Ancient Fish May Have Walked Underwater: Sea Creature Had Pelvis & Hip Socket Enabling It To 'Walk' With Fins
Fossils of an ancient fish with a pelvis and hip socket may have evolutionary biologists recasting their theories as to how and even when vertebrates first walked onto land. Now it looks like fish themselves were already embodying the changes necessary to make that thrilling transition. Four-limbed land creatures, which up till now have been getting all the glory of being the first to develop the kind of appendages that made crawling possible, may have to give it up to the lowly fish.
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With sharp teeth set in a broad, flat head, the nine-foot long Tiktaalik roseae fish, whose fossils are described in the online version of the Proceedings of the National Academy of Sciences, Monday, hunted in shallow fresh water about 375 million years ago. Freakish even in their day, their large fore-fins had shoulders, elbows, and partial wrists, supporting them on the river bottom much like a crocodile. And though the Tiktaalik roseae had the scales, lobed fins, and gills of fish, they also sported mobile necks, robust ribcages and primitive lungs — like, well, nothing that yet existed. "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 [four-limbed animals, which would include birds]," said Dr. Neil Shubin, Distinguished Service Professor of Anatomy at the University of Chicago and corresponding author of the study, in a press release. "But it looks like this shift actually began to happen in fish, not in limbed animals."
"It's the first time we have had decent fossil evidence of the sequence of changes leading from the 'fish' pelvic fin and girdle to that of tetrapods," Dr. Jennifer A. Clack, Professor and Curator of Vertebrate Palaeontology, University Museum of Zoology, Cambridge told the International Science Times in an email. "This change is one of the most profound that we see across the fish-tetrapod transition, but up till now we only had very scanty evidence for how and when it took place. The material shows that it was a gradual, not a sudden change."
Up till now, Shubin and co-author Dr. Edward Daeschler had only fossils of the front portion of Tiktaalik that they had dug up on an expedition to northern Canada about 10 years ago. After a recent expedition to the dig site, they discovered the Tiktaalik's tush, which contained the pelvis as well as partial pelvic fin material. "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 in a statement. "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 well-preserved fossils of a Tiktaalik roseae pelvis and partial pelvic fin suggest that the evolution of hind legs originated with the development of enhanced hind fins. The pelvic girdle was nearly identical in size to its shoulder girdle, just like in four-limbed creatures. 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 a powerful and advanced fin function. And though as of yet no one has found a femur bone to tell the tale, pelvic fin material, including long fin rays, indicates that Tiktaalik roseae's hind fin was at least as long and as complex as its fore-fin.
"The new material of the pelvis of Tiktaalik also shows that it started at the proximal end (nearest the body) with the pelvis, while other part of the appendage were still essentially fish-like," Clack told the International Science Times. "This is what we also see in the pectoral limb. We could speculate that the fins were used in a form of bottom-walking using an alternating gait, as we see in some modern bottom-dwelling fishes."
Indeed, Tiktaalik's more advanced pelvic girdle, hip joint, and fin 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, who will be hosting a three-part PBS series based on his book Your Inner Fish, in April, 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."
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