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Posted by on Jan 14, 2014 in Evolution, Science | 8 comments

Discovery of New Tiktaalik Roseae Fossils Reveals Key Link in Evolution of Hind Limbs

Science Daily:

Jan. 13, 2014 — The discovery of well-preserved pelves and a partial pelvic fin from Tiktaalik roseae, a 375 million-year-old transitional species between fish and the first legged animals, reveals that the evolution of hind legs actually began as enhanced hind fins. This challenges existing theory that large, mobile hind appendages were developed only after vertebrates transitioned to land. The fossils are described by scientists in the Proceedings of the National Academy of Sciences, online on Jan. 13.

This is an updated illustration of Tiktaalik roseae in its natural environment. (Credit: University of Chicago, Neil Shubin)

“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 and corresponding author of the study, which marks his inaugural article as a member of the National Academy of Sciences. “But it looks like this shift actually began to happen in fish, not in limbed animals.”

Discovered in 2004 by Shubin and co-authors 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, Tiktaalik roseaerepresents the best-known transitional species between fish and land-dwelling tetrapods.

A lobe-finned fish with a broad flat head and sharp teeth,Tiktaalik looked like a cross between a fish and a crocodile, growing up to a length of 9 feet as it hunted in shallow freshwater environments. It had gills, scales and fins, but also had tetrapod-like features such as a mobile neck, robust ribcage and primitive lungs. In particular, its large forefins had shoulders, elbows and partial wrists, which allowed it to support itself on ground.

However, only specimen blocks containing the front portion ofTiktaalik have been described thus far. As the researchers investigated additional blocks recovered from their original and subsequent expeditions to the dig site in northern Canada, they discovered the rear portion of Tiktaalik, which contained the pelves as well as partial pelvic fin material. The fossils included the complete pelvis of the original ‘type’ specimen, making a direct comparison of the front and rear appendages of a single animal possible.

The scientists were immediately struck by the pelvis, which was comparable to those of some early tetrapods. The Tiktaalikpelvic girdle was nearly identical in size to its shoulder girdle, a tetrapod-like characteristic. It possessed a prominent ball and socket hip joint, which connected to a highly mobile femur that could extend beneath the body. Crests on the hip for muscle attachment indicated strength and advanced fin function. And although no femur bone was found, pelvic fin material, including long fin rays, indicated the hind fin was at least as long and as complex as its 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.”

Tiktaalik pelves were still clearly fish-like, with primitive features such as an undivided skeletal configuration, as opposed to the three-part pelvic girdle of early tetrapods. However, the expanded size, mobility and robusticity 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.

Shubin will be hosting a three-part TV series based on his book “Your Inner Fish,” on PBS in April 2014, tracing the origins of the human body through the DNA of living animals and the legacies of now-extinct, but biologically important species such as Tiktaalik roseae.

  1. Neil H. Shubin, Edward B. Daeschler, and Farish A. Jenkins, Jr. Pelvic girdle and fin of Tiktaalik roseaePNAS, January 13, 2014 DOI: 10.1073/pnas.1322559111

 

University of Chicago Medical Center (2014, January 13). Discovery of new Tiktaalik roseae fossils reveals key link in evolution of hind limbs.ScienceDaily. Retrieved January 13, 2014, from http://www.sciencedaily.com­/releases/2014/01/140113154211.htm

  • Seth R. Massine

    I love how there are no comments on this article yet…hmm. Fascinating stuff, though. I read Your Inner Fish, and it was very illuminating. Just imagine how many fossils are under our feet right now; each a chapter in the book of life. Great post, and great blog. Keep up the awesome work

    • http://www.skepticink.com/tippling/ Jonathan MS Pearce

      Thanks for commenting Seth!v Stick around for some fun! I read Shubin’s book some time ago and have just finished Coyne’s Why Evolution Is True, which has been on my shelf for ages. They are both great books, but I particularly enjoyed Coyne’s for being comprehensive – a good section on biogeography.

      • Seth R. Massine

        I loved Coyne’s book. Very broad and comprehensive. Have you read Dawkin’s The Greatest Show on Earth? That’s another gem

        • http://www.skepticink.com/tippling/ Jonathan MS Pearce

          I listened to the audio book. It’s not really a case of gaining new knowledge with these books, but the synthesis and bringing it all together in one place.

          That said, I did learn about the Gondwana situation with patterns of fossilised fallen trees from glacial movement in Coyne’s which is really nice evidence of a split continent etc.

          • Seth R. Massine

            That’s how I’m in it as well. Cohering information helps paint a more vivid and fascinating view of reality. On a different note, I’ve been researching free will–determinism lately (I used to be a dualist, but that view was shattered), and was wondering if you could recommend any good books on the subject? That is, written by someone who isn’t trying to trick themselves into believing that there’s any credence to free will, of any variety.

          • http://www.skepticink.com/tippling/ Jonathan MS Pearce

            Well, it would be remiss of me not to advocate getting hold of mine…. – from the sidebar just over there! Free Will?

            Other than that, well, it depends what sort of area: philosophy, neuroscience, psychology…

          • Seth R. Massine

            I’m gonna order your book John, thank you :)

          • http://www.skepticink.com/tippling/ Jonathan MS Pearce

            Thanks mate! It is a good introduction from which you can launch into more in depth works, though they will become drier, the more hardcore they get.