Amphibious Ancestors
Vertebrates' transition to dry land took some fancy footwork
Imagine a scale-covered fish that uses fleshy limbs that end in fins to haul itself out of the water. Its mosaic of body features also includes sturdy ribs, the first vertebrate neck, and both gills and lungs. Paleontologists recently unearthed fossils of such a creature, which met their expectations for a proposed missing link between fish and the earliest land vertebrates. These relics derive from an era that corresponds to a 9-million-year gap in the vertebrate fossil record.
The new fossils turned up in an Arctic region decades ago pinpointed as a likely location for a transitional creature that would be well adapted to life in the shallows but also mobile on land. Some paleontologists are predicting that the species will become an evolutionary icon as important as Archaeopteryx, the first bird.
These fossils and a reexamination of others found decades ago are providing insights into ancient vertebrates’ transition from water to land. Scientists are also developing a new understanding of how some of the earliest land vertebrates moved.
Up north
Situated far north of the Arctic Circle, Canada’s Ellesmere Island is a cold, harsh place. Temperatures rarely rise above the freezing point, soil and vegetation are sparse at best, and the weather is so foul that paleontologists can conduct fieldwork for only a month or so each summer. Even during those short stints, blustery conditions can make prospecting for bones difficult.
After spending several days during the 2004 field season hammering at the ice-covered rocks in one of Ellesmere’s bluffs, researchers spied a portion of a fish skull protruding from the stone. “That’s ideal, having the snout sticking out, because in the cliff behind it is likely the rest of the animal,” says Neil H. Shubin, a University of Chicago paleontologist who was on that expedition.
Deeper digging in the rocks—laid down as floodplain sediments some 382 million years ago—did indeed reveal the near-complete remains of a 1.3-meter-long fish unlike any previously discovered. About 8 m away, the researchers found another specimen, this one twice as long as the first. And then, they found yet another, close in size to the first.
“Within 2 weeks, we had three specimens of a creature that we knew was sitting at the cusp of the transition between aquatic and land-dwelling animals,” says Shubin. The fossils revealed all the hallmarks expected of a missing link between fish and land vertebrates. The team gave the animal the genus name Tiktaalik, which means “large, freshwater fish” in Inuktitut, a native language of the region that includes Ellesmere Island.
The upper surface of the Tiktaalik body was covered with distinctive bony scales similar to those of other fish of the same era. Grooves along certain bones at the rear of the creature’s skull hinted that Tiktaalik had well-developed gills, says Edward B. Daeschler of the Academy of Natural Sciences in Philadelphia.
However, the fossils don’t include an operculum, or gill cover, that would have protected the delicate structures and moved oxygenated water across them. The lack of that bony structure freed the animal’s skull from its shoulder girdle, giving the creature’s spinal column extra flexibility and, in essence, a neck. Tiktaalik is the only fish known to have had a neck, notes Daeschler. He, Shubin, and their Harvard colleague Farish A. Jenkins Jr. describe the creature in the April 6 Nature.
Tiktaalik probably relied on both gills and lungs to obtain oxygen. The creature’s broad, flat head, which was shaped like a bellows, would have enabled it to gulp air efficiently, the technique used by early air breathers to transfer oxygen into their primitive lungs.
Other physical features highlight Tiktaalik‘s transitional position between fish and land vertebrates. The creature’s four fleshy appendages contained bones analogous to the upper- and lower-limb bones of all subsequent tetrapods. These protolimbs could flex at the shoulder, elbow, hip, and knee just as the limbs of people and other modern land animals do. That range of motion and the length of the protolimbs enabled Tiktaalik to assume a posture in which the appendages bear weight.
Unlike the fins of its ancestors, those at the end of Tiktaalik‘s protolimbs included bones analogous to those in human wrists and fingers. Analyses of how those Tiktaalik bones fit together suggest that the creature’s fins could flex outward about 90°, placing them in a position like that of a person’s hand when doing a push-up, says Shubin.
Such a configuration would have provided a stable yet flexible extremity that could conform to a variety of surface textures and terrains. Therefore, Tiktaalik was well adapted to walking along the bottoms of lakes and streams, wading in the shallows, and even walking on land if necessary. “It was one of the world’s first multitaskers,” quips Shubin.
Says Jennifer A. Clack, a vertebrate paleontologist at the University of Cambridge in England, “This really is what our ancestors looked like when they began to leave the water.”
The creature’s body structure hints that Tiktaalik could also have spent considerable time out of water. Unlike the rodlike ribs of a typical fish, those of Tiktaalik are broad, flat, and overlapping. Such an arrangement provides a stiffer body, which would have better supported the creature when it made excursions onto land. Fish that never left the water, where buoyancy counteracts gravity, wouldn’t have needed such ribs, says Daeschler.
Into the gap
About 385 million years ago, what is now Ellesmere Island was part of a continent that straddled the equator and reached what are today subtropical latitudes. Scientists have dubbed that landmass Euramerica because it later broke apart to form Europe and North America. The fern-filled rain forests that covered the region teemed with wingless insects and other arthropods, and the tropical rivers and coasts were crowded with fish.
