Q.Archaeopteryx is known as a missing link because it is a fossil and shares characters of both –

Palaeontology is the study of ancient life through fossils — the preserved remains of plants and animals from the past, found within sedimentary rock, amber, ice or peat bogs — and provides the most direct and convincing evidence for evolution, since older, more primitive fossil forms are consistently found in older, deeper rock layers while more advanced forms occur in more recent, shallower ones. Fossils occur as actual remains (bodies preserved intact in permafrost or amber, e.g. the Woolly Mammoth), moulds (hardened impressions left after the original remains decayed away, e.g. footprints), casts (mineral matter that later filled a mould's cavity) or compressions (a thin carbon film outlining external features, with no internal structure preserved). Palaeontology is valuable for reconstructing phylogeny, studying extinct forms and their habits, and — most importantly for evolutionary theory — supplying connecting-link (missing-link) fossils that show characters of two otherwise distinct groups, such as Seymouria (between amphibians and reptiles) and Archaeopteryx (between reptiles and birds). Archaeopteryx lithographica, a crow-sized, toothed, …