The shock study shows that the evolution mammalian forelimbs began to emerge 270 million years ago, some 40 million years before the first dinosaurs emerged during the Triassic period. The limbs in question belonged to a group of ancient mammals known as synapsids, which split away from sauropsids who went on to include dinosaurs, birds, crocodiles, and lizards, about 312 million years ago. The research also shows that no other vertebrate has evolved so many different limbs – for example, birds have wings, and practically all lizards walk on all fours.
Study author Jacqueline Lungmus, a research assistant at Chicago’s Field Museum and a doctoral candidate at the University of Chicago said: “Aside from fur, diverse forelimb shape is one of the most iconic characteristics of mammals.
“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.”
In spite of this, the earliest synapsids, called pelycosaurs, looked like massive bulking lizards, but they are in fact more closely related to us than they are the dinosaurs.
Study co-author and Field Museum curator Ken Angielczyk, said: “If you saw a pelycosaur walking down the street, you wouldn’t think it looked like a mammal – you’d say, ‘That’s a weird-looking crocodile.'”
The team examined the upper arm bones of hundreds of fossil remains of therapsids, another mammalian ancestor, and pelycosaurs, to reach their conclusion.
Ms Lungmus added: “Modern mammals are the only surviving therapsids. This is the group that we’re part of today.
“Therapsids were the first members of our family to really branch out. Instead of just croc-like pelycosaurs, the therapsids included lithe carnivores, burly-armed burrowers, and tree-dwelling plant-eaters.
“This is the first study to quantify forelimb shape across a big sample of these animals.
The therapsids are the first synapsids to increase the variability of their forelimbs. This study dramatically pushes that trait back in time.
“Prior to this study, the earliest that paleontologists had been able to definitively trace back mammals’ diverse forelimbs was 160 million years ago.
“Our work has pushed this back by more than a hundred million years.”
“This is something that’s so cool about our evolutionary lineage.
“These animals are in the same group as us. Part of what makes this research compelling is that these are our relatives.”