A 2026 study of Permian and Triassic trackways shows that dinosaur-line archosauriforms did not switch from lizard-like sprawling to upright walking in one leap. Their posture changed in stages: limbs moved closer to the body, trackways narrowed, strides lengthened, feet became more toe-walking and cursorial, and the hands shrank. The transition began in the Late Permian and was substantially complete by the Middle Triassic, before true dinosaurs became the dominant bipedal experimenters.
What the 2026 footprint study found
The animals in question were archosauriforms, the broad evolutionary group that includes dinosaur ancestors as well as crocodile-line relatives. Lorenzo Marchetti and colleagues describe a stepwise acquisition of three linked traits: digitigrady (supporting weight on the toes), cursoriality (a foot proportioned for efficient running or sustained walking) and an erect stance with the limbs positioned beneath the body.
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This sequence matters because it places the crucial postural change before true dinosaurs. Upright walking was not an innovation that suddenly appeared with the first dinosaurs; it was built through earlier archosauriform evolution and later provided the mechanical foundation for repeated bipedalism in dinosaur-line animals.
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Pace angulation is the key geometric signal
Pace angulation is the angle between successive footprints made by the same side of an animal in a trackway. In the geometric model tested for archosauriform locomotion, a sprawling animal projects its legs sideways. Its prints sit farther from the body line, the trackway is relatively wide and successive steps form a smaller angle. When the limbs rotate underneath the body, the trackway narrows and the stride lengthens, increasing pace angulation.
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That makes a trackway a record of movement rather than merely a catalogue of bones. It captures how an animal actually placed its feet during one episode of locomotion, including the relationship between limb position, stride and body progression.
Foot shape adds independent evidence
Geometry is not the only clue. The 2026 analysis also identifies relatively longer middle digits, toe-supported contact and reduced hand impressions as parts of the same shift. A longer central digit can improve forward travel, digitigrade contact reduces the area of the foot on the ground, and smaller hands are consistent with forelimbs becoming less important as weight-bearing supports. Together, these traits indicate increasing cursoriality and help distinguish a genuine locomotor trend from a change caused only by trackway geometry.
| Feature | More sprawling condition | More erect, cursorial condition |
|---|---|---|
| Limb position | Projects farther sideways from the body | Held closer to and more beneath the body |
| Trackway | Wider | Narrower |
| Pace angulation | Lower | Higher |
| Foot contact | Flatter-footed contact is more compatible | Digitigrade, toe-supported contact |
| Digits and hands | Less specialized for rapid forward travel; larger hand impressions are possible | Relatively longer middle digits and reduced hands |
| Later evolutionary implication | Less suited to sustained cursorial travel | Creates conditions that can support later bipedality |
The numerical trail from the Late Permian to the Late Triassic
Kubo and Benton’s 2009 analysis measured 461 published Permian and Triassic trackways. Their median pace-angulation values rise sharply across the interval:
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| Interval | Median pace angulation | What the value indicates in the comparative model |
|---|---|---|
| Late Permian | 87.5° | Predominantly sprawling geometry |
| Early Triassic | 139.5° | Large early shift toward a narrower, more erect trackway |
| Middle Triassic | 149° | Continued refinement and spread of erect walking |
| Late Triassic | 156.5° | High-angle trackways consistent with strongly erect locomotion |
The largest numerical jump occurs between the Late Permian and Early Triassic, from 87.5° to 139.5°. The newer study describes the broader evolutionary pattern as stepwise and completed by the Middle Triassic, so the best reading is a rapid initial reorganization followed by continued refinement and expansion of erect walking among Triassic archosauriforms rather than a single instantaneous transformation.
A stepwise sequence of locomotor change
Late Permian: a sprawling starting point
The earliest trackways in the sequence record animals whose limbs were still carried substantially to the sides. Their lower pace angulation and wider trackways fit a sprawling gait. This was the starting condition for the archosauriform lineage before the end-Permian crisis.
Early Triassic: the rapid first shift
After the end-Permian mass extinction, trackways show a marked increase in pace angulation. The limbs were being brought closer to the body, while feet and digits were becoming better suited to forward travel. The 2009 analysis concluded that an erect archosaur posture emerged within the first six million years of post-extinction recovery—much faster than earlier skeletal interpretations, which had suggested a 15–20 million-year transition.
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Middle Triassic: erect walking established
By the Middle Triassic, the combined footprint evidence indicates that erect walking was established across relevant archosauriforms. Digitigrade contact, relatively long middle digits and smaller hands appear as coordinated parts of the change, not isolated novelties.
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These modifications did not automatically make every archosauriform bipedal. They improved limb alignment and forward efficiency, however, creating anatomical and mechanical conditions that later dinosaur-line animals could exploit when evolving habitual or facultative bipedality.
Where the tracks were found and who made them
The 2026 report identifies relevant footprints first found at GEOPARC Bletterbach in the Italian Dolomites, a UNESCO World Heritage site. The study names a distinct track type, Pachychirotherium gardenaensis, and compares its proportions with archosauriform hands and feet. The authors assign it to Archosauriformes, probably an erythrosuchid or an animal with very similar limb morphology.
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Comparable Early Triassic Protochirotherium footprints are known from Germany, Poland and Morocco. These geographic comparisons help show that the locomotor pattern was not restricted to one Italian outcrop.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why exact trackmaker names remain uncertain
A footprint is a behavioral trace, but it is not a skeleton with a label attached. Trackways are generally assigned to broad groups by comparing digit counts, hand and foot shape, proportions and known skeletons. Several related animals can produce similar prints, and one species may leave different-looking tracks at different speeds or on different substrates.
Pace angulation is therefore a strong comparative indicator, not a literal photograph of limb posture. Gait, pelvic rotation, limb proportions and body size can all influence the angle. The safest conclusion is that the trackways document a population-level trend toward more erect locomotion among archosauriforms, while species-level attribution remains provisional.
Why the timing matters after the end-Permian extinction
The end-Permian mass extinction occurred about 252 million years ago. The rapid rise in trackway angulation during the earliest Triassic places the postural reorganization within the first phase of ecological recovery. Efficient, more upright walking could have helped archosauriforms move over longer distances and exploit changing environments, although the footprints alone do not identify a single selective cause.
What they do establish is timing: the locomotor system was being reorganized before dinosaurs diversified widely. Fossil skeletons show what an animal could have done; tracks show what it actually did on the ground.
Study details and the significance of trace fossils
The report by Lorenzo Marchetti and colleagues, “The new archosauriform track Pachychirotherium and the stepwise acquisition of digitigrady, cursoriality and erect stance in the group between the Permian and Triassic,” appears in Palaeontology (2026), DOI 10.1111/pala.70089.
“It is amazing to see how many details on the biology and locomotion of archosauriforms are preserved in fossil footprints, and this becomes even more important when the skeletons are scarce. In fact, trace fossils are the only fossils to directly register the locomotion of living animals.”
Lorenzo Marchetti, Museum für Naturkunde Berlin
That distinction explains why the trackway evidence is so valuable. Bones can reveal joint structure and possible ranges of motion, but a sequence of prints preserves the animal’s posture, stride and weight transfer during movement. In this case, hundreds of trackways turn a gradual evolutionary change into a measurable pattern: sprawling geometry in the Late Permian, a major early Triassic shift and increasingly erect walking through the Middle Triassic and later Triassic.
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