The fossil organisms which show characters of two different groups of animals are called missing links. In other words, they are fossil connecting links. The fossil connecting links include: Archaeopteryx, Icthyostega, Seymouria etc.
It was discovered by Wagner in 1961 from the Jurassic lithographic limestones of Bavaria in Germany. Two other fossils  of the same bird were discovered in the same place, one in 1877 and the other in 1956. It lived about 140 million years ago in the late Jurassic period. Archaeopteryx
It is an extinct bird; it was about the size of the crow and existed in the forest. It is a transitional stage showing the characters of both reptiles and the birds.
IchthyostegaIt is a primitive fossil amphibia included in the group Stegocephalia. It is lived during the late Devonian and Carboniferous periods. In addition to the amphibian characters . it was provided with many piscine characters, So it forms a connecting link between  pisces and amphibians. The piscine characters of Ichthyostega are autostylic jaw suspension, presence of internal ears. lateral line canals, labrynthodont dentition, fin rays in the caudal region and well developed limbs, pectoral and pelvic girdles.
Seymouria (extinct)is a missing link between amphibia and reptiles.
It was lizard like animal lived about 250 million years ago.  It combines amphibian and reptilian characters. Its amphibian characters include short limbs, labrinthine teeth and lateral line canals. The reptilian characters include  two sacral vertebrae, an interclavicle and cleidoic eggs. Seymouria clearly a transitional stage in the evolution of amphibians into reptiles. 
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