In 1859, when Darwin published his legendary text On the Origin of Species, he knew something was missing. His theory of descent with modification through natural selection was logically sound, but the concrete evidence was lacking. While he predicted that the fossil record would show species with intermediate or transitional features, it was virtually unexplored at the time. Knowing what he knew about rarity of fossilization and incomplete nature the geologic record, Darwin actually imagined the record was so poor, it was likely paleontologists would never find the transitional fossils.
Darwin knew the lack of fossils would bring out extreme criticism of his theory.
Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.
Now, in 2015, paleontologists have discovered a myriad of fossils that could be considered intermediates, despite the fact that some very publicly still state these fossils don’t exist. Only 2 years after the publication of On the Origin of Species was one of the most famous transitional fossils discovered—Archaeopteryx. Here are 4 of the most informative transitional fossils, a small fraction of those lending strong support to Darwin’s theory of evolution.
This fossil could almost be considered the smoking gun in Darwin’s theory of natural selection—except he didn’t see it until after his book had been published. Arguably one of the world’s most famous fossils, the first Archaeopteryx was discovered in Germany in 1861. It possesses a combination of traits that clearly place it as a transitional form between non-avian dinosaurs and birds. Its similarities to non-avian dinosaurs include a long feathered tail and small teeth. Unlike non-avian dinosaurs however, Archaeopteryx also has flight feathers and wings, just like a modern bird. The discovery of the furcula, or fused clavicle bone, in Archaeopteryx was a firm confirmation of the relationship between birds and dinosaurs, as they are the only two groups to have this anatomical feature.
Pakicetus and other amphibious whales
Whales once walked on land. Fossil evidence from a variety of different species, such as Pakicetus, indicates that the earliest relatives of enormous whales like humpbacks were comparatively small land mammals. Discovered in Pakistan, Pakicetus lived during the Eocene (50 million years ago) and although it primarily lived its life on land, it is linked as a whale relative through its unique inner ear shape. Only whales have such an enhanced region of the ear called an “auditory bulla”–and Pakicetus has this too. Other fossil whale relatives that were likely amphibious between land and sea include Ambulocetus and Remingtonocetus. Basilosaurids lived around 40 million years ago and are the first known obligate aquatic whales, indicating that is when whales made their permanent transition to the ocean.
The ancestry of manatees and dugongs was a long standing mystery in paleontology. In 2001, the discovery of Pezosiren portelli provided a huge clue. The fossil was discovered in Jamaica, and at 50 million years old represents the oldest member of the group Sirenia (sea cows). It is clear Pezosiren had 4 legs it used to walk on land, but it also had heavy ribs that could indicate it lived part time in water, much like a hippopotamus. This species likely represents the transitional form of sea cows as it maintained the general body plan of a sea cow, but just without flippers.
Looking all the way back 375 million years ago, there is an intermediate fossil that represents the transition of vertebrate life from water to land. Tiktaalik roseae, discovered in Nunavut in 2004, is an ancient fish called a sarcopteryigian, or lobe-finned fish. Although it bears many similarities to fish like gills, scales, and fins, other key characteristics link Tiktaalik to land animals. While it did have fins, the bones inside the fins are homologous to the bones of the human hand and wrist, indicating it may have been able to bear weight. The animal also had a mobile neck and a strong ribcage, two critical traits that allowed four-legged (tetrapod) creatures to move onto land. Tiktaalik makes sense evolutionarily in the progression of other early tetrapods like the more aquatic Panderichthys and the clearly amphibious Acanthostega.