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, 4 (3), e4591

Bird-like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace


Bird-like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace

Andrew R C Milner et al. PLoS One.


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.

Methodology/principal findings: 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/significance: 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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Figure 1
Figure 1. Location of the St. George Dinosaur Discovery Site at Johnson Farm (SGDS) (green star) in Washington County, southwestern Utah.
The site and others within the 1 km2 mentioned in the text are within the boundaries of the City of St. George.
Figure 2
Figure 2. Stratigraphic section of the Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm.
Resting trace and trackway SGDS.18.T1 is in the “Top Surface” of the Main Track-Bearing Sandstone Bed (indicated by the blue arrow) in the Whitmore Point Member of the Moenave Formation.
Figure 3
Figure 3. Schematic map of the “Top Surface” tracksite (SGDS.18).
Beige shaded areas represent the “Top Surface” of the Main Track-bearing Sandstone Bed; gold shaded areas are unexcavated; brown areas represent areas removed after mapping to examine lower horizons. The Eubrontes trackway that includes the crouching trace is highlighted in red.
Figure 4
Figure 4. Eubrontes trackway with resting trace (SGDS.18.T1) in the Whitmore Point Member of the Moenave Formation, St. George, Utah.
A, Overhead, slightly oblique angle photograph of SGDS.18.T1 resting trace. Note normal Eubrontes track cranial to resting traces (top center) made by track maker during first step upon getting up. Scale bar equals 10 cm. B, Schematic of SGDS.18.T1 to scale with A: first resting traces (manus, pes, and ischial callosity) in red, second (shuffling, pes only) traces in gold, final resting traces (pes and ischial callosity) in green, and tail drag marks made as track maker moved off in blue. Note long metatarsal (“heel”) impressions on pes prints. C, Direct overhead photograph and D, computerized photogrammetry with 5 mm contour lines of Eubrontes trace SGDS.18.T1. Color banding reflects topography (blue-green = lowest, purple-white = highest); a portion of the berm on which the track maker crouched is discernible. Abbreviations: ic = ischial callosity, lm = left manus, lp = left pes, rm = right manus, rp = right pes, td = tail drag marks.
Figure 5
Figure 5. Stereophotographs of SGDS.18.T1 crouching trace.
Elevation exaggerated to emphasize individual tracks. Placards on surface are markers used in generation of photogrammetric image in Figure 4D.
Figure 6
Figure 6. Schematic diagrams of Late Triassic-Early Jurassic theropod tracks.
A, Eubrontes, referred specimen, right pes (AC 45/1; traced from [38]). B, Gigandipus, holotype, left pes (AC 9/16; traced from [47]). C, Anchisauripus, holotype, left pes (AC 4/6; traced from [38]). D, Grallator, holotype, left pes (reversed image of natural cast) (AC 4/1a; traced from [38]). E, Dilophosauripus, holotype, ?left pes (UCMP 79690; traced from [50]). F, Kayentapus, right pes from holotype trackway (UCMP 83668; traced and modified from [50]). Scale bar equals 5 cm. AC = Appleton Cabinet, Amherst College, Amherst, Massachusetts, United States of America; UCMP = University of California Museum of Paleontology, Berkeley, California, United States of America.
Figure 7
Figure 7. Restoration of Early Jurassic environment preserved at the SGDS, with the theropod Dilophosaurus wetherilli in bird-like resting pose, demonstrating the manufacture of SGDS.18.T1 resting trace.
By Heather Kyoht Luterman.

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