Vertebrate life began in the water, but around 340-360 million years ago, four-limbed creatures, or tetrapods, made the transition onto land. In the years that followed, some species adapted to terrestrial life, while others turned back to the water and readapted to an aquatic lifestyle.
A new study of these early amphibians, published in the journal PLOS ONE and led by Penn paleontologist Aja Carter, suggests that these environmental shifts left an impression--on the shape of the animals' spines.
"I'm interested in how the shapes of the vertebrae affect how animals move," she says. "Our findings suggest that, in at least one part of the vertebrae, the shape of the bones correlated with the environment in which the animals were living." Those associations, Carter says, may be a result of the different physical demands of living on land versus in the water.
Diversity, now and then
Many characteristics distinguish creatures as diverse as humans, dogs, snakes, and even Stegasaurus, but one thing that unites them is the basic shape of their vertebrae.
"They all have this hockey puck-shaped thing, the centrum, fused to these sticky-outy parts, the neural arch, the transverse processes," says Carter, a paleontologist who earned her doctoral degree from Penn's Department of Earth and Environmental Science in the School of Arts & Sciences and is now a postdoctoral fellow in the School of Engineering and Applied Science. "But that's not always how it's been."
Go back further in evolutionary time, Carter says, starting even before dinosaurs arose, and one will find a highly diverse set of vertebral shapes. Yet it's not clear why all this diversity was present, or why vertebrates today have landed on one basic vertebral structure almost without exception.
Scientists have long debated whether a particular vertebral shape was important for these animals to colonize land or subsequently recolonize water. To explore the question, Carter focused on early amphibians, a group known as temnospondyls, which were the most diverse tetrapods living in the Carboniferous through the late Permian, or about 360 to 250 million years ago. Some lived exclusively on land, some exclusively in water, and some were amphibious. In addition, some that were known to be terrestrial had evolved from ancestors that were aquatic, and vice versa.
"If we want to understand macro-evolutionary trends, we need a lot of specimens," Carter says. "There are about 290 species of temnospondyls, and they come in different sizes, and we know a lot about them. So this was a great group to focus on."
Shapes by the numbers
Though previous studies had used qualitative techniques to assess the relationship between vertebral form and a land- or water-based lifestyle, Carter and colleagues opted to use a quantitative approach known as geometric morphometrics. This strategy quantifies shape by assigning points to the outline of a structure. In the current study, they used a two-dimenisonal approach, imagining the outline of the vertebrae viewed from the side.
The researchers focused in particular on the shape of two parts of the vertebrae, the intercentrum, a weight-bearing structure, and the neural arch, sites of muscle attachment. Many in the field presumed that a land-based lifestyle required a stiffer spine than one in water, so the team's prediction was that terrestrial temnospondyls would have longer neural arches and longer intercentra to restrict flexibility through the spine.
Yet that's not what they discovered. Instead, they found no association between the neural arches and the environment in which they animals lived. "That suggests there is some plasticity," Carter says. In other words, there is not just one type of neural arch that enables successful movement on land.
The intercentrum shape, however, did correlate with environment and not in the way that that earlier scientists had predicted. Terrestrial species tended to have shorter, more curved intercentra, while aquatic and semi-aquatic animals had intercentra that were taller and flatter.
"The intercentra are weight-bearing, and these seem to fall out based on environment," says Carter. "But the neural arches' shape don't seem to bend to the constraints that we think of in terms of terrestrial versus aquatic."
Indeed, in terrestrial species, the researchers found some with a high degree of spinal flexibility, contrary to previous beliefs, Carter says. "In fact, in our results, a lot of terrestrial taxa seemed to have spines that were a lot less stiff than their aquatic counterparts."
In addition, aquatic species that were known to have ancestors that had lived on land maintained morphologies more akin to their terrestrial counterparts. "They don't go back," she says.
Questions for the future
Carter notes, however, that there are other ways to gain spinal flexibility, including increasing the total number of vertebrae, but the new findings still buck earlier notions of what morphologies enabled a successful move to the land.
"This tells us there is more diversity than what these labels--terrestrial, aquatic--are saying, when it comes to vertebral composition and shape," Carter says.
Carter acknowledges that paleontological studies such as these leave a lot of room for doubt. Measurements are coming from fossils that may have been reshaped during the fossilization process, for example. Plus, muscle attachments, lost to the fossilization process, would have a significant impact on movement. So she won't be surprised if future studies challenge these findings.
"Science is both iterative and overturned all the time," she says.
Yet in the not-so-distant future, in her new position at Penn Engineering, Carter hopes to build paleontologically inspired robots in which she could test how differently formed vertebrae impact funcational movement.
"I'm learning that's going to be a difficult challenge, but I'm excited to work on it," she says.
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Aja Carter is a Vice Provost Postdoctoral Fellow for Academic Diversity in the University of Pennsylvania School of Engineering and Applied Science.
Carter's study coauthors were Temple University's S. Tonia Hsieh and Penn's Lauren Sallan and Peter Dodson.
Journal
PLoS ONE