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What Are Transitional Fossils

Transitional fossils are the fossilized remains of transitional forms of life that tangibly and demonstrably encode an evolutionary transition. Thus, transitional fossils are characterized by their retention of primitive (plesiomorphic) traits in contrast with their more recently evolved characteristics (the phenotype and genotype).

The term "missing link" is a popular slang term for such transitional forms, but is misleading. The term is particularly used in popular media, but is inaccurate and confusing, partly because it implies that there exists a single undiscovered fossil that is needed to confirm the transition. In contrast, the continual discovery of more and more transitional fossils is further refining and validating evolutionary transitions. Transitional fossils are numerous and varied throughout the tree of life, including those between primates and early humans, contrary to the claims of creationists who deny evolution.

Evolutionary theory considers all populations of organisms to be in transition, whether changes be slow, as in genetic drift, or fast, as when a changing environment imposes significant adaptive pressures. A transitional form of life is one that demonstably illustrates a particular intermediate evolutionary stage of change or adaptation.

Transitional fossils usually coexist with gaps in a sequence in the fossil record. The probabilities of fossilization pretty much precludes the discovery of detailed sequences of fossils spanning millions of years. However, fine gradations of fossils between species and genera are abundant in the fossil record, as are coarser sequences between higher taxa.

Fossil transitionals between kingdoms and phyla:

The Cambrian fossils Halkiera and Wiwaxia have features that connect them with each other and with the modern phyla of Mollusca, Brachiopoda, and Annelida. In particular, one species of halkieriid has brachiopod-like shells on the dorsal side at each end. This is seen also in an immature stage of the living brachiopod species Neocrania. It has setae identical in structure to polychaetes, a group of annelids. Wiwaxia and Halkiera have the same basic arrangement of hollow sclerites, an arrangement that is similar to the chaetae arrangement of polychaetes. The undersurface of Wiwaxia has a soft sole like a mollusk's foot, and its jaw looks like a mollusk's mouth. Aplacophorans, which are a group of primitive mollusks, have a soft body covered with spicules similar to the sclerites of Wiwaxia (see Conway Morris 1998, 185-195).

Cambrian and Precambrain fossils Anomalocaris and Opabinia are transitional between arthropods and lobopods.

Wikipedia has an extensive section listing notable transitional fossil sequences.