Darwin’s solitary illustration of evolutionary branching has left a lasting impression in the minds of readers. From an ancestral form, speciation occurs and the diversity of descendants increases. This can be visualised as a cone of morphological variation, extending from the source. However, the Cambrian Explosion provides empirical evidence against this concept, as a large number of organisms appear abruptly. (For more, see here) Yet it has been tempting for Darwinists to interpret the Cambrian species in terms of a number of cones of increasing diversity that all have their origins deeper in the Precambrian. This was something Stephen Jay Gould attempted to counter by proposing an “inverted cone” model. But there is a need for a third model to be on the table for consideration. For some years, it has been recognised that two groups of animals have very similar disparities in the Cambrian and the Recent. These are the arthropods and the priapulid worms.
“Both are reported to have comparable morphological disparity in the Cambrian and the Recent. This is important for our understanding of the manner in which metazoans radiated, because it implies that Cambrian animals had already explored a variety of ‘design’ options similar to that realized by their present-day counterparts. [. . .] This challenges the traditional ‘cone of increasing diversity’ (or, more precisely, it shifts this evolutionary model back in time) in favour of an approximately cylindrical model of bodyplan diversity from the Cambrian to the Recent. It also implies that the magnitude of Cambrian morphological diversity requires some form of explanation: whether in terms of Pre-Cambrian evolution at a small size, the gradual Pre-Cambrian differentiation of internal bodyplans decoupled from the appearance of external (and fossilizable) characters in the Cambrian, or some other mechanism.” (p.2057)
For more, go here.