Filling the Terrestrial Gap
The recent discovery above the Arctic Circle of remarkably well preserved fossils from a new species of ancient fish provides a key marker in the evolutionary transition of fish to limbed animals.
|Dr. Ted Daeschler of The Academy of Natural Sciences in Philadelphia co-led the expedition to the Canadian Arctic that discovered a new species of ancient fish.
Credit: The Academy of Natural Sciences
In two related articles highlighted on the April 6 cover of the journal Nature, Dr. Ted Daeschler of The Academy of Natural Sciences in Philadelphia, co-leader of the expedition to Ellesmere Island, and his colleagues announced the discovery of 375-million-year-old fossils with numerous features that place them squarely at the evolutionary transition from fish to limbed animals. The new species has a skull, neck, ribs and part of a fin like the earliest limbed animals, but also has fins and scales like a fish.
The new species, named Tiktaalik roseae, shows that the evolution from life in water to life on land happened gradually in fish living in shallow water.
For about a century, scientists have been able to trace the broad outline of the millions-of-years-long transition of lobe-finned fish to limbed animals (tetrapods). The new find, however, is the most compelling evidence yet of an animal that was on the verge of the transition from water to land. "The find is a dream come true," said Daeschler, the Academy’s curator of vertebrate biology. "We knew that the rocks on Ellesmere Island offered a glimpse into the right time period and were formed in the right kinds of environments to provide the potential for finding fossils documenting this important evolutionary transition."
Tiktaalik was a predator with sharp teeth, a crocodile-like head, and a flattened body that lived in what was then a subtropical climate. The quality of the fossils allowed the team to examine the joint surfaces on many of the fin bones and figure out that shoulder, elbow and wrist joints were capable of supporting the body like limbed animals. "Tiktaalik blurs the boundary between fish and land animals," said Dr. Neil Shubin of the University of Chicago, the other co-leader. "This animal is both fish and tetrapod; we jokingly call it a fishapod."
The expedition uncovered the fossils in 2004 in a remote valley of Ellesmere Island, more than 600 miles north of the Arctic Circle, in Canada’s Nunavut Territory. It was the fourth summer they had spent there amassing a diverse array of fossil fish dating to the late Devonian Period (380-365 million years ago). Tantalizing fragments uncovered in 2000 convinced the scientists to return to the site.
The fossils were recovered from the layered rock of the so-called Fram Formation, the deposits of meandering stream systems formed some 375 million years ago when North America was part of a supercontinent straddling the equator. These fossils and previously known fossil relatives suggest the evolution from fish to tetrapod occurred on this landmass. "This kind of shallow stream system seems to be the place where many features of land living animals first arose," said Daeschler.
The skeletal structure of Tiktaalik and the nature of the deposits where it was found suggest an animal that lived on the bottom of shallow waters and perhaps even out of the water for short periods. "The skeleton of Tiktaalik indicates that it could support its body under the force of gravity whether in very shallow water or on land," said Dr. Farish A. Jenkins of Harvard University, another collaborator. "This represents a very critical early phase in the evolution of all limbed animals, including us."
|Progression from land to sea, legs to fins, walking to swimming. "It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted?…Breeders habitually speak of an animal’s organization as something quite plastic, which they can model almost as they please." –Darwin, Origin of Species|
Naming the fossils
Instead of using the traditional Latin or Greek to name the fossil, the team consulted Nunavut residents, who suggested Tiktaalik (tic-TA-lick), the Inuktikuk word for large, shallow water fish. The second part of the name, roseae, honors an anonymous supporter. Other funding came from the National Science Foundation, National Geographic Society and the researchers’ home institutions.
Tiktaalik has a flattened, triangular-shaped skull reminiscent of the earliest tetrapods. Although the lower jaws and snout have fish-like features, the rear portion of the skull looks more like a limbed animal. The skull is significantly shortened behind the eye sockets and has deep notches in its rear margin. The bones that connect the skull to the shoulders in fishes are not found in Tiktaalik, also hinting at its tetrapod-like nature. An intermediate stage in the transition from fin to limb is also exhibited in the bones of the pectoral fins, which show robust skeletal elements indicative of powerful and mobile appendages, flexible at shoulder, elbow and wrist, while retaining a reduced set of the thin rods found in fish fins. The wide, flattened body of Tiktaalik is also tetrapod-like but is covered by scales as in fish.