Dino spoor, that is. A recently reported finding in PLoS ONE clarifies a number of questions about how certain dinosaurs held their front limbs (zombie/Frankenstein-position palm-down vs. huggie-wuggie palms-facing-each-other). This research confirms …
that early theropods, like later birds, held their palms facing medially, in contrast to … 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.
Figure 7 from the paper. Restoration by Heather Kyoht Luterman 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.
The find is from southwestern Utah. In particular, the tracks were found in the Whitmore Point Member of the Moenave Formation (WP), which in turn is one of about nine or so formations that are exposed in Zion and Kolob canyons in Zion National Park. The WP Member itself is about 100 meters thick. The Moenave Formation and together with the Kayenta formation (just above it) are considered to be Lower Jurassic in age. The base of the Moenave formation is a disconformity caused when the basin was uplifted, and thus eroded, for about ten million years. Subsequent to this shallow seas to the north of this region repeatdly expanded or shifted into this area, and the sediments of the Moenave formatoi represent lake, river, and flood plain (river-side and beyond) sediments that were part of this sea basin.
Because of the constant (in geological time) shifts between environments, the Moenave Formation possesses layers bearing fossils and traces of a wide range of sediments. Within the WP Member itself, there are plant fossils in some of the lower layers, and fish fossils throughout. Dinosaur bones have been found in the upper most layer. But in many layers, from the lowest to nearly the uppermost, there at tracks. The tracks discussed in this paper are from the lower part of the formation.
Figure 2 from the paper: 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.
The reason that I’m pointing all of this out is to give an (accurate) impression of the significance of this basin (see this discussion). There are many hundreds of meters of sediment at Zion and other nearby locations (including the Grand Canyon) that tell the story of major changes in the landscape, and that preserve long, well represented records of life. Immense geological time is represented here, as well as the occasional brief and fleeting moments, like when some dinosaur lays down to rest and leaves behind an impression of its body, which happens, against all odds, to be preserved as a trace fossil. It is a paleontologists dream:
Twenty-five track-bearing horizons contained within a small area (1 km2) in St. George, Utah, contain a diverse, theropod-dominated ichnofauna. The most fossiliferous and diverse surface … is preserved within the St. George Dinosaur Discovery Site at Johnson Farm … museum. Mudflat, shoreline, and periodically submerged surfaces coincide on the same bedding plane as evidenced by mud cracks, ripple marks (current, symmetrical, wind-driven, interference, and wave-formed), erosive mega-ripples, load and flute casts, rill and tool marks of various sizes, raindrop impressions, and invertebrate and vertebrate ichnites. [an “ichnite” is a fossilized foot print.] This suite of sedimentary features formed on a beach or shoal along the shores of an Early Jurassic freshwater body (Lake Dixie) that underwent seasonal regressive-transgressive fluctuations. The majority of theropod trackways on this surface trend north-south, paralleling the paleoshoreline. The 22.3 m long SGDS.18.T1 trackway … includes the unique crouching traces….
Figure 4 from the paper. 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.
Because early Jurassic dinosaurs of the type that left these tracks had relatively undifferentiated feet, it is impossible to assign these tracks to species. The tracks themselves, grouped together from different locations but looking similar, form what is called an “ichnotaxon” … a species or set of species as represented by tracks of similar morphology. Indeed, when dealing with dinosaurs, perhaps we should say that a given ichnotaxon of this type may even represent a set of genera. The paper itself provides a lengthy discussion of this issue, if you want to delve into it.
The paper concludes that …
… other ostensible theropod manus [manus = front foot] prints are either dubiously attributable to theropods, dubiously made by the manus of a pes-print [pes = back foot] maker, or uninformative with regard to the track maker’s forelimb functional morphology. Because the crouching traces in the trackway [studies here] match the architecture of known theropods, we support the alternative interpretation that most, if not all, other prints showing manus impressions instead pertain to ornithischian or other non-theropodan dinosaurs or dinosauriforms with functionally tridactyl pedes. [This trackway] therefore includes the only unambiguous theropod manus impressions recognized to date and indicates that the avian orientation of the manus, with medially-facing palms, evolved very early within the Theropoda. Less parsimoniously, this posture evolved in immediate dinosaur ancestors; absence in other dinosaurs would thus constitute reversals.
The lack of marks in [this trackway] made by the distal thoracic and pelvic limbs and the ventral portion of the pelvis indicate that, while resting, even the earliest theropods adopted a modern ratite-like [bird-like] posture with the legs folded symmetrically beneath the body such that the weight of the body was distributed between each metatarsus and pes. … The clear symmetry of [this trackway] demonstrates that even some of the oldest, basal-most theropods engaged in this additional avian-style behavior, which therefore also evolved very early in the theropod lineage or was retained in theropods from pre-dinosaurian archosaurs.
Background and references:
Andrew R. C. Milner, Jerald D. Harris, Martin G. Lockley, James I. Kirkland, Neffra A. Matthews (2009). Bird-Like Anatomy, Posture, and Behavior Revealed by an Early Jurassic Theropod Dinosaur Resting Trace PLoS ONE, 4 (3) DOI: 10.1371/journal.pone.0004591