The unique arms of mammals began to evolve even before dinosaurs existed



PICTURE: Photographs of arm bones of seven types of parents of primitive mammals. The three bones on the left belong to an old group called pelycosaurs, and the bones are …
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Credit: (c) Jacqueline Lungmus, Field Museum

Bats fly, whales swim, gibbons sway from tree to tree, horses gallop and humans dangle on the phone – the different habitats and lifestyles of mammals depend on our unique forelimbs. No other group of vertebrate animals has developed so many different types of arms: on the other hand, all birds have wings and just about all lizards walk on all fours. Our fore members are a big part of what makes mammals special, and in a new study in the Proceedings of the National Academy of SciencesScientists have discovered that our first parents had begun to move to various fore legs 270 million years ago, 30 million years before the earliest dinosaurs existed.

"In addition to fur, the varied shape of the foreleg is one of the most iconic features of mammals," says lead author Jacqueline Lungmus, research assistant at the Field Museum in Chicago and Ph.D. the University of Chicago. "We were trying to understand where this comes from, whether it's a recent feature or it's a particular feature of the group of animals to which we belong since the beginning. "

To determine the origin of mammal weapons today, Lungmus and his co-author, Field Museum curator Ken Angielczyk, have examined the fossils of ancient mammalian parents. About 312 million years ago, terrestrial vertebrates split into two groups: sauropsids, which later included dinosaurs, birds, crocodiles and lizards, and synapsids, the group whose mammals are part of it. An essential difference between sauropsides and synapsids lies in the configuration of the openings in the skull where the jaw muscles attach themselves. While the first synapsids, called pelycosaurs, were closer to humans than dinosaurs, they looked like big reptiles. Angielczyk notes: "If you saw a pelycosaur walking down the street, you would not think it looks like a mammal – you'd say," It's a weird-looking crocodile. "

About 270 million years ago, however, a more diverse (and sometimes hairy) lineage of our family tree emerged: the therapsids. "Modern mammals are the only survivors, and this is the group we belong to today," says Lungmus. Therapidides were the first members of our family to truly diversify. Instead of being limited to crocodile-like pelycosaurs, therapids included light carnivores, sturdy burrowers and plant eaters.

Lungmus and Angielczyk investigated whether this explosion of diversity was accompanied by a corresponding explosion in different forms of fore limbs. "This is the first study to quantify the shape of the forelegs on a large sample of these animals," said Lungmus. The team examined the upper arm bones of hundreds of fossil specimens representing 73 types of pelycosaurs and therapids, taking measurements near the point where the bones joined the shoulder and elbow. They then analyzed the shapes of the bones using a technique called geometric morphometry.

When they compared the shape of the bones in the arms, the researchers found that the bones of the thereasides showed much more variation than the pelycosaurs. They also noted that the upper arm, near the shoulder, was particularly varied in the therapies – a feature that might have left them moving more freely than the pelycosaurs, whose bones were not. voluminous and well-fitted shoulders probably gave them range of motion.

Lungmus and Angielczyk discovered that a wide variety of different forms of fore limbs had evolved within the therapside 270 million years ago. "Therapaps are the first synapsids to increase the variability of their forelimb – this study dramatically repels this trait over time," says Lungmus. Prior to this study, paleontologists were the first to be able to retrace definitively the diversity of the forelegs of mammals 160 million years ago. With the work of Lungmus and Angielczyk, it has been pushed back more than a hundred million years.

The researchers note that the study helps explain how mammals have developed the traits that have made us what we are today. "A lot of what we do every day is related to how our previous members have evolved – even to simple things like holding a phone," said Angielczyk.

"It's very cool in our evolutionary lineage," says Lungmus. "These animals are part of the same group as us, and one of the reasons this research is convincing is that it is our parents."


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