"Missing link" is a term for a transitional form from the fossil record that connects two different species to an earlier ancestor. The missing link may have features common to both species; for example an early avian fossil could have feathers much like a modern bird but still retain the bony tail and teeth of a dinosaur. This would be considered a missing link between dinosaurs and birds.
The missing link is a popular and not a scientific concept. Scientists studying the fossil record have long known that not every species that lived was 'lucky' enough to leave behind a fossil. More importantly, populations are constantly changing and species are statistical constructs and not ideal-types; therefore, there is not scientific meaning to the notion of a "transitional form."
This being said, a number of fossils exist that do indicate a link between earlier and later forms of animal. The lobe-finned fish Eusthenopteron is thought to be the first step towards land-dwelling amphibians; fossils of feathered dinosaurs in China seem to indicate that feathers were commonplace even before true birds evolved; and recently reexamined fossils of some amphibians have shown that some bones of these animals bear striking resemblances to those of fish.
Paleontologists still seek the missing link between ancestral humans and monkeys.
Transitional Fossil Wikipedia
Scientists hail stunning fossil of a 47-million-year-old, lemur-like creature BBC - May 19, 2009
Ida ... 'Missing link' primate likely to stir debate MSNBC - May 19, 2009
Blog: Myth of the Missing Link Live Science - May 20, 2009
Blog: Why 'Ida' Inspires Navel-Gazing at Our Ancestry Live Science - May 20, 2009
Ancient Human Ancestor 'Ida' Discovered - Missing Link? Live Science - May 19, 2009
New Fossil Primate Links Humans, Lemurs? National Geographic - May 19, 2009
Common Ancestor Of Humans, Modern Primates? 'Extraordinary' Fossil Is 47 Million Years Old Science Daily - May 19, 2009
In what could prove to be a landmark discovery, a leading paleontologist said scientists have dug up the 47 million-year-old fossil of an ancient primate whose features suggest it could be the common ancestor of all later monkeys, apes and humans.
"Human"-Faced Missing Link Found in Spain? National Geographic - June 11, 2009
An 11.9-million-year-old fossil ape species with an unusually flat, "surprisingly human" face has been found in Spain.
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