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The backbone is the Swiss army knife of mammalian locomotion. It can work in all kinds of ways, which allows living mammals to have remarkable diversity in their movements.
They can run, swim, climb and fly, all thanks, in part, to the vast reorganization of their spine, which has occurred over approximately 320 million years of evolution.
Open any anatomy textbook and you will find the long-held hypothesis that the evolution of the mammalian spine, which is only capable of sagittal (up and down) movements, evolved from a spine that functioned similarly to that of living reptiles, which move around. laterally (side by side). This so-called “lateral to sagittal” transition was based entirely on superficial similarities between non-mammalian synapsids, extinct mammalian precursors, and modern day lizards.
In an article published on March 2 in Current biology, a team of researchers led by Harvard University challenges the “lateral to sagittal” hypothesis by measuring the shape of the vertebrae on a large sample of living and extinct amniotes (reptiles, mammals and their extinct relatives). Using state-of-the-art techniques, they map the impact of evolutionary changes in shape on spinal function and show that non-mammalian synapsids moved their backbone in a way that was distinctly theirs and quite different from any living animal.
The team, led by first author Katrina E. Jones, former postdoctoral fellow, Department of Organism and Evolutionary Biology, Harvard University, found that although the degree of sagittal flexion increases during mammalian evolution, the spine of early synapsids was optimized for stiffness. and the evolutionary transition to mammals did not include a stage characterized by a reptile-like lateral flexion. Instead, they found that modern lizards and other reptiles have unique morphology and skeletal function that does not represent ancestral locomotion, and that the early ancestors of mammals did not move like a lizard, like scientists do. applied for it before.
“The long-held idea that there was a transition in mammalian evolution directly from lateral flexion to sagittal flexion is far too simple,” said senior author Stephanie Pierce, Thomas D. Cabot, associate professor in the Department of Organic and Evolutionary Biology and Curator of Vertebrate Paleontology. at the Harvard University Museum of Comparative Zoology. “Lizards and mammals diverged from each other millions of years ago and they each made their own evolutionary journey. We show that living lizards do not represent any sort of ancestral morphology or function that the two groups would have had in common so long ago.
Co-author Ken Angielczyk, MacArthur Curator of Paleomammalogy, Negaunee Integrative Research Center, Field Museum of Natural History, agreed: “Reptiles evolve as long as mammals and because of that there is just as much time to as changes and specializations accumulate for reptiles. If you look at the vertebrae of a modern lizard or crocodile, their vertebrae are actually quite different from those of the earliest ancestors of mammals and reptiles that lived around the same time around 300 million years ago. Living mammals and reptiles have accumulated their own set of specializations over the course of evolutionary time.
Jones and his co-authors, including former Harvard graduate student Blake Dickson, PhD ’20, began by measuring the shape of the vertebrae of a range of reptiles, mammals, salamanders, and some non-mammalian fossil synapsids. . The specimens came from museum collections around the world, with modern animal skeletons primarily from the Museum of Comparative Zoology (MCZ), and fossil synapsids from the MCZ, the Field Museum of Natural History, and various other museums in the United States. , in Europe, and South Africa.
“We first had to quantify the shape of the vertebrae and it’s actually a bit tricky,” Jones said. “Each spine is made up of several vertebrae and when you have different numbers of vertebrae, their shapes and functions can divide in different ways.”
They selected five vertebrae at equivalent locations on each of the spinal columns and measured their shapes across the different animals in three dimensions. The results showed quantitatively that the non-mammalian synapsid vertebrae are very different from the vertebrae of modern mammals, and critically also from the vertebrae of lizards and other reptiles.
Next, the researchers looked at how vertebrae functioned using data from their previous work that compared spinal shape to the degree of spinal movement in lizards and living mammals, providing a crucial link between form and function. The data allowed the researchers to map the variation in spinal function across the large sample of animals, including fossils, allowing them to reconstruct the precise combination of functional traits that described each group of animals.
“Our team’s approach to data analysis is exciting because it can reveal how different shapes of backbones can lead to different functional tradeoffs,” Pierce said. Reptiles, for example, are very good at lateral flexion, but are unable to move their spines up and down like mammals. “In addition to lateral and sagittal flexion, we also looked at other functions of the spine and then determined the optimal combination of tradeoffs for mammals, reptiles, and non-mammalian synapsids,” Pierce said.
“We were able to show that non-mammalian synapsids have a different combination of functions in their backbone for both living reptiles and mammals,” Jones said, “and during this evolution they weren’t just crossing the reptile. laterally to mammalian-like sagittal flexion, they were actually on a completely distinctive path in which they evolved from a distinct condition.
“History expects the synapse ancestors of mammals to make the same compromises as modern reptiles. But it turns out they have an entirely different set of tradeoffs, ”Angielczyk said. “The hope that reptiles retain ancestral locomotor patterns that existed over 320 million years ago is too simple.”
The results show that the skeletons of non-mammalian synapsids were in fact quite rigid and completely different from those of lizards which are very flexible in the lateral direction. In addition, during the evolution of mammals, new functions were added to this rigid ancestral foundation, including sagittal flexion in the posterior back and forward torsion. Adding these new functions has been essential in building the functionally diverse mammalian backbone, allowing modern mammals to run really fast and rotate their spines to heal their fur.
“By rigorously analyzing the fossil record, we are able to reject the simplistic lateral to sagittal hypothesis for a much more complex and interesting evolutionary story,” Pierce said. “We are now revealing the evolutionary path to the formation of the unique mammalian spine.”
The study is part of a series of ongoing projects investigating the evolution of the mammalian spine, reconstructing its development, morphology, function and evolution. “We still don’t have the full story,” Jones said, “but we’re getting closer.”
Researchers are now using three-dimensional modeling of vertebrae to understand how mammalian ancestors moved. “We are currently testing our previous studies with three-dimensional CAD-assisted models,” Jones said. “So far, it’s working pretty well and seems to confirm what we’ve found in this document.”
Header image credit: Ken Angielczyk
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