Ancient legged fish provides missing link in limb evolution

New evidence suggests that the closest known aquatic relative of our land-dwelling, legged ancestors evolved hind legs long before they went terrestrial. These animals may have even been capable of walking underwater, according to Science Daily and Live Science.

 

The findings reveal that a major step in hind leg evolution began in fish – meaning hind legs may have gotten their start as enhanced rear fins. This challenges previous theories that big, mobile, rear appendages evolved only after our backboned animal ancestors transitioned onto land.

 

The creature in question is a 375-million-year-old fish called the Tiktaalik roseae, which was discovered in northern Canada’s Ellesmere Island in 2004. Resembling something of a cross between a crocodile and a fish, it has lobe-shaped fins, sharp teeth, and a wide, flat head. At 9 feet long, scientists believe it preyed on animals that lurked in shallow freshwater.

 

^{_{Credit: Neil Shubin, University of Chicago}}

 

Fish or crocodile? 

 

The extinct Tiktaalik has been classified as a fish because of its fins, scales, and gills. Strangely enough, it also bears characteristics seen in modern tetrapods – creatures with four limbs such as mammals, birds, reptiles, and amphibians. Some such features include primitive lungs, a robust ribcage, as well as a flexible neck.

 

The ancient organism was able to support itself on ground thanks to its large forefins, shoulders, elbows, and primitive wrists. This makes Tiktaalik an excellent example of a transitional species between finned fish and legged creatures that lived on land. It also marks that evolutionary leap from water to land for animals with backbones.

 

Previous study of other fossils from the same period as Tiktaalik revealed creatures with weak back appendages but strong front legs. This seemed to imply that the oldest ancestors of tetrapods relied heavily on their front limbs to move around, and that their hips and hind legs developed only after they started living on land.

 

Even previous research on Tiktaalik revealed a strong pair of front limbs. It was not until recently that they discovered the hind limbs – when the scientists began closely examining additional blocks of rock taken from the same dig site where the first specimen of Tiktaalik was found. It took them this long to analyze the blocks because they initially thought the blocks had no further bone content. Additionally, proper removal of the rock enveloping the fragile fossil was a long and difficult process.

 

Legs, legs, legs

 

“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 paleontologist at the University of Chicago and the study’s lead author, Neil Shubin. “But it looks like this shift actually began to happen in fish, not in limbed animals.”

 

The researchers found the Tiktaalik’s well-preserved rear portion to contain hips and enough material for a partial pelvic fin. This enabled them to directly compare the back and front appendages like never before.

 

The scientists discovered that the creature’s pelvic bones were big and strong, just like early tetrapods’.

 

“I was expecting to find a diminutive hind fin and pelvis,” he said. “Seeing the whopping pelvis set me back a bit – I looked at it again and again, because I was quite surprised.”

 

The size of the creature’s pelvic girdle is almost the same as its shoulder girdle. This is a significant feature because it is very similar to the internal structure of tetrapods. Such a configuration helped the Tiktaalik support its strong hind limbs.

 

Tiktaalik also had a prominent hip joint of the ball-and-socket kind, which in turn was connected to a highly-movable femur that could extend below the body, much like the thighbone of tetrapods.

 

The hip bone possessed crests to which muscles were attached, a sign that the fins had advanced functionality and strength.

 

“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 co-author Edward Daeschler. “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.”

 

No femur bone was found. They did, however, discover pelvic fin material that consisted of fin rays of considerable length. This suggests the hind fin could have been as long and intricate as the front one.

 

“We had long thought that expanded hind limbs and hips were features of limbed animals,” stated Shubin. “Tiktaalik shows that our closest fish relatives had expanded hips and hind fins; hence, this feature may well have arisen in fish.”

 

The hip of Tiktaalik is undivided and therefore fishlike, unlike early tetrapods’, which come in three parts. Despite this difference, the pelvic girdle, hip joint, and fin are large, mobile, and sturdy enough to allow not only swimming, but other motor behaviors such as underwater walking as well.

 

“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.”

 

He added: “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 admits that Tiktaalik is not the single ancestor of every terrestrial, limbed vertebrate in the world today. It is not the “sole, direct ancestor,” he explained. “It is more like our closest cousin.”

 

The study appears in the January 13 issue of the scientific journal, Proceedings of the National Academy of Sciences of the United States of America.  — TJD, GMA News