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5 hours ago
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Early mammal evolution of a more upright posture was a transformative event. New research shows that this development, however, didn’t occur in a straight line, but was a highly complex and dynamic evolutionary process.
All mammals trace their origins to the earliest stem mammals more than 300 million years ago – nearly 100 million years before the first dinosaurs. These mammal ancestors, called synapsids, evolved from early reptiles.
Some synapsids grew to be massive, dominating the Earth until the greatest mass extinction event in history about 252 million years ago at the end of the Permian period.
That mass extinction ushered in a new age where our early mammal ancestors continued to evolve as mostly small, scampering, nocturnal creatures under the shadow of the colossal dinosaurs.
Mammals are now the dominant creatures on land, largely thanks to the evolutionary foundations of those pioneering stem mammals. One major factor was the development from the reptile-like sprawling mode of locomotion to a more upright (parasagittal) posture.
The study, published in PLOS Biology, shows that the musculoskeletal changes required for this development occurred in a much more complex way than previously thought.
“The origin of upright mammalian posture is a key part of their evolutionary story,” says lead author Robert Brocklehurst from Harvard University’s Museum of Comparative Zoology (MCL) in the US. “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.”
Brocklehurst’s team analysed the humerus (upper arm) bones of more than 200 species of tetrapod – 4-legged animals. They compared fossil bones to those of living animals, including salamaders, reptiles, upright therian mammals and monotremes.
Monotremes proved to be critical in the study because this strange mammal group – today represented only by echidnas and platypuses – shares many reptilian features including egg-laying and a more sprawled stance.
They examined humerus length, torsion, muscle leverage, bending strength and rotational ability. The relationship between the bone’s shape and function was then modelled on a computer to visualise different evolutionary paths of mammal ancestors.
“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 senior author Stephanie Pierce, also at MCL. “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 co-author Kenneth Angielczyk from the University of Chicago, Illinois also in the US. “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 research points to a period of diversification in stem mammals, rather than a straight-line path to an upright posture.
“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,” the authors write. “These fossils weren’t stepping-stones, they were animals evolving to explore a wide range of ecologies, niches and habitats.”
