Paleontology studies the fossil remains of extinct organisms and establishes their similarities and differences with modern organisms. Paleontological data make it possible to learn about the plant and animal world of the past, reconstruct the appearance of extinct organisms, and discover the connection between the oldest and modern representatives of flora and fauna.
Compelling evidence of changes in the organic world over time is provided by comparing fossils from earth layers of different geological eras. It allows you to establish the sequence of occurrence and development of different groups of organisms. For example, in the most ancient strata, the remains of representatives of types of invertebrate animals are found, and in later strata, the remains of chordates are found. Even younger geological formations contain the remains of animals and plants belonging to species similar to modern ones.
Paleontology data provide a great deal of material information about the continuity between different systematic groups. In some cases, it was possible to establish transitional forms between the most ancient and modern groups of organisms, in others it was possible to reconstruct the phylogenetic series, i.e., the series of species successively replacing one another.
On the shores of the Northern Dvina, a group of bestialous reptiles was found. They combined the signs of mammals and reptiles. Beast-toothed reptiles are similar to mammals in the structure of the skull, spine and limbs, as well as in the division of teeth into fangs, incisors and molars.
Of great interest from an evolutionary point of view is the find of Archeopteryx. This animal the size of a pigeon had the signs of a bird, but still retained the features of reptiles. Signs of birds: hind limbs with tarsus, feathers, general appearance. Signs of reptiles: a long row of caudal vertebrae, abdominal ribs and the presence of teeth. Archeopteryx could not be a good flyer, since he has poorly developed chest bone (without keel), pectoral muscles and muscles of wings. The spine and ribs were not a rigid bone system, stable during flight, as in modern birds. Archeopteryx can be considered a transitional form between reptiles and birds. Transitional forms combine at the same time signs of both ancient and more evolutionarily young groups. Another example is ichthyostegs, a transitional form between fresh-water cysteper fish and amphibians.