Neoproterozoic Era: Ediacaran Period 

635–539 Ma

The Ediacaran: at the dawn of animals

Technically speaking, the Ediacaran Period is not part of the Paleozoic Era, or even the Phanerozoic Eon, instead representing the earliest currently ratified period at the end of the Neoproterozoic Erabut its evolutionary story is just too important to skip. Many of the organisms shown here are sometimes called vendobionts, and may just reflect a bizarre experiment in the earliest evolution of complex multicellular life. Some of them, however, may truly signify stem representatives of animal clades—or those extinct organisms that display some but not all, the morphological features that come to define later descendants. For examples shown here, while the large, frondose Charniodiscus and purse-shaped Ernietta may not have well-known or recognized animal descendants later in the fossil record, the slug-like Kimberella peeking around the corner and leaving traces of scratch marks in its wake as it munches on the abundant microbial mat surfaces has several morphological features that suggests it was a stem-mollusk—today’s group that contains the snails, slugs, and a wide diversity of other organisms. Aside from the first forays into multicellularity, the Ediacaran Period saw another fossil-record-changing evolutionary development: the origin of shell-building, or biomineralization—the remnants of which would, from this point onward, make up the bulk of fossil materials. The dense accumulation of what look like stacked cones are known as Cloudina, one of the very first animals to grow an external skeleton out of minerals. The evolutionary advantages of such shells are easily imagined, for instance offering added structural support to grow upright from the ocean floor as well as offering shelter or protection to the animal inside from any hungry predators that may be lurking. Interestingly enough, even these very first shell-builders may show signs that early predators figured out a way to defeat the shelly defense system—if you look closely, at least one of our Cloudina has a hole drilled through its external shell. While the soft tissues that stuck out of Cloudina’s shells are merely speculated, researchers at Mizzou were the first to describe some of this animal’s internal soft tissues in the form of remarkably preserved digestive tracts—the oldest known in the fossil record! The shape of these guts helped tell us that the animals living inside of these abundant stacked-cone tubes were most likely stem ancestors of annelid worms, some of which still live in tubes today.