Dynamic locomotor capabilities revealed by early dinosaur trackmakers from southern Africa

Abstract

Background: 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.

Methodology/principal findings: 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 degrees ), and plantigrade and digitigrade foot posture.

Conclusions/significance: 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.

Figure 1. The Moyeni trackway locality and its stratigraphic and phylogenetic context.

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 on a timescale. Subsequent analyses have resolved Lesothosaurus at the base of Thyreophora , but most agree that it is positioned near the base of clade uniting Thyreophora, Ornithopoda, and Marginocephalia. The Moyeni tracksite (marked by yellow band) preserves tracks made by early dinosaur trackmakers, well after the initial divergence of saurischians and ornithischians (early Late Triassic) but well before the origin of flight (Upper Jurassic) and prior to three independent acquisitions of quadrupedal posture (Early–Middle Jurassic) , . The graded stratigraphic range for Marginocephalia reflects uncertainty in its first appearance date; icons atop diagram are representative theropod and ornithischian dinosaurs .

Figure 2. Map of major geological features and tracks at the Moyeni tracksite.

Trackways of Anomoepus and Grallator are in solid green and blue, respectively; all other trackways are in 50% grey. Trackway numbering (Arabic numerals) begins with the first recognized step of each trackmaker. Grallator tracks 1 and 2 are not shown because they are now underneath a retaining wall . Red wavy lines indicate ripple marks; solid light grey fields indicate algal-matted surface. The inset highlights postural and gait changes in the Anomoepus trackway, which was made by a basal ornithischian. A shift from wide-gauge to narrow-gauge posture (between tracks 13 and 14) is marked by shift in pace angulation (blue dotted line) and was accompanied by a brief pause, during which the tail registered on the substrate when tracks 12 and 13 were impressed. The shift from a quadrupedal to a bipedal gait (between tracks 15 and 16) occurs atop the algal matted surface. Note that Grallator track 11 (blue) overprints Anomoepus track 15, indicating it was made later. Grid pattern forms 1 meter squares for both maps. Abbreviations: lm, left manus; lp, left pes; mt, metatarsus; rm, right manus; rp, right pes; t, tail; Roman numerals indicate pedal digits.

Figure 3. Moyeni dinosaur tracks.

Photographs of plaster casts (positives) of Grallator track 6 (A) and Anomoepus track 8 (B) made at the Moyeni tracksite by the authors. Tracks are shown at the same scale (10 cm), and hatching pattern indicates broken surfaces. The Grallator hind foot print was made by pedal digits II–IV; the trackmaker's phalangeal formula was 3–4–5. Digits I and V did not contact the substrate. The rugose texture surrounding the print is the algal mat. The Anomoepus manus–pes couple registers all five manual digits (i–v), four pedal digits (I–IV), the metatarsus (mt), and toe drag marks (dm). Additional structures to the left of the pes are incidental marks made by a different trackmaker.

Figure 4. Dynamic locomotor adjustments to paleosurface heterogeneity. A, “flexed-ungual” locomotion in the Grallator trackway, which is attributed to a theropod.

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 Allosaurus. B, postural changes in Anomoepus. Panels show our interpretation of limb posture during plantigrade, wide-gauge locomotion (top), and digitigrade, narrow-gauge locomotion (bottom) in lateral (left) and anterior (right) views. Silhouettes are based on limb proportions and skeletal morphology common to basal ornithischians –.

Figure 5. Arc of rotation of ungual during the step cycle.

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).