Traveling through time

by Rachel Wasylyk

Channeling a childhood love of dinosaurs, Richard Blob analyzes the movements of living animals to understand shifts in evolution and their consequences in the present.

mudskippers and salamanders

A mudskipper (left) uses two fins to propel its body, as though using crutches. A tiger salamander walks by moving its front and back limbs on opposite sides of the body, as a dog does. Measuring the mechanical forces in each helps researchers make “snapshots” of evolutionary change. Image courtesy of Sandy Kawano.

While vertebrates today live in a wide range of environments, all of their ancestors originated from aquatic habitats. Animals with limbs tended to shift toward life on land while those with fins often did not make the same transition until much later in evolutionary history.
Sandy Kawano, a Ph.D. student working with Richard Blob, set out to determine what made certain animals successful over others during this momentous event. “We want to figure out how the move from fins to limbs contributed to animals living on land, and what the consequences of the anatomical changes observed in the fossil record were,” Kawano explains.

By comparing the forces that act on fins and limbs as animals travel over land, the researchers hope to understand why limbs were more successful in the transition to life on land. Kawano used a multidimensional force plate to measure horizontal and vertical forces on two species of vertebrates. Mudskipper fish use two fins to propel their bodies along the ground, much like a human walking with crutches. Tiger salamanders walk by moving their front and back limbs on opposite sides of the body in a pattern similar to that of a dog. Kawano found that both animals experienced vertical forces from the ground in response to the pressure they exerted on the plate.

But the horizontal component of the force on the mudskipper was significantly angled towards the body, while the force on the salamander only exhibited a slight angle. These differences in distribution affect how the skeleton bears the forces. With less stress on their appendages, vertebrates with limbs may have found it easier to transition toward life on land.

The physical differences in appendages may have led to the subsequent changes in the animal’s morphology and its place in the ecosystem, Blob says. “This is an exciting possibility to begin to understand, much like a time machine to the past.”

fossils

Sandy Kawano studies fossils of Seymouria, 270 to 280 million years old, to learn how animals became fully terrestrial. Seymoria was an amphibian, but its reptilian features helped it live on dry land. These specimens are at the Carnegie Museum of Natural History in Pittsburgh. Image courtesy of Sandy Kawano.

Insights into the past

Comparing living animals to fossils presents numerous challenges. Because morphology changes gradually, fossils are often only snapshots of a larger picture and may not reveal critical information about the organism’s size, shape, weight, or habitat. Scientists use patterns evident in living animals to infer the functions of animals preserved in fossils. To do this, scientists extract data from the forces exerted on the living specimens and map them on the structures of the fossils. In this way, Blob is able to infer whether a specific creature might have been able to support its body on land using its appendages. Through a thorough analysis of numerous species that span the transition from aquatic to terrestrial animals, the team may be able to pinpoint a window of time when an evolutionary change occurred.

“It’s like an enticing puzzle to figure out when major events happened in the past,” Blob says.

Blob and his team are now focused on analyzing a broad range of fossils to narrow in on the window of time that the transition occurred. Taking a cross section of the specimen is a good way to examine the morphology of a fossil, but may render the sample useless to others. Since these remnants are typically rare and highly cherished, the researchers needed a new and more effective evaluation method.

Collaborating with the Clemson School of Architecture’s digital design studio, Kawano used a 3-D laser scanner to create computer models of the fossils. From here, the image can be imported into an animation program to perform in-depth analyses of the fossil morphology and even estimate probable range of movements of the bones. The models become like virtual marionettes, allowing scientists to implement and test theories on the digital creations.

In the future, the researchers say, a 3-D printer, which uses printing technology to build up solid objects in three dimensions, could help scientists recreate physical samples of existing fossils. This would allow researchers to enlarge, cut, analyze, and reproduce specimens without destroying the original fossils. Numerous museums are interested in pursuing such endeavors to replicate and distribute models to other exhibits, and Blob plans to pursue this aspect of the work. So his time machine to the past could soon have a lot of new passengers.

mudskippers

Mudskippers are amphibious fish that can use their pectoral fins to walk on land and are especially adapted to intertidal habitats, where they hide under wet seaweed or in tidal pools. Richard Blob and Sandy Kawano study the fish to learn how animals made their way onto land. Image courtesy of Sandy Kawano.

Richard W. Blob is a professor of biological sciences in the College of Agriculture, Forestry, and Life Sciences. Rachel Wasylyk, a 2012 graduate and former editor of Decipher, a student-led research magazine at Clemson, is now a marketing coordinator and freelance writer based in Charlotte, North Carolina.

This research was made possible by grants from the National Science Foundation (IOS 0517340 and 0817794), Sigma Xi, Clemson University (Stackhouse Fellowship and Professional Enrichment Grant), the American Society of Ichthyologists and Herpetologists (Raney Award), and the Society of Vertebrate Paleontology (Estes Award). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NSF.

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