Neither Fish nor Tetrapod

Neither Fish nor Tetrapod

Several times a year popular science articles breathlessly announce the discovery of a “missing link,” but in the case of the recently identified fossils of an odd creature hailing from the Canadian territory of Nunavut, this overused term is compellingly accurate. In an April 2006 issue of Nature, paleontologists revealed the discovery of a 375-million-year-old transitional species whose anatomical traits bridge the gap between fish and tetrapod (four-legged vertebrate). Nicknamed the fishapod, its formal name is Tiktaalik roseae, from the Inuit name for a large shallow-water fish.

Tiktaalik joins several other significant transitional fossils—the most famous of which is Archaeopteryx, the part-bird, part-reptile considered the “missing link” between birds and dinosaurs, which was discovered in 1860, just two years after Darwin published The Origin of Species.

The transformation of aquatic creatures into land animals took place during the Devonian period, about 410 to 356 million years ago. But before the discovery of the 375-million-year-old Tiktaalik fossils, there had been no actual fossil evidence to illustrate this crucial evolutionary moment. According to paleontologist Neil Shubin of the University of Chicago, “We are capturing a very significant transition at a key moment of time. What is significant about the animal is that it is a fossil that blurs the distinction between two forms of life—between an animal that lives in water and an animal that lives on land.”

Tiktaalik resembles a huge scaly fish with a flat, crocodile snout. What amazed scientists was its pectoral fins, which contain bones forming the beginnings of a shoulder, elbow, wrist, hand, and even nascent fingers. Shubin describes the fin as “basically a scale-covered arm,” asserting that “here’s a creature that has a fin that can do push-ups.” Tiktaalik could pull its own weight, dragging itself along in shallow water and onto dry land, much like a seal.

Tiktaalik also distinguishes itself from a fish by the existence of a primitive neck and ribs. As Harvard University paleontologist Farish A. Jenkins explains, “Out of water, these fish encountered gravitational forces very different from the relative buoyancy they enjoyed in an aquatic setting. Restructuring of the body to withstand these forces is evident in the ribs, which are plate-like and overlap like shingles, forming a rigid supporting mechanism for the trunk.” And while a fish has no need of a neck—in water, its entire torso easily falls into place behind its head when changing directions—Tiktaalik's developed neck allowed it to move its head while its body, constrained by the stronger pull of gravity on land, remained stationary. According to Edward Daeschler of the Academy of Natural Sciences, the combination of these radically new anatomical features with classic fishlike traits demonstrates that “evolution proceeds slowly…in a mosaic pattern with some elements changing while others stay the same.”

Fact Monster/Information Please® Database, © 2005 Pearson Education, Inc. All rights reserved.

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