Transitioning from a sprawling stance, similar to that of lizards, to the upright posture seen in modern mammals marked a significant evolutionary shift. This transformation involved substantial changes in limb structure and function within synapsids - a group that includes both mammals and their distant relatives - and ultimately led to the evolution of therian mammals, such as marsupials and placentals. Despite years of research, questions regarding the exact mechanics, timing, and causes of this change have remained unanswered.
In a new study published in *Science Advances*, researchers at Harvard University shed fresh light on the evolution of mammalian posture. Using advanced fossil analysis and biomechanical modeling, they discovered that this transition was far more intricate and occurred later than previously thought.
Lead author Dr. Peter Bishop, a postdoctoral fellow, and senior author Professor Stephanie Pierce, both affiliated with Harvard's Department of Organismic and Evolutionary Biology, studied the biomechanics of five modern animals that exhibit a range of limb postures. The subjects included a tegu lizard (sprawled stance), an alligator (semi-upright), and a greyhound (upright).
"By first studying these modern species, we greatly improved our understanding of how an animal's anatomy relates to the way it stands and moves," said Bishop. "We could then put it into an evolutionary context of how posture and gait actually changed from early synapsids through to modern mammals."
Their study extended to eight fossil species from across four continents, representing 300 million years of evolutionary history. The team examined species from the tiny proto-mammal Megazostrodon to the larger Ophiacodon and included iconic animals such as the sail-backed Dimetrodon and the saber-toothed Lycaenops. By developing digital biomechanical models of each species' musculature and skeletal structure, the researchers could simulate the force each species' hindlimbs could apply to the ground.
"The amount of force that a limb can apply to the ground is a critical determinant of locomotor performance in animals," Bishop explained. "If you cannot produce sufficient force in a given direction when it's needed, you won't be able to run as fast, turn as quickly, or worse still, you could well fall over."
Their simulations generated a three-dimensional "feasible force space," capturing each limb's performance capabilities. "Computing feasible force spaces implicitly accounts for all the interactions that can occur between muscles, joints and bones throughout a limb," said Pierce. "This gives us a clearer view of the bigger picture, a more holistic view of limb function and locomotion and how it evolved over hundreds of millions of years."
While the concept of feasible force space has existed since the 1990s, this study is the first to apply it to fossils to gain insight into how extinct species might have moved. The researchers also developed user-friendly computational tools, allowing other paleontologists to investigate their own hypotheses and potentially aiding engineers in designing bio-inspired robots for navigating rough terrains.
The study identified key signals of locomotion, revealing that modern species optimized their force generation around postures typical of their movement patterns. This assurance allowed Bishop and Pierce to feel confident that their findings accurately reflected how extinct species may have moved.
The researchers' analysis showed that changes in locomotion capabilities among extinct species were neither consistent nor linear; instead, they fluctuated over millions of years. Some extinct animals appeared adaptable, shifting between sprawled and upright postures, while others displayed a regression to sprawled postures before mammals evolved. This observation suggests that the upright stance associated with modern mammals evolved later than previously believed, most likely near the common ancestor of therians.
These findings also help clarify long-standing mysteries in the fossil record. For instance, they explain why many early mammalian ancestors retained limb and joint asymmetries typical of sprawling animals and why early mammal fossils are often found in a spread-eagle pose - consistent with a sprawled posture - whereas modern mammals are found lying on their sides.
"It is very gratifying as a scientist, when one set of results can help illuminate other observations, moving us closer to a more comprehensive understanding," Bishop said.
Professor Pierce, whose lab has studied the evolution of mammalian anatomy for nearly a decade, noted that these findings align with changes observed in other body structures, like the vertebral column. "The picture is emerging that the full complement of quintessentially therian traits was assembled over a complex and prolonged period, with the full suite attained relatively late in synapsid history," she explained.
The study also suggests that evolutionary milestones, such as the shift to upright posture, were complex and could have been influenced by significant events. A notable example is the Permian-Triassic mass extinction, which may have driven synapsids into evolutionary shifts as other groups, such as dinosaurs, rose to dominance. The authors speculate that this "ecological marginalization" may have altered synapsid evolution.
Whether this hypothesis holds, Pierce emphasized that new digital modeling and computing advances are invaluable for deciphering the intricate story of mammalian evolution. "Using these new techniques with ancient fossils allows us to have a better perspective of how these animals evolved, and that it wasn't just this simple, linear evolutionary story," she remarked. "It was really complicated and these animals were probably living and moving in their environments in ways that we hadn't appreciated before. There was a lot happening and mammals today are really quite special."
Research Report:Late acquisition of erect hindlimb posture and function in the forerunners of therian mammals
Related Links
Harvard University, Department of Organismic and Evolutionary Biology
Darwin Today At TerraDaily.com
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