From Annalee Newitz at Ars Technica:
The Cambrian Explosion gets a lot of play because it was the first time multicellular creatures ruled the planet. What few people (other than geologists and paleontologists) realize is that there was an even crazier time for early life. It came during the Ordovician period, right after the Cambrian came to a close 485 million years ago. The Ordovician Radiation, also called the Great Ordovician Biodiversification Event (GOBE), saw a quadrupling of diversity at the genus level (that’s the category one step above species). Life also started occupying new ecological niches, clinging to plants floating in the ocean’s water column and burrowing deep into the seabed. More.
All life, so far as is known, was under water and the often-preserved graptolite was a classic example of what has been found so far:
Graptolite, any member of an extinct group of small, aquatic colonial animals that first became apparent during the Cambrian Period (542 million to 488 million years ago) and that persisted into the Early Carboniferous Period (359 million to 318 million years ago). Graptolites were floating animals that have been most frequently preserved as carbonaceous impressions on black shales, but their fossils have been found in a relatively uncompressed state in limestones. They possessed a chitinous (fingernail-like) outer covering and lacked mineralized hard parts. When found as impressions, the specimens are flattened, and much detail is lost. (Britannica)
Now that evolution is becoming a more serious, less ideological subject, a question arises: What difference do the life forms that were never preserved make in our understanding of an era?
See also: Not just the Cambrian? The Ordovician “age of fishes” was an “explosion” of diversity too?
Due to more oxygen. So that means that the remarkable Cambrian explosion did not even have the benefit of as disproportionately large an amount of oxygen as is commonly assumed. One senses that there are more surprises to come.