Mammalian ancestors moved in their own unique way

The backbone is the Swiss Army knife of mammalian movement. It can function in all sorts of ways with which living mammals have remarkable diversity in their movements. They can run, swim, climb and fly, in part due to the extensive reorganization of their spine, which took place during about 320 million years of evolution.

If you open any anatomy manual, you will find the long-held hypothesis that the evolution of the mammal’s spine, which is unique to sagittal (up and down) movements, evolved from a spine that was similar to that of living reptiles, which move laterally (from side to side). This so-called “lateral-to-sagittal” transition was based entirely on superficial similarities between non-mammalian synapsids, the extinct mammalian precursors, and contemporary 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 vertebral shape over a broad sample of living and extinct amniotic fluid (reptiles, mammals and their extinct relatives). Using cutting-edge techniques, they map the impact of evolutionary changes in shape on the function of the spine and show that non-mammalian synapses shift their spine in some way and are completely different from any living animal.

The team, led by first author Katrina E. Jones, former postdoctoral researcher, Department of Organic and Evolutionary Biology, Harvard University, found that although the degree of sagittal bending does increase during mammalian evolution, the backbone of the earliest synapsid is optimized is for stiffness. and the evolutionary transition to mammals did not include a stage characterized by reptilian lateral flexion. Instead, they discovered that modern lizards and other reptiles have a unique spinal morphology and function that does not represent ancestral motion, and that the earliest ancestors of mammals did not move like a lizard, as scientists have previously said. .

“The long-standing idea that there is a transition in mammalian evolution directly from lateral 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. in the Museum of Comparative Zoology at Harvard University. ‘Lizards and mammals differed millions of years ago and each went on their own evolutionary journey. We show that living lizards do not have any form of morphology or function of representing the ancestors that the two groups would have had in common so long ago. ‘

Co-author Ken Angielczyk, MacArthur Curator of Paleomammology, Negaunee Integrative Research Center, Field Museum of Natural History, agrees: “Reptiles evolve just as long as mammals and therefore there is only so much time for changes and specializations to build for. If you look at the vertebrae of a modern lizard or crocodile, their vertebrae actually differ from early ancestors of mammals and reptiles that lived at the same time about 300 million years ago.Both live mammals and reptiles have their own set of specializations about evolutionary time . ‘

Jones and co-authors, including former Harvard student Blake Dickson, Ph.D. ’20, begins by measuring the shape of the vertebrae of a series of reptiles, mammals, salamanders and some fossil non-mammal synapsids. The specimens come from museum collections around the world, with modern animal skeletons, mainly from the Museum of Comparative Zoology (MCZ), and fossil synapsids from the MCZ, the Field Museum of Natural History and several other museums in the US, Europe, and South Africa.

“We had to quantify the shape of the vertebrae first, and it’s actually a little tricky,” Jones said. “Each spine is made up of several vertebrae and if you have different numbers of vertebrae, they can divide their shapes and functions in different ways.”

They selected five vertebrae at equivalent locations from each of the vertebral columns and measured their shapes in three dimensions across the different animals. The results showed quantitatively that 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 examined how the vertebrae may have functioned using data from their previous work that compared the vertebral shape with the degree of vertebral motion in living lizards and mammals, providing an important relationship between form and function. The data enabled the researchers to map variation in vertebral function in the broad sample of animals, including the fossils, which enabled them to reconstruct the exact combination of functional characteristics that each group of animals describes.

“Our team’s approach to data analytics is exciting because it can demonstrate how different spinal shapes can result in different functional compromises,” Pierce said. Reptiles, for example, are very good at lateral flexion, but cannot move their spine up and down like mammals. “In addition to the lateral and sagittal flexion, we also examined other spinal functions and then determined the optimal combination of compromises for mammalian, reptile, and non-mammalian synapses,” Pierce said.

“We were able to show that synapsed non-mammals have a different combination of functions in their spine than living reptiles and mammals,” Jones said, “and in the course of evolution they did not just move through the reptile-like lateral to the mammalian sagittal bending, they were actually on a completely characteristic path in which they evolved from a separate state. ‘

“The historical expectation is that the synapsed ancestors of mammals make the same set of compromises as modern reptiles do. But it seems that they have a completely different compromise,” Angielczyk said. “The expectation that reptiles would retain ancestral movement patterns that existed more than 320 million years ago is too simple.”

The results show that the spine of non-mammalian synapsids was actually quite stiff and completely different from lizards that adapt very much in the lateral direction. Furthermore, during the evolution of mammals, new features were added to this stiff ancestral substrate, including the sagittal flexion in the posterior spine and anterior rotator cuff. The addition of these new features played an important role in building the functionally diverse spine of mammals so that modern mammals could run really fast and rotate their spine to care for their fur.

“By carefully analyzing the fossil record, we can reject the simplistic lateral-to-sagittal hypothesis for a much more complex and interesting evolutionary story,” Pierce said. “We are now unveiling the evolutionary path to the formation of the unique mammalian backbone.”

The study is part of a series of ongoing projects on the evolution of the spine of the mammal, summarizing its development, morphology, function and evolution. “We do not have the whole story yet,” Jones said, “but we are getting close.”

The researchers now use three-dimensional modeling of the vertebrae to understand how the ancestors of mammals moved. “We are now testing our previous studies with CAD-supported three-dimensional models,” Jones said. “So far, it works pretty well and seems to support what we found in this article.”