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Illustration showing an elderly mammalian parent, Thrinaxodon, who was part of the first group to have an extra fourth section of their spine. Credit: April Neander
Mammals are unique in many ways. We are warm and agile compared to our reptilian parents.
But a new study, funded by the National Science Foundation (NSF) and led by Stephanie Pierce and Katrina Jones, researchers at Harvard University, suggests that we are unique more: the composition of our spines. Researchers describe their discovery in an article published this week in the journal Science.
"The spine is basically like a series of beads on a string, with each pearl representing a single bone – a vertebra," said Pierce, curator of Vertebrate Paleontology at Harvard. "In most four-legged animals, like lizards, the vertebrae all look the same.
"But the dorsal spines of mammals are different, the different sections or regions of the spine – such as the neck, thorax and lower back – take very different shapes, they work separately and can adapt to different lifestyles and climbing. "
While the backbones of mammals are specialized, the regions underlying them are considered to be old, dating from the first terrestrial animals.
Mammals have made the most of the existing anatomical model, or at least scientists have believed. However, the new study challenges this idea by examining the fossil record.
Edaphosaurus, a parent of the first mammals that lived about 300 million years ago, had a more primitive spine with only three different regions. Credit: Field Museum
"There are no live animals today that record the transition from a" lizard-like "ancestor to a mammal," said Jones, lead author of the study. "To do this, we need to dive into the fossil record and look at extinct mammalian precursors, non-mammalian synapsids."
These ancient ancestors hold the key to understand the origin of the specific characteristics of mammals, including the spine.
But studying fossils is not easy. "Fossils are rare and finding missing animals with more than 25 vertebrae is incredibly rare," Jones said.
To tackle this problem, researchers examined museum collections around the world to study the best preserved animal fossils that lived 320 million years ago.
"Looking back, an early change in mammalian vertebral columns was an important first step in their evolution," said Dena Smith, NSF's Program Director, Earth Sciences Division. "Changes in the spine over time have allowed mammals to become the myriad of species we know today."
Skeletons of a modern dog and cat – note the regions with different forms of bone that make up the spine. Credit: Field Museum
Pierce and Jones, along with co-author Ken Angielczyk of the Field Museum in Chicago, examined dozens of fossil spines, as well as more than 1,000 live animal vertebrae, including mice, alligators, lizards and amphibians.
They wanted to know if the vertebral areas of mammals were as old as we thought or if the mammals were doing something unique.
"If the vertebral regions had remained unchanged throughout the evolution, as expected, we would expect to see the same regions in non-mammalian synapsids that we see in mammals today," Pierce said. .
But that does not seem to be the case. When the researchers compared the positioning and shape of the vertebrae, they found something surprising. The spine had acquired new areas during the course of mammalian evolution.
"The first non-mammalian synapsids had fewer areas than live mammals," Jones said.
About 250 million years ago, a new region evolved near the shoulders and front legs. Dramatic changes have also begun to appear in the forelimbs of animals known as non-mammalian therapsids.
According to scientists, these simultaneous developments probably took place alongside changes in the way creatures walked and ran.
The three stages of mammal skeleton evolution on a phylogenetic tree. Bottom right: Edaphosaurus; left center: Thrinaxodon; up: a modern mouse. Credit: Stephanie E. Pierce, Museum of Comparative Zoology, Harvard University
"There seems to be some kind of interference during development between the tissues that form the vertebrae and the shoulder blade," Pierce said. "We believe that this interaction has resulted in the addition of a region near the shoulder while the earlier members of our ancestors have evolved to take on new forms and functions."
Later, a region emerged near the basin. "It's this last region, the lumbar region without a queen, that seems to be able to adapt the most to different environments," said Pierce.
The final step in the construction of the backbone of a mammal may be related to modifications of the Hox genes, important for the spinal regions early in their development.
"We have been able to make connections between changes in skeletons of extinct animals and ideas in modern biology and developmental genetics," Jones said. "This combined approach helps us understand what makes a mammal a mammal."
Explore more:
Tiny fossils reveal how reduction was essential for successful evolution
More information:
K.E. Jones et al., "Fossils reveal the complex evolutionary history of the regionalized mammalian spine", Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aar3126
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