The surprising discovery that hind legs evolved from a kind of enhanced hind fin disputes the current belief that mobile rear appendages formed after vertebrates made their way to land.
Published in the Proceedings of the National Academy of Sciences, the study hinges on new fossils belonging to a 375 million-year-old fish species known as Tiktaalik roseae, first discovered in 2004 in northern Canada.
With a broad flat head and sharp teeth, the 9-foot monster that lurked in shallow freshwater resembled something of a cross between a fish and a crocodile. Though it had gills, scales and fins, it also had a mobile neck, sturdy rib cage and rudimentary lungs. Furthermore, its forefins were associated with shoulders, elbows and even partial wrists that enabled it to support itself on ground.
The back half of the creature remained a mystery, however, until now.
Ted Daeschler of Drexel University and Neil Shubin of the University of Chicago, both of whom were involved in Tiktaalik's discovery, have analyzed a new collection of specimens containing partial pelvic fin material and the pelves, the structure at the base of the spine the where the hind limbs attach.
According to the researchers, the pelvis was especially striking, its size comparable to those of some early tetrapods, or four-limbed vertebrates. Like tetrapods, the pelvic girdle, or bone complex supporting the hind limbs, was almost equal in size to the shoulder girdle, or set of bones supporting the upper limbs. It also had a prominent ball and socket hip joint connected to a highly mobile femur and crests on the hip for muscle attachment indicating both strength and a high degree of fin function. Although the researchers didn't find a femur bone, those remains that were present suggested that the hind fin was no shorter or less complex than 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," said Daeschler, the vice president for collections and associate curator of vertebrate zoology at his school's Academy of Natural Sciences.
"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."
The current understanding of vertebrates' transition from fins to limbs rests on "a 'front-wheel drive' idea that these animals were primarily using their front limbs for locomotion," he explained. "Hind limbs were small and not particularly involved in new patterns of movement."
Based on their findings, however, "it looks like this shift actually began to happen in fish, not in limbed animals," Shubin said.
According to Shubin, one possibility is that Tiktaalik used its hind fins for paddling, adding that it may also have used them to walk. The African lungfish, he notes, has a comparably large pelves and is able to walk underwater, as he and his team revealed in 2011.
"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.
Going forward, Daeschler said his group will continue to study vertebrate fossils from the Canadian Arctic as well as Pennsylvania that date back to the Late Devonian era, some 375-359 million years ago.
"Many of these are relevant to the fin-limb transition in the sense that we are learning about the ecosystems that were the crucible for these important evolutionary events," he told Nature World News in an email. "We will certainly continue exploring and discovering new fossils, there is much to learn."
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