That mosaic forms cannot be interpreted as transitional forms without having been examined can be shown especially well by the example of the duck-billed platypus (fig. 18). Between which two evolutionary groups does it belong? If it would be on its way from a reptile to a mammal with a placenta, then the bird like features of a horned beak and a flattened tail would not fit. That is why theoretically, the duck-billed platypus can not be seen as a transitional species, rather it is placed on a side branch. Generally we can conclude that a transitional form within a phylogenetic tree must fit into the space which lies between the two groups which need to be connected. Obviously this is not the case with the duck-billed platypus as most evolutionists agree. The Archaeopteryx, is commonly interpreted as a transitional form, however, it also has several characteristics that do not fit (LINK to Archaeopteryx).
A further criterion for the question whether a mosaic form can be interpreted as a transitional form must also be considered. The evolutionary view expects that intricate organs would evolve step by step. There should be examples of transitional forms with the details of an organ or form changing into another form For instance, predecessors and transitional forms of bird feathers should exist. Some think that feathers are derived from Reptilian scales. The difference between these two structures is enormous. Based on what can be recognized from fossil Archaeopteryx feathers, their feathers are the same as modern day bird feathers. In this case, the second criterion is not met. Some dinosaurs do have feather-like structures that are possible candidates for transition of individual characteristics. These discoveries are dealt with later. This page focuses on the fundamental aspects.
We can summarize as follows:
Mosaic forms Transitional forms
Intermediate form connecting link
combines characteristics of fits without contradiction into a
different groups phylogenic taxonomy
have transitions in single characteristics
The problem is illustrated in fig. 20.
Why are transitional forms missing? Missing suitable evolutionary transitional forms are often justified by the claim that transitions take place relatively fast, within small populations and within a small geographic area. That is why there are so few transitional examples found in the fossil record. This is the case of the so-called “punctuated equilibrium” (punctualism). The plausibility of these arguments is discussed later.