropod lineage than previously recognized, and may characterize all theropods.
Milner
Andrew R C
AR
St George Dinosaur Discovery Site at Johnson Farm, St George, Utah, United States of America. amilner@sgcity.org
Harris
Jerald D
JD
Lockley
Martin G
MG
Kirkland
James I
JI
Matthews
Neffra A
NA
D000818:Animals; D001522:Behavior, Animal; D001717:Birds / Q000033:anatomy & histology* / Q000502:physiology; D025061:Dinosaurs; D005580:Fossils