How are fossils used to study evolution

All animals and plants that are more than 10,000 years old are considered fossils. These fossilized remains of organisms are documents of evolution. The individual sedimentary rocks can be imagined as stacked pages of an ancient book, many of which have been torn out and others hardly contain any characters. These gaps have various causes and explain why the maintenance of continuous series of evolutionary steps as fossils tends to be the exception. This makes fully preserved skeletons all the more important because they make an important contribution to questions of evolution.

Processes as a filter
The so-called fossil report - the sum of all documented fossil occurrences in an area - is like such an incomplete book. Before an organism can become a fossil, various physical and biological processes act as filters. The most common fossils are marine organisms such as mollusks, echinoderms or corals, but they only made up a small part of the community at that time. In today's shallow marine ecosystem, around 20 to 30% of the organisms have a hard, calcareous or chitinous outer or inner skeleton, the rest have no hard parts that can be transmitted. This biological filter falsifies the fossil record a priori. A complete tradition presupposes certain conditions. In addition to the skeleton, rapid embedding or sufficient coverage by sediments is important. Otherwise the shell of certain algae or fungi will be crushed relatively quickly, caught by the current in the ground or damaged by mechanical wave energy. If the sediment cover is insufficient, the shell can be exposed again by currents or brought back to the surface by burrowing organisms. These erosive processes can be compared with the torn out book pages: If there is enough oxygen in the pore water of the sediment, bacteria ensure the rapid breakdown of the soft tissues. If the calcium carbonate is undersaturated, the chemically unstable hard parts are dissolved and, in the best case, a hollow shape is retained. This is like the fading of the printing ink in our ancient book. In short: in order for a complete organism to be handed down, a rapid and sufficient sediment cover and an environment that is as low in oxygen as possible are required. In this way, not only the shell, but in extremely rare cases also soft parts can be preserved.

A rare bird
Let us turn to a very rare and special fossil that can be considered a prime example of evolution: the Archeopteryx, in which not only the bones but also the body appendages, in this case the feathers, have been handed down. It was found shortly after the publication of Charles Darwin's work of the century "On the Origin of Species". On his research trip with the "Beagle", Darwin dealt intensively with the revolutionary ideas of the British geologist Charles Lyell. That is why his theory of evolution also contains statements about the very old age of the earth. An important contribution to the debate at that time was made by a fossil skeleton "of an animal covered with feathers" that was found in the quarries of Solnhofen (Bavaria) in 1861 (Fig. 1). The Munich paleontologist Andreas Wagner first identified it as a pterosaur and gave it the name Gryphosaurus (puzzle lizard). For him, as a staunch anti-Darwinist, there could be no link between bird and reptile. Shortly before, Hermann von Meyer had described an isolated bird's feather from the same rock layers and placed it in the new genus Archeopteryx. While the real nature of the fossil was debated in Germany, negotiations with the owner resulted in it being sold to the British Museum. As soon as it arrived, it was scientifically described by Darwin's arch-rival Sir Richard Owen and made known as Archeopteryx macrura. Its image as the link between reptile and bird did not meet with enthusiasm, neither in science nor in the public, rather the fossil seemed ominous as proof of Darwin's theory of evolution at the time. Above all, it challenged the religious view, according to which birds and reptiles are static groups that God placed on our planet as unchangeable forms.

Theoretical dispute
A heated argument broke out between Owen, the well-respected leading scientist and intimate of the royal family, and Thomas Henry Huxley, arguably the most ardent defender of Darwin and his new theory. Owen, a deeply religious paleontologist, despised Darwin's idea of ‚Äč‚Äčnatural selection. He believed in continuous generation, in which life forms would emerge from archetypal blueprints from time to time. He insisted that Archeopteryx was the oldest bird to have evolved from a long-tailed pterosaur like Ramphorhynchus through transformation (Fig. 2). In 1868, Owens opponent Huxley published a study in which he described Archeopteryx as a real bird. He even compared it with Compsognathus, a small predatory dinosaur from Solnhofen, which he also regarded as the "missing link" between reptiles and birds. Huxley believed that birds were descended from dinosaurs and was basically right. In the 1970s, the US paleontologist John Ostrom revived the debate when he showed that birds are believed to have descended from the dromaeosaurids (Fig. 3). He too should be right: around ten years ago, new observations and findings showed that certain dinosaurs had real feathers, including the group that Ostrom had studied. The discovery of feathered dinosaurs in China was instrumental in reinvigorating the debate. Is Archeopteryx the oldest bird or "just" a dinosaur with feathers? Some bones on the skull and the splayed ulna (ulna) of the wrist are unambiguous bird features, the rest of the skeleton shows clear signs of a theropod dinosaur, i.e. a being that moved on two legs. Archeopteryx is now regarded by many experts as a sister group of the real birds, which have evolved from the group of maniraptor dinosaurs (small, nimble hand robbers). Feathers, however, were "invented" earlier in evolution. Today the recently described Epidexipteryx is considered to be the oldest and phylogenetically fundamental theropod dinosaur that had courtship feathers but was unable to fly. This find from China is between 150 and 170 million years old. The newest specimen of Archeopteryx has a highly flexible second toe like the dromaeosaurids, which are characterized by sickle-shaped, sharp claws. This underpins the origin of theropods, but blurs the distinction between basic troodontid (nocturnal forms with large eye sockets) and dromaeosaurid dinosaurs; In addition, it is ultimately questioned whether all birds descend from a common archetype. What began with the London Archeopteryx, the great debate about Darwin's theory, is still very topical today. However, the fossils are sometimes so numerous that we can prove evolution despite incomplete records - even if the debate about a possible relationship between birds and dinosaurs keeps coming back to life.