The transition from sprawling (reptile-like) to more upright (parasagittal) posture and locomotion was a transformative event in mammalian evolution. A study published June 24th in the open-access journal PLOS Biology by Dr. Robert Brocklehurst and Professor Stephanie Pierce at Harvard University, USA and colleagues suggests that parasagittal posture evolved via an indirect, dynamic, and radiating process.
Non-mammalian synapsids (tetrapod vertebrates), the ancestors of extant mammals, underwent major musculoskeletal reorganization, including modifications to the forelimbs. However, when and how these anatomical transformations translated into mammal-like limb posture and upright locomotion is unknown. Prior research based on qualitative observations of the fossil record had hypothesized the evolutionary transition from sprawling to upright as a direct progression with discrete postural stages.
In order to better understand the origins and evolutionary pathway of mammalian posture, researchers analyzed humerus bone from over 200 species of tetrapods. They compared fossilized synapsid humeri to an array of extant salamander, reptile, monotreme and upright therian mammal humeri, examining humeral length, torsion, muscle leverage, bending strength, and radius of gyration. The researchers then computationally modeled the relationship between bone shape, function, and posture to visualize different evolutionary scenarios for how therian mammal upright posture evolved.
The researchers found that ancestral synapsids had a sprawling posture, but their anatomy and movement were distinct from extant sprawlers. The upright posture of modern-day mammals involved a fundamental reorganization of the musculoskeletal system and expansion of differences in forelimb function, suggesting that mammals started walking upright later than previously thought. The study had several limitations, such as uncertainty around phylogenic branch lengths and estimated dates of taxa divergence. Future studies are needed to develop greater specificity and certainty around limb posture in different taxa.
According to the authors, "Previous hypotheses posit the synapsid 'sprawling-parasagittal' transition as a series of discrete postural shifts; our study supports the view of synapsid evolution as a series of successive radiations, with major clades exhibiting considerable functional (and postural) variation. Our data on humerus morphology and functional trait evolution suggest that parasagittal posture evolved late, within stem therians."
"The origin of upright mammalian posture is a key part of their evolutionary story", adds Dr Robert Brocklehurst, lead author of the study. "However, there's been a lot of uncertainty as to when and how the upright postures of modern mammals evolved. People have been working on this problem for over 100 years, but historically the focus has been on bone shape in a few exceptionally preserved fossils. We knew if we wanted to understand the big picture of posture evolution in mammals and their ancestors we would need to see as many fossils as possible, and really get to grips with bone function and mechanics, not just shape".
"By assembling an unprecedented dataset spanning the full breadth of synapsid evolution, we achieved the resolution necessary to disentangle the transformation from our sprawling synapsid ancestors to the upright-limbed mammals of today," says Professor Stephanie Pierce, senior author of the paper. "Our results show that the hallmark forelimb posture and function of modern mammals emerged surprisingly late in synapsid evolution—this delayed acquisition ultimately laid the foundation for the extraordinary ecological success of mammals."
"The ancient synapsid forerunners of mammals are often compared with modern reptiles because they had sprawling limbs that look something like those of lizards or crocodiles. However, our study showed that most synapsid limbs functioned differently than those of modern reptiles," says Dr. Kenneth Angielczyk, a co-author of the study. "They're not just copies of reptiles, but distinctive animals in their own right that are a little different from anything that's alive today."
The authors add, "While the ancestors of mammals did generally get more upright as time went on, there was a lot of variation in each major group of mammalian ancestors. These fossils weren't stepping-stones, they were animals evolving to explore a wide range of ecologies, niches and habitats."
In your coverage, please use this URL to provide access to the freely available paper in 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
Author countries: United States
Funding: Funding was provided by the US National Science Foundation (grant DEB1754459 to SEP and grant DEB1754502 to KDA), and by the Harvard Museum of Comparative Zoology (Grant-In-Aid of Undergraduate Research to MM). Funding for publication costs provided by a grant from the Wetmore Colles Fund. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
