A groundbreaking new analysis of fossil forelimb bones has upended longstanding ideas about when and how the ancestors of mammals shifted from sprawling, reptile-like locomotion to the upright gait characteristic of modern mammals. Published today in PLOS Biology, the study led by Dr. Robert Brocklehurst and Professor Stephanie Pierce of Harvard University shows that the evolution of parasagittal (upright) posture was neither a simple, linear progression nor an early adaptation—but rather a late, dynamic radiation of diverse postural strategies among stem mammals.
Key Findings at a Glance
- Parasagittal posture evolved late in the stem-therian lineage, much later than previously thought.
- Early synapsids exhibited a unique “intermediate” sprawling gait, distinct from that of modern reptiles.
- Forelimb bone shape and mechanical function diversified in successive radiations, not in discrete stages.
- Modern mammalian posture likely emerged in a burst of evolutionary experimentation within stem therians.
Background: From Sprawling to Upright
Today’s mammals stand and move with limbs tucked beneath their bodies, a posture that delivers efficient weight-bearing, agile locomotion, and the anatomical foundation for running and jumping. By contrast, most modern reptiles and amphibians “sprawl,” with limbs extended to the sides. While paleontologists have long recognized that early synapsids—mammal ancestors that thrived 300–250 million years ago—also sprawled, the point at which their descendants adopted a parasagittal gait remained murky. Traditional narratives posited a direct, step-by-step shift: sprawling → semi-erect → upright.
“People have been framing this as a straightforward progression for over a century,” says Dr. Brocklehurst. “But the fossil record is fragmentary, and our assumptions rested on a handful of well-preserved specimens. We wanted to cast a much wider net and examine what bone function really tells us about posture across the tree of synapsids.”
Methods: A Massive Humerus Dataset and Functional Modeling
To capture the breadth of synapsid postural diversity, Brocklehurst, Pierce, and colleagues assembled data on the humerus (upper arm bone) from more than 200 species—ranging from early non-mammalian synapsids through monotremes and marsupials to placental mammals, as well as modern sprawling amphibians and reptiles for comparison.
For each specimen, they measured:
- Humeral length and torsion (twist of the bone)
- Muscle moment arms (leverages for major limb muscles)
- Cross-sectional bending strength
- Radius of gyration (distribution of bone mass about its axis)
Combining these metrics with computer simulations, the team reconstructed likely limb postures and loading regimes, mapping them onto an evolutionary tree to visualize how posture and function shifted through time.
“We weren’t just looking at bone shapes,” explains Professor Pierce. “We translated those shapes into mechanical traits that reflect how the limb functioned in life—how much torque muscles could generate, how much stress the bone could resist, and so on. That’s what lets us test competing evolutionary scenarios quantitatively.”
Results: A Radiating, Indirect Pathway to Upright Posture
Contrary to the simple linear model, the analysis revealed:
- Intermediate Synapsid Gait Is Unique
Early synapsids did sprawl, but their humeral mechanics diverged from modern reptiles. Traits like torsion angles and muscle leverages indicate a distinct, intermediate mode—neither true sprawling nor fully erect. - Successive Functional Radiations
Rather than one uniform march toward upright posture, major synapsid clades (e.g., caseids, gorgonopsians, cynodonts) experimented with different limb functions. Some lineages showed early increases in limb leverage akin to partial elevation, while others retained or even exaggerated sprawling traits. - Late Emergence of True Parasagittal Gait
Only in the stem-therian lineage—precursors to marsupials and placentals—did forelimb traits coalesce around the modern parasagittal condition. This shift occurred well after many key mammalian features, such as differentiated teeth and middle-ear bones, had already evolved. - Convergent Functional Strategies
Multiple stem-therian subgroups independently acquired elements of upright posture, but only one lineage combined all traits into the habitual, underslung gait of extant mammals.
“The upshot is that mammalian posture didn’t evolve in a neat sequence,” says co-author Dr. Kenneth Angielczyk. “Instead, synapsids repeatedly branched into new locomotor ‘experiments,’ adapting to varied ecologies. Only later did one branch lock in the fully upright strategy that underpins mammal success today.”
Implications for Mammalian Evolution and Ecology
The timing of upright posture acquisition matters because it reshapes interpretations of other evolutionary milestones. High-stride, agile locomotion may have arisen around the same time as the first small-bodied, nocturnal therian mammals, potentially aiding resource partitioning in an environment dominated by diurnal reptiles and the lingering decline of dinosaur synapsids.
“This study rewrites the choreography of the mammalian takeover,” notes Dr. Brocklehurst. “Rather than a punctual postural revolution, we see a drawn-out series of ‘tries and retries,’ peaking in the stem therians. That delay may help explain why early mammals remained small and inconspicuous for so long.”
Study Limitations and Future Directions
The authors acknowledge:
- Phylogenetic Uncertainties: Divergence dates and branch lengths in synapsid family trees remain debated, which can influence trait-evolution reconstructions.
- Incomplete Fossil Preservation: Many key species are known only from fragmentary remains, limiting sample sizes in certain clades.
- Soft-Tissue Inference: Muscle reconstructions rely on bone landmarks but cannot capture full soft-tissue complexity.
“Fleshing out these details requires new fossils, refined phylogenies, and integration with biomechanical modeling of whole limbs,” says Professor Pierce. “But our humerus-based approach lays a sturdy foundation for mapping the tempo and mode of posture evolution in unprecedented detail.”
Link to the Open-Access Paper
Readers can access the full study—Adaptive landscapes unveil the complex evolutionary path from sprawling to upright forelimb function and posture in mammals—free on PLOS Biology: https://plos.io/4khp9mP
Citation:
Brocklehurst RJ, Mercado M, Angielczyk KD, Pierce SE (2025) Adaptive landscapes unveil the complex evolutionary path from sprawling to upright forelimb function and posture in mammals. PLoS Biol 23(6): e3003188. https://doi.org/10.1371/journal.pbio.3003188
Funding and Disclosures
This work was supported by NSF grants DEB-1754459 (SEP) and DEB-1754502 (KDA); the Harvard Museum of Comparative Zoology; and the Wetmore Colles Fund for publication costs. The authors report no competing interests. All data and code used in the study are publicly available per PLOS’s open-science policy.
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