Evolution: reptile and mammal backbones differ (Evolution)

by David Turell , Tuesday, March 02, 2021, 19:05 (1150 days ago) @ David Turell

They evolved in different directions:

https://phys.org/news/2021-03-mammal-ancestors-unique.html

"Open any anatomy textbook and you'll find the long-standing hypothesis that the evolution of the mammal backbone, which is uniquely capable of sagittal (up and down) movements, evolved from a backbone that functioned similar to that of living reptiles, which move laterally (side-to-side). This so called "lateral-to-sagittal" transition was based entirely on superficial similarities between non-mammalian synapsids, the extinct forerunners of mammals, and modern-day lizards.

"In a paper published on March 2 in Current Biology, a team of researchers led by Harvard University challenge the "lateral-to-sagittal" hypothesis by measuring vertebral shape across a broad sample of living and extinct amniotes (reptiles, mammals, and their extinct relatives). Using cutting-edge techniques they map the impact of evolutionary changes in shape on the function of the vertebral column and show that non-mammalian synapsids moved their backbone in a manner that was distinctly their own and quite different from any living animal.

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"'Lizards and mammals diverged from one another millions of years ago and they've each gone on their own evolutionary journey. We show that living lizards don't represent any sort of ancestral morphology or function that the two groups would have had in common so long ago."

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"'Reptiles have been evolving just as long as mammals and because of that there's just as much time for changes and specializations to accumulate for reptiles. If you look at the vertebrae of a modern lizard or crocodile their vertebrae are actually very different from early ancestors of mammals and reptiles that lived at the same time around 300 million years ago. Both living mammals and reptiles have accumulated their own set of specializations over evolutionary time."

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"'We were able to show that non-mammalian synapsids have a different combination of functions in their backbone to both living reptiles and mammals," Jones said, "and in the course of that evolution they weren't just traversing from the reptile-like lateral to the mammal-like sagittal bending, they were actually on a completely distinctive path in which they were evolving from a separate condition."

"'The historical expectation is that the synapsid ancestors of mammals were making the same set of tradeoffs that modern reptiles do. But it turns out that they have an entirely different set of tradeoffs," Angielczyk said. "The expectation that reptiles would retain ancestral locomotor patterns that existed over 320 million years ago is too simple."

"The results show the backbones of non-mammalian synapsids were actually quite stiff and completely unlike those of lizards which are very compliant in the lateral direction. Further, during the evolution of mammals, new functions were added to this stiff ancestral foundation, including sagittal bending in the posterior back and twisting up front. The addition of these new functions was pivotal in building the functionally diverse mammalian backbone, allowing modern-day mammals to run really fast and rotate their spine to groom 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 evolution story," Pierce said. "We are now revealing the evolutionary path towards the formation of the unique mammalian backbone.'"

Comment: A clear exposition of the continuity in evolutionary changes, but dhw objects to this process finally leading to humans.