Many examples have been put forward as instances that bridge the gaps between extant or fossil taxa. Before we give consideration to any of these we should back up to evaluate the philosophical question of origins and gaps.
No knowledgeable person would question the widespread and pervasive nature of gaps between taxa, modern or fossil. This is the norm. To propose a fossil or extant form to be an evolutionary intermediate between two fossil or modern taxa, one should first establish the criteria that would enable one to recognize the form as intermediate. What characteristics should a valid evolutionary intermediate exhibit before we can consider it to be a true transitional form between two disjunct taxa?
One could always argue that the resolving power of the fossil record is inadequate for the preservation of every intermediate between two end forms. But if that is true, we must abandon the certainty that all-too-often attends the assertion that any particular form is a true evolutionary intermediate.
If we limit our consideration to only a particular condition (i.e. length of tail, or some other measurable trait) and ignore other features, we can never make the assertion that the condition is intermediate. It may be an example of parallel evolution or in some other relationship to the derived form, rather than in a direct lineage.
When we attempt to include all characteristics in any consideration of intermediates (cladistic analysis), we end up with some conditions that are appearing in the right order (the ones evolutionists traditionally would concentrate on), and a suite of other conditions, some of which are ancestral, and some of which are derived, and some of which appear not to fit any evolutionary lineage. Although the cladistic approach is often said to support evolution, such an assertion is tautologic, since cladistic analysis depends upon evolutionary assumptions in its constructions. Evolutionary cladists (there are some cladists who refuse to assume evolutionary relationships as a premise) are forced to invoke the "principle" of "parsimony" (choosing the path with the least number of falsifying characteristics) in order to maintain their evolutionary view of relationships. But these are hardly the attributes one would wish to push as evidence for intermediate status.
2. If "A" is found earlier in the fossil record than "C", and "B" is intermediate, then the stratigraphic criteria for "B" as an intermediate are satisfied. If "B" is found either earlier than or at the same time as "A" or later than or at the same time as "C", the intermediate status of "B" is compromised.
If a fossil form meets these two criteria, namely intermediate characteristics and intermediate stratigraphic position, it would seem to represent a plausible intermediate form.
2010 Arthur V. Chadwick, Ph.D.