1.3: The Rise of Nonskeletal Fauna
This mentioned above glaciation possibly, in turn, triggered the evolution of Earth, because, in the Ediacaran period (the last period of Proterozoic), animals and other multi-cellular organism appear. There are three most unusual things about Ediacarian ecosystems. First, they were filled with creatures as similar to contemporary life as would (not yet discovered) extra-terrestrial life be. In other words, they (like Pteridinium , see Fig. 2.2.30) had no similarity with the recent fauna and flora. Second, all these Ediacaran creatures were soft, nonskeletal. This last fact is even more striking because, in the next period (Cambrian), almost all animals and even algae had skeletal parts.
There were different types of ecosystems in Neoproterozoic. However, in essence, they all consisted of these soft creatures (it is not easy to say what they were, animals, plants of colonial protists). They thrived for about 90 million years and then suddenly declined (some left-overs existed in Cambrian, though). This decline is the third bizarre thing. Weird because later ecosystems almost always left descendants, even famous dinosaurs went extinct but left the great group of birds, their direct “offspring”.
Why they went extinct, it is not clear. Several factors could be blamed: oxidization of ocean, the appearance of macroscopic carnivores, increased transparency of water. The last could relate with two first by means of pellet production . Many recent small plankton invertebrates pack their feces in granules (pellets), which speedily fall to the ocean bottom. In Ediacaran, there was probably no pellet production, and therefore ocean water was mostly muddy. When first pellet producers appear, water start to be increasingly more transparent, which raised oxygen production by algae and, as the next step, allowed more and bigger animals to exist. Bