Late Triassic Sauropodomorph Trackways

Overview

• Study re-examines Late Triassic (Norian) tetradactyl trackways in Lesotho (Lower Elliot Formation).
• Focus on ichnogenera Pseudotetrasauropus and Tetrasauropus, their stratigraphy, morphology, and locomotor implications.
• Tracks date from ~219.6 Ma to ~209.6 Ma, representing early records of basal sauropodomorph locomotion in Gondwana.
• Methods: field measurements, casts, photogrammetry, 3D models, gait and speed estimates (Alexander 1976).

Geological Context

• All sites lie in the lower Elliot Formation (Stormberg Group, main Karoo Basin).
• Four main tracksites: Phuthiatsana (>219.6 ± 2.5 Ma), Maphutseng (~215.4 ± 2.5 Ma), Ha Falatsa and Libataolong (~209.6 ± 1.4 Ma).
• Track-bearing surfaces vary in substrate saturation and preservation; sedimentology and radiometric dating used to constrain ages.

Sites and Trackways (Summary Table)

Notes: PL = pes length; heteropody = manus vs pes size difference; DM = drag trace.

Track Morphology — Key Observations

• Common pes features across sites:
  • Tetradactyl pes with digits I–IV; digit III usually longest.
  • Blunt distal ends for digits II–IV (except HF1 with sharp claw impressions).
  • Posterior "heel" or basal pad located on external margin ~27–30 cm below tip of digit IV.
  • Pes long axis roughly parallel to trackway midline.
• Manus features:
  • Pseudotetrasauropus manus: wider than long, low fidelity, generally clawless impressions, variable orientation.
  • Tetrasauropus manus (HF1): longer than wide, pronounced medially-directed claw traces, pronated orientation, manual impressions external and anterior to pes.
• Drag traces:
  • Several trackways show U-shaped, arcuate or linear drag traces (DM1–DM5).
  • Interpreted variably as manus dragging or other autopod/tail movement; PT1 traces likely manus-related.

Ichnotaxonomy (Assignments)

• Pseudotetrasauropus (Ellenberger, 1972) — assigned trackways: PT1, PT2, PT3, MP1, LB1, LB2.
  • Ichnospecies:
    • P. bipedoida: bipedal forms (LB1, LB2, PT3); PL ~40–46 cm; narrow gauge.
    • P. jaquesi: quadrupedal forms (PT1, PT2, MP1); larger PL up to ~56 cm; strong heteropody.
  • Rationale: Similar pes morphology across bipedal and quadrupedal trackways; manus presence explains quadrupedal ichnospecies.
• Tetrasauropus unguiferus (Ellenberger, 1972) — assigned to HF1.
  • Diagnostic: entaxonic pes with medially directed robust claw traces; wider intermanus distance; pronated, clawed manus.

Locomotor Interpretations

• Gait and speed:
  • Hip height approximated as 4× pes length; stride/h ratios indicate walking gaits for all measured trackways.
  • Estimated walking speeds range ~2.1–5.7 km/h depending on trackway.
• Bipedal vs quadrupedal:
  • Both locomotor modes recorded across ~10 Ma interval; suggests facultative quadrupedality in some basal sauropodomorphs.
  • Quadrupedal Pseudotetrasauropus shows parasagittal hindlimb posture with variable forelimb placement.
  • Tetrasauropus suggests a wider, flexed forelimb stance with fleshy feet and pronounced manual claws.
• Limb posture and pad hypothesis:
  • Depth-pressure patterns indicate presence of a plantar (fatty) pad distributing weight, especially in quadrupedal trackways.
  • Pseudotetrasauropus shows strong hindfoot weight-bearing; Tetrasauropus shows greater loading along external pes margin (digit IV area).
  • Manus traces suggest reduced phalanges or fleshy cushions in forefeet for large-bodied forms.

Sedimentological and Preservation Controls

• Substrate saturation, pooling, and rheology strongly influence track morphology and presence/absence of manus impressions.
• Plastic substrates produce sediment ridges, expulsion flaps, and exaggerated digit I impressions.
• Weathering has reduced morphological detail at several localities (notably PT3 and HF1 in situ).

Comparisons Within OPEK Plexus and Other Ichnotaxa

• OPEK plexus includes Otozoum, Pseudotetrasauropus, Evazoum, Kalosauropus, Tetrasauropus.
• Differences highlighted:
  • Pseudotetrasauropus: digits largely straight/forward (not medially curved), digit III longest.
  • Otozoum: digits curve medially; complex pad impressions; often different digit-length relationships.
  • Tetrasauropus fits otozoid features by medially oriented claw traces, but also shows distinct differences.

Potential Trackmakers

• Likely basal sauropodomorphs (non-sauropodan prosauropods) based on pedal morphology and stratigraphic context.
• Candidate genera from Lower Elliot Formation include Kholumolumo, Melanorosaurus, Blikanasaurus, Plateosauravus, Eucnemesaurus.
• Stratigraphic association:
  • Kholumolumo remains are stratigraphically close to Maphutseng tracksite (possible maker of some Pseudotetrasauropus).
  • Blikanasaurus (robust pes) is a plausible candidate for Tetrasauropus due to robust, clawed impressions.
• Caution: direct body fossil–trackmaker matches are tentative due to incomplete fossil manus/pes records and substrate effects.

Key Terms and Definitions

• Heteropody: difference in size/role between manus and pes impressions.
• Entaxonic: weight-bearing bias toward medial side of foot/pes.
• PTR (Pes Trackway Ratio): gauge measure of trackway width relative to pes width.
• GAD (Gleno-acetabular Distance): estimate from trackway used to infer body proportions.
• Plantar pad (fatty pad): fleshy soft-tissue structure inferred from midfoot depth-pressure patterns.
• OPEK plexus: grouping of related tetradactyl ichnotaxa (Otozoum, Pseudotetrasauropus, Evazoum, Kalosauropus, etc.).

Conclusions

• The Lesotho Lower Elliot Formation records both bipedal and quadrupedal basal sauropodomorph trackmakers between ~219.6 and ~209.6 Ma.
• Pseudotetrasauropus and Tetrasauropus are distinct ichnogenera with differing manus placement, claw traces, and inferred forelimb postures.
• Evidence supports facultative quadrupedality and the presence of fleshy plantar pads in these early sauropodomorphs.
• Rare drag traces provide new insight into limb and possibly tail movements during locomotion.
• These trackways document an important stage in the evolution of sauropodomorph locomotion preceding full sauropod graviportal adaptations.

Action Items / Next Steps (as implied by study)

• Continue high-resolution stratigraphic correlation between tracksites and body fossil localities.
• Acquire more complete body-fossil manus/pes specimens from matching stratigraphic levels to refine trackmaker assignments.
• Further experimental and biomechanical work to model plantar pad function and manus weight-bearing in basal sauropodomorphs.
• Preserve and digitize remaining in situ surfaces and casts to prevent further loss and enable comparative analyses.