This post is a contribution to the blog swarm "Blog for Darwin," held from February 12-15, 2009, to celebrate the 200th anniversary of Charles Darwin's birth.

When Charles Darwin published On the Origin of Species in 1859, introducing the world to the revolutionary concept of evolution through natural selection, he noted that "perhaps ... the most obvious and gravest objection which can be urged against my theory" was the apparent lack of "transitional" fossils in the geological record. He speculated that the reason that "geology ... does not reveal [a] finely graded organic chain" of intermediate forms was because of the paucity of fossil discoveries at the time.

In one sense, every species that has ever existed is a "transitional" form; it reflects characteristics of its ancestors, and its characteristics will be reflected in its descendants. But humans are impressed by the dramatic, and in the years since Darwin's first book on evolution was published, some very dramatic fossils, intermediate between two large classes, have been discovered.

The most famous of these is Archaeopterix, which depending on one's interpretation is either a a bird with teeth and clawed hands or a feathered, flying dinosaur. But another lesser-known fossil is equally dramatic as an example of a transitional form: the marvelous Ichthyostega.

Ichthyostega is an intermediate form between fishes and amphibians from the late Devonian period, about 365 million years ago. One could describe it as a fish with legs that could walk around on land, or one could say it was an amphibian with the head and tail of a fish. Since fish have their weight supported by the water, Ichthyostega faced the problem of supporting its weight on land, which was accomplished by thickened, overlapping ribs — a clumsy, primitive solution necessary because it had so recently evolved from its lungfish-like ancestors.

Ichthyostega had seven toes on each hind foot, notable since the number of toes on tetrapod* feet stabilized at five early in their evolution. Its shoulder and hip had adaptations to help it move about on land. Its hind limbs could support its body as a juvenile, but were likely too weak to support its full weight on land in adulthood. It is hypothesized that juveniles could leave the water to escape predators, but adults only partially pulled themselves out of the water to sun themselves.

Ichthyostega is one of many fossils that fill in the natural history of the evolution of tetrapods from fish. The story is still not complete: "Romer's gap" (named after paleontologist Alfred Sherwood Romer) is a 20 million year period of missing fossils between Ichthyostega and its contemporaries and the next-oldest known tetrapods.

Gaps in the fossil record like this were a worry to Darwin, who recognized them as a potential rebuttal of his seminal theory. Yet absence of evidence is not evidence of absence; the most parsimonious explanation for Romer's gap remains the one Darwin put forth — the incompleteness of the geological record. Charles Darwin's extraordinary theory is now secure as one of the most well-established, concrete principles of biology.

*Tetrapods are four-legged vertebrates and their descendants; in other words, all vertebrates except fish.

More information on Ichthyostega

• Ichthyostega from the Tree of Life web project
• Cladogram showing Ichthyostega's place in the evolution of tetrapods (Palaeos database)

Ichthyostega image information

• Top right: Reconstruction of a juvenile Ichthyostega from the National Museum of Natural Science in Taiwan. Photo by Alton Thompson. (CC) Some rights reserved.
• Left: Pencil drawing of Ichthyostega. From Wikimedia Commons, licensed under the GNU Free Documentation License (GFDL).
• Bottom right: Drawing of Ichthyostega skeleton. The forelimbs are incompletely fossilized and the hand morphology is unknown. From the Tree of Life web project (link above), after Ahlberg et al. 2005. (CC) Some rights reserved.