January 14, 2014
New Fish Fossil Reveals How Fins May Have Become Hind Limbs
[ Watch the video: Tiktaalik roseae fossils reveal evolution of hind limbs ]
April Flowers for redOrbit.com - Your Universe OnlineThe evolution of hind legs actually began as enhanced hind fins, according to the newly discovered, well-preserved pelvis and a partial pelvic fin from Tiktaalik roseae—a 375 million-year old transitional species between fish and the first legged animals.
The findings, published in Proceedings of the National Academy of Sciences (PNAS), challenge the existing theory that large, mobile hind appendages were developed only after vertebrates transitioned to land.
"Previous theories, based on the best available data, propose that a shift occurred from 'front-wheel drive' locomotion in fish to more of a 'four-wheel drive' in tetrapods," said Neil Shubin, PhD, Robert R. Bensley Distinguished Service Professor of Anatomy at the University of Chicago. "But it looks like this shift actually began to happen in fish, not in limbed animals."
Tiktaalik roseae were discovered in 2004 by Shubin and Edward Daeschler, PhD, Associate Curator of Vertebrate Zoology at the Academy of Natural Sciences of Drexel University, and the late Farish A. Jenkins, Jr., PhD, of Harvard University. The species represents the best-known transitional species between fish and land-dwelling tetrapods.
Tiktaalik was a lobe-finned fish with a broad flat head and sharp teeth that looked like a cross between a fish and a crocodile. The fish could grow up to a length of 9 feet as it hunted through shallow freshwater environments. Along with gills, fins and scales, the Tiktaalik had tetrapod-like features such as a mobile neck, robust ribcage and primitive lungs. The fish also had shoulders, elbows and partial wrists on its large forefins, which allowed it to support itself on ground.
Researchers have described only specimen blocks containing the front portion of Tiktaalik, until now. The current research team investigated additional blocks recovered from the original and subsequent expeditions to the dig site in northern Canada, finding the rear portion of Tiktaalik. These rear portions contained the pelvis as well as partial pelvic fin material. The complete pelvis of the original "type" specimen was also included in the discovery, making a direct comparison of the front and rear appendages of a single animal possible.
The pelvis, which was comparable to those of some early tetrapods, immediately struck the scientists as unique. The pelvic girdle of Tiktaalik was nearly identical in size to its shoulder girdle -- this is a distinct tetrapod-like characteristic. The fish also possessed a prominent ball and socket hip joint, which connected to a highly mobile femur that could extend beneath the body. Advanced fin function and strength were indicated by crests on the hip for muscle attachment. No femur bone was found, however, pelvic fin material -- including long fin rays -- indicated the hind fin was at least as long and complex as the forefin.
"This is an amazing pelvis, particularly the hip socket, which is very different from anything that we knew of in the lineage leading up to limbed vertebrates," Daeschler said. "Tiktaalik was a combination of primitive and advanced features. Here, not only were the features distinct, but they suggest an advanced function. They appear to have used the fin in a way that's more suggestive of the way a limb gets used."
With primitive features such as an undivided skeletal configuration -- as opposed to the three-part pelvic girdle of early tetrapods, the Tiktaalik pelvis was still clearly fish-like. The expanded size, robusticity, and mobility of the pelvic girdle, hip joint and fin of Tiktaalik made a wide range of motor behaviors possible.
"It's reasonable to suppose with those big fin rays that Tiktaalik used its hind fins to swim like a paddle," Shubin said. "But it's possible it could walk with them as well. African lungfish living today have similarly large pelves, and we showed in 2011 that they walk underwater on the bottom."
"Regardless of the gait Tiktaalik used, it's clear that the emphasis on hind appendages and pelvic-propelled locomotion is a trend that began in fish, and was later exaggerated during the origin of tetrapods," Shubin said.
Image Below: This is a close up view of the hip bone of Tiktaalik roseae. Credit: University of Chicago, Kevin Jiang