There were 75-100 phyla, but over time some disappeared. This graph depicts the Cambrian explosion where most phyla appeared and how over time some phyla disappeared. This is graphically the pattern that is central to Darwin’s doubt. It destroys the notion of Darwinian gradualism:
Remarkably, as phyla disappeared, new species appeared. This is a curious pattern. Some have labeled this phenomenon “disparity precedes diversity”.
In biological classification:
phylum is a taxonomic rank below kingdom and above class. Traditionally, in botany the term division is used instead of “phylum”, although in 1993 the International Botanical Congress accepted the
Phyla can be thought of as grouping organisms based on general specialization of body plan.
A body plan is the blueprint for the way the body of an organism is laid out. Each species of multicellular organism—plant, fungus, red algae, slime mold, among others—has a body plan. This article is about animal body plans.
An animal’s symmetry, its number of body segments and limbs are all aspects of its body plan. One of the key issues of developmental biology is the evolution of body plans as different as those of a starfish, or a mammal, which come from a close common biological heritage-both are deuterostomes.