The sudden appearance of animal structural types in the Cambrian period, and the lack of new types since 500 million years since the Cambrian is discussed in the book -What Evolution is, (2001), by Ernst Mayr, who is the professor Emeritus in the Museum of Comparative Zoology at Harvard University. he wrote on page 279:
" How can we explain the two great puzzles in the phylogeny of animals?
The first puzzle is the sudden appearance of 60 to 80 different structural tyes ( body plans) of animals in the early Cambrian, and the second puzzle is why no major new types originated in the 500 million years since the Cambrian.
It is now clear that the seemingly sudden origin ( within 10-20 million years) since the Cambrian ( beginning 544 million years ago) is an artifact of preservation. By use of the molecular clock, the origin of the animal types can be placed at about 670 million years ago, but the animals living between 670 and 544 million years ago are not preserved as fossils because they were very small and without skeletons.
The reason why no major new types originated in the ensuing 500 million years is more complex and only partly understood. However, molecular geneticics has led to an explanatory suggestion. Development is tightly controlled in the now living organisms by very precise "working teams" of regulatory genes. In the Precambrian, there were apparently only a few such genes, which did not control development as tightly as later on. This allowed a frequent occurrence of rapid major restructuring of the structural types. By the end of the Cambrian, the dominance of these regulatory genes had been fully established and the origin of completely new structural types had become difficult, if not impossible. One must always remember that the changes prior to the Cambrian did not occur suddenly, but over a period of several hundred million years , even though not documented in the fossil record."
Brien
" How can we explain the two great puzzles in the phylogeny of animals?
The first puzzle is the sudden appearance of 60 to 80 different structural tyes ( body plans) of animals in the early Cambrian, and the second puzzle is why no major new types originated in the 500 million years since the Cambrian.
It is now clear that the seemingly sudden origin ( within 10-20 million years) since the Cambrian ( beginning 544 million years ago) is an artifact of preservation. By use of the molecular clock, the origin of the animal types can be placed at about 670 million years ago, but the animals living between 670 and 544 million years ago are not preserved as fossils because they were very small and without skeletons.
The reason why no major new types originated in the ensuing 500 million years is more complex and only partly understood. However, molecular geneticics has led to an explanatory suggestion. Development is tightly controlled in the now living organisms by very precise "working teams" of regulatory genes. In the Precambrian, there were apparently only a few such genes, which did not control development as tightly as later on. This allowed a frequent occurrence of rapid major restructuring of the structural types. By the end of the Cambrian, the dominance of these regulatory genes had been fully established and the origin of completely new structural types had become difficult, if not impossible. One must always remember that the changes prior to the Cambrian did not occur suddenly, but over a period of several hundred million years , even though not documented in the fossil record."
Brien