The earliest known tetrapods lived in regions that became Latvia and Scotland about 376 million years ago. So, paleontologists suspected that the vertebrates of the water-to-land transition lived in the waters of Euramerica. Waters of the future Latvia produced a fish called Panderichthys, previously the closest known relative to early tetrapods. Its protolimbs were fleshy and ended in fins. So, Shubin and his colleagues looked to the 382-million-year-old rocks of the once-nearby Ellesmere Island for an earlier transitional creature.
The well-preserved remains of Tiktaalik go a long way, but not quite the whole way, toward filling the gap in the fossil record between the earliest tetrapods and the lobe-finned fish that preceded them, says Clack. Even though the bones in Tiktaalik‘s fin resemble those of tetrapod digits, they’re still very much part of a fin. If the digits of early tetrapods evolved from these bones, the process must have involved considerable changes in anatomical development, Clack notes.
Much of the importance of transitional fossils such as Tiktaalik lies in how they resemble and differ from their nearest neighbors on life’s family tree, Clack comments in the April 6 Nature.
Tiktaalik wasn’t a full-time land dweller, which indicates that the evolutionary development of weight-bearing limbs began early. But why would aquatic animals leave the water? Land life presents many difficulties. Besides having to deal with gravity, animals on land run the risk of desiccating and are exposed to damaging wavelengths of solar radiation that are blocked or diminished by water.
Plants had colonized dry land, and arthropods such as giant millipedes and wingless insects scurried through the resulting ecosystems. However, the tooth structure of Tiktaalik suggests that it—and possibly other such experiments in evolution—weren’t exploiting those previously untapped food sources, says Shubin. It’s more likely that such creatures, not wanting to become a meal themselves, were escaping aquatic predators, he proposes.
Water quality might have been another evolutionary pressure driving fish out of water. The evolution of plants that shed their leaves on a regular basis resulted in large amounts of organic material ending up in rivers, streams, and coastal waters, Clack notes. The decomposition of that material, along with the plants and animals that those nutrients nourished, robbed the water of oxygen. Because gases aren’t as soluble in warm water as they are in cold water, long-term warming of the climate exacerbated the decrease in oxygen dissolved in the world’s waters.
At the same time, plants, including those that had expanded onto land, were pumping ever-increasing amounts of oxygen into Earth’s atmosphere (SN: 12/17/05, p. 395: Available to subscribers at Changes in the Air). This made breathing air a more efficient way to obtain oxygen.
On the move
As Tiktaalik‘s successors spent more time on land, fins evolved into digits. The best known of these early land-dwelling tetrapods are Ichthyostega and Acanthostega, both of which lived about 365 million years ago in what is now Greenland. Nevertheless, some aspects of Ichthyostega‘s anatomy, including the structure of its ear, suggest that the creature spent a significant amount of time in the water (SN: 9/13/03, p. 173: Available to subscribers at Fossils’ ear design hints at aquatic lifestyle).
Paleontologists have unearthed hundreds of specimens of Ichthyostega. However, all those fossils are fragmentary, and no single specimen includes a complete spinal column, says Clack. Previous reconstructions of Ichthyostega, which were typically based on just a few bones, portrayed all the creature’s vertebrae, from its neck to its tail, as being similar. However, a new analysis by Clack and her colleagues Per Ahlberg and Henning Blom of Uppsala University in Sweden indicates that those interpretations are probably wrong.
In their study, the researchers scrutinized the six Ichthyostega specimens that include substantial portions of the creature’s vertebral column. Clack and her colleagues found that the vertebrae in some segments of Ichthyostega‘s spinal column were shaped differently than those in other segments. In particular, the bony flanges that protrude upward from the vertebrae—structures called neural arches—were tilted at different angles in four different regions. These differences hadn’t been seen in previous reconstructions.
Ichthyostega is the earliest known tetrapod to have had a spinal column segregated into such sections, says Clack. She, Ahlberg, and Blom described their reconstruction in the Sept. 1, 2005 Nature.
Like Tiktaalik but unlike fish, Ichthyostega had wide ribs, reports Clack. The ribs were so broad that, per her team’s reconstruction, each would have overlapped three of its neighbors. This bony girdle would have provided support for the creature’s body, but it would also have diminished its side-to-side flexibility.
Bony protrusions on Ichthyostega‘s spinal vertebrae would also have constrained sideways movement. However, the size and shape of muscle attachments on the spinal vertebrae suggest that the creature could flex its lower back as people do when bending forward to touch their toes.
Therefore, Ichthyostega probably had one of two gaits, says Clack. The creature could have walked with diagonally synchronized limb movements—that is, by moving its front-right limb and left-rear limb together, then moving its front-left limb and right-rear limb. Many modern animals travel in this way.
Alternately, Ichthyostega could have moved along, inchworm-style.
Detailed analyses of the surfaces in Ichthyostega‘s joints may shed additional light on the creature’s walking style, the researchers note.
Researchers speculate that Tiktaalik propelled itself through the water with its tail, but they haven’t yet investigated how the creature moved on land.
Although Tiktaalik has shrunk the gap in the fossil record between tetrapods such as Ichthyostega and the lobe-finned fish that preceded them, that gap hasn’t gone away, says Clack. Another breach exists between partially aquatic species such as Ichthyostega and Acanthostega and the fully terrestrial tetrapods that arose millions of years later.
Clack is optimistic that further research will fill those holes. “The world is peppered with large areas of unexplored rocks of the right age to yield more transitional fossils,” she says. The discovery of Tiktaalik may herald many groundbreaking finds to come.
