The bones of early mammals reflect the diversity of ways they moved, but current locomotor categories do not. Anne Kort is working to change that.
Back to the basics and the basics of backs: researchers study the evolution of mammalian spines
The box of lumbar vertebrae had already been on quite the trek. Housed in the University of Wyoming, they were shipped cross-country to Bloomington and were now scaling up the Midwest in the trunk of Anne Kort’s car to Minneapolis.
Kort was on her way to the University of Minnesota to analyze the bones with a micro-CT scanner in hopes of understanding how early mammals moved. Outside, temperatures approached freezing, too cold for 20-million-year-old bones. So, the 2023 IU Ph.D. awardee stopped at her mother-in-law’s home in Madison, Wisconsin, for the night. The next morning, she and the bones bid her host goodbye and carried on with their journey to the CT scanner.
What she found from those 3D scans —and from scans of 48 other species— was that placental mammals had varied shapes of lumbar vertebrae that allowed them to move in specialized ways and that this range of motion likely developed earlier than previously thought. Kort’s findings, published in the Evolutionary Journal of the Linnean Society, encourage researchers to think differently about how early mammals may have moved and to reconsider traditional locomotor classifications.
From slithering to standing
The lumbar vertebrae are a set of three to eight bones in the lower back that protect the spinal cord, guide the movement of various muscles, and support much of the body’s weight. They’re also the part of the spine that allows you to bend up and down, a range of motion unique to mammals (with a few exceptions, like crocodiles).
“Snakes and lizards are the classical example of moving side to side,” described David Polly, a Professor of Earth and Atmospheric Sciences within the College of Arts and Sciences at Indiana University Bloomington and Kort’s advisor for the project. “Mammals somewhere in our evolution went from moving that way to moving up and down," he said.
Kort first became interested in this transition after spotting an unusual spine in a cat-like mammal skeleton in the Natural History Museum of Utah. She noticed that its vertebrae fit together almost like a lock and key rather than the stack of disks seen in modern mammals. Kort reported her findings to Polly, who was immediately intrigued.
“I had never really seen that,” said Polly. The pair hypothesized that the first animals to develop up and down motion of lumbar vertebrae may have been stiff and stilted: constrained by these interlocking bones.
Yet, as Kort studied, scanned, and segmented more fossils, she found that those early placental mammals were as mobile as anything alive today. “It seems like that lumbar spine mobility was a pretty well-established feature further back than has previously been appreciated,” she said, maybe as early as the Mesozoic Era 65 to 250 million years ago.
Limitations of locomotor categories
Kort’s research revealed a second surprise. She classified each species according to their type of locomotion (running, climbing, crawling, etc.) then had a computer program do the same based solely off of the shape of each species’ lumbar vertebrae.
The computer failed, correctly classifying only seven of the 31 species tested.
But for Kort, the exercise was a great success. It demonstrated the impressive diversity of motion within locomotor groups. “Those locomotor categories have their uses, but they’re very limited,” said Kort. Take, for example, the case of sloths and squirrels.
Sloths are slow climbers. Their spines are adapted for slow movement and long periods of hanging from tree branches. Squirrels, on the other hand, are specialized for leaping, weaving through obstacles, and actively climbing. “Those are both arboreal,” said Kort, “but they’re doing it in very different ways.”
Kort’s goal is to create more comprehensive categories of locomotion that consider the unique ways that animals move. Now an assistant professor and postdoctoral scholar at the University of Michigan, Kort remains connected with Polly and IU’s William R. Adams Zooarchaeology Laboratory collections.
She visited Bloomington in December of 2023 to scan mammalian pelvic bones and will be back again, with or without a box of million-year-old bones in tow.