In 2006, scientists discovered fossilized skeletons of an animal with several interesting features. Fossil evidence indicated that the animal not only had scales, fins, and gills, but also had lungs, a full set of ribs, and limb bones arranged to support the animal’s weight. The illustration below shows what the animal, called Tiktaalik, probably looked like based on the fossil evidence. Which of the following statements best explains why the Tiktaalik fossils are an important piece of evidence for evolution?
A. They include skeletons of both males and females.
B. They are complete skeletons of a carnivorous animal.
C. They allow scientists to estimate the animal’s lifespan.
D. They show a transitional form between fish and land-dwelling vertebrates.