Conceived and designed the experiments: JAW CAM RMHS. Performed the experiments: JAW CAM RMHS. Analyzed the data: JAW CAM RMHS. Contributed reagents/materials/analysis tools: JAW CAM RMHS. Wrote the paper: JAW CAM RMHS.
The authors have declared that no competing interests exist.
A new investigation of the sedimentology and ichnology of the Early Jurassic Moyeni tracksite in Lesotho, southern Africa has yielded new insights into the behavior and locomotor dynamics of early dinosaurs.
The tracksite is an ancient point bar preserving a heterogeneous substrate of varied consistency and inclination that includes a ripple-marked riverbed, a bar slope, and a stable algal-matted bar top surface. Several basal ornithischian dinosaurs and a single theropod dinosaur crossed its surface within days or perhaps weeks of one another, but responded to substrate heterogeneity differently. Whereas the theropod trackmaker accommodated sloping and slippery surfaces by gripping the substrate with its pedal claws, the basal ornithischian trackmakers adjusted to the terrain by changing between quadrupedal and bipedal stance, wide and narrow gauge limb support (abduction range = 31°), and plantigrade and digitigrade foot posture.
The locomotor adjustments coincide with changes in substrate consistency along the trackway and appear to reflect ‘real time’ responses to a complex terrain. It is proposed that these responses foreshadow important locomotor transformations characterizing the later evolution of the two main dinosaur lineages. Ornithischians, which shifted from bipedal to quadrupedal posture at least three times in their evolutionary history, are shown to have been capable of adopting both postures early in their evolutionary history. The substrate-gripping behavior demonstrated by the early theropod, in turn, is consistent with the hypothesized function of pedal claws in bird ancestors.
The earliest dinosaurs were small, bipedal forms that are first recorded ca. 228 million years ago in the Late Triassic, when the Earth's continental landmasses were interlocked as Pangea
The thumbnail map of Africa shows areal extent of main Karoo Basin (grey) and the country of Lesotho (yellow). The phylogeny depicts the basic interrelationships of major dinosaur clades
Extensive, well-preserved fossil tetrapod trackways of the Stormberg Group in the Karoo Basin of southern Africa provide significant information about faunal diversity and turnover during the early Mesozoic in southern Pangea
Originally, Ellenberger
The Moyeni tracksite records more than 250 tetrapod footprints and associated invertebrate traces on a 100 m2 sandstone surface (
Trackways of
We interpreted the sedimentary sequence below and above the track-bearing surface as having accumulated on the inside bank of a meander loop rather than in a lacustrine setting, with the main trackway surface preserved on one of several scroll bars making up a low-angle point bar
Although Ellenberger
The fossilized tracks and trackways at Moyeni provide direct evidence of how early dinosaurs behaved in life. However, because trackways record the interaction of soft tissues with the substrate—typically only the undersides of the manus and pes—trackmaker identifications are imprecise and rely on a combination of stratigraphic, geographic, and morphological coincidence with body fossils
The map in
The two dinosaur trackmakers recorded numerous tracks and trackways at Moyeni. We focus on two lengthy trackways of
Photographs of plaster casts (positives) of
Few noticeable changes are apparent along the one definitive theropod trackway preserved at Moyeni, despite substantial differences in substrate. The trackway consists of 25 tridactyl pes prints that traverse the point bar top and curve gently down the slope onto the rippled surface at or very near the water's edge (
Images at right show the impression of terminal end of the third pedal digit in tracks (tr) 5, 16, 17, and 24. Differences in the shape of terminal impression reflect deeper penetration of the ungual into the substrate, as shown in corresponding schematic interpretations of digit III at right. In the first and last panel, the tip of the ungual makes a narrow, pointed impression. In the middle two panels, the ungual has penetrated deeper into the substrate so that its base makes a rounded impression at the surface. Silhouette morphology and proportions based on
Two lines of evidence support this interpretation over an alternative explanation for these tracks as a natural consequence of a forward shift in body weight. First, the pre-ungual portion of the pes is shallowly impressed and pre-ungual phalanges are identical in size and shape to previous steps in the trackway. Second, flexion of the second and third ungual phalanges measurably lengthened the second and third digits (absolutely and relative to the fourth digit) compared to those impressed on the bar top (
Flexed ungual tracks are longer than typical tracks because the ungual leaves a longer impression when it is submerged within the sediment (compare upper and lower lines).
In contrast to the theropod trackway, the basal ornithischian trackways at Moyeni shift between three distinct locomotor styles, each of which is associated with a different substrate consistency and slope. North American
The first 13 steps in the
The last 5 steps are “narrow gauge” with the limbs positioned along the midline, underneath the body. The transition between the straddling, wide-gauge, plantigrade posture and a parasagittal, narrow-gauge, digitigrade posture was punctuated by a tail impression made during a brief pause (
The wide-gauge walking recorded in the rippled portion of the trackway indicates a greater range of abduction than implied by
In summary, in the riverbed,
High-fidelity preservation of long, continuous trackways of early dinosaurs crossing a heterogeneous paleosurface at Moyeni documents real time responses to substrate quality and inclination. This particular set of taphonomic and sedimentological circumstances is rare and allows detailed reconstruction of the locomotor behavior of the trackmakers. Nonetheless, we infer that the locomotor behaviors inferred from the Moyeni tracks are likely to be common to other basal ornithischians and theropods, due to their morphological similarity to contemporaneous North American track-types
Despite the unevenness of the ground, the theropod (
In marked contrast, the
Ellenberger translation. This is an English-language translation of two portions of Ellenberger (1974) that name and describe the ichnotaxa discussed the text. The first part is the author's description of
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Measurements of two dinosaur trackways from Moyeni, Lesotho. Includes measurements of lengthy trackways of the dinosaur ichnotaxa
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We thank T. Baumiller, S. Gatesy, and P. Meyers for discussions, and M. Carrano, M. D'Emic, T. Ikejiri, D. Pol, and E. Rainforth for comments on previous drafts of this paper. F. Knoll and A. R. C. Milner provided thoughtful and thorough reviews that improved this paper. We thank B. Miljour for assistance with the figures. We thank B. Britt and J. Madsen for lending us a cast of an