The ability of mammals to develop unique arms that match their diverse lifestyles and habitats had started long before even the first dinosaurs walked the Earth.
In an article featured in Proceedings of the National Academy of Sciences, researchers in the United States found evidence suggesting that early mammals developed different forelimbs some 270 million years ago. That was 30 million years before dinosaurs appeared on the scene.
"Aside from fur, diverse forelimb shape is one of the most iconic characteristics of mammals," said Jacqueline Lungmus, a scientist at Chicago's Field Museum and co-author of the study.
"We were trying to understand where that comes from, if it's a recent trait, or if this has been something special about the group of animals that we belong to from the beginning."
Early Mammals: Pelycosaurs And Therapsids
Lungmus teamed up with Field Museum curator Ken Angielczyk to find out how mammals developed their unique forelimbs. They began their investigation by examining the fossils of the animals' ancient ancestors.
Two distinct groups of land-dwelling vertebrates lived on Earth 312 million years ago. The first of these was the sauropsids, where birds, crocodiles, dinosaurs, and lizards belonged to. The second group was the synapsids, where early mammals eventually became a part of.
Sauropsids and synapsids differed from one another based on the pattern of openings in their skull, which is where jaw muscles are attached to.
The earliest synapsids, known as pelycosaurs, were believed to be a close relative to early humans. However, they physically appeared more reptilian rather than mammalian.
"If you saw a pelycosaur walking down the street, you wouldn't think it looked like a mammal," Angielczyk noted. "You'd say, 'That's a weird-looking crocodile.'"
A more diverse and, sometimes, a furry line of mammals known as therapsids came along some 270 million years ago. Lungmus said modern mammals, including humans, can trace their lineage to these early animals.
Therapsids were the first to develop more mammalian features rather than the reptilian appearance of the pelycosaurs. These mammals went on to include groups such as burly-armed burrowers, lithe carnivores, and tree-dwelling plant-eaters.
The researchers examined whether the arrival of these different groups of mammals corresponded with the development of unique arms.
Origin Of Mammals' Unique Arms
Lungmus and Angielczyk looked at the upper arm bones fossil specimens from 73 different kinds of pelycosaurs and therapsids. They measured the area where bones joined the animals' elbows and shoulders. They also analyzed the shape of the bones using geometric morphometrics.
After comparing the mammal bones, the scientists discovered more variation in therapsid bones than in pelycosaur bones. They saw that therapsids had varying bones in the upper part of their arm, located near their shoulders.
The team believes this feature was what allowed therapsids to move more freely compared to pelycosaurs, who had larger and more tightly-fitting bones in their shoulders that severely limited their movement.
Therapsids might have developed a wide variety of forelimb shapes 270 million years ago. Lungmus said these mammals were the first synapsids who were able to increase the variability of their forelimbs. Their study dramatically pushes this mammalian trait "back in time."
Earlier paleontologists believed mammals developed their diverse forelimbs some 160 million years ago. However, Lungmus and Angielczyk's findings suggest that the event started a hundred million years before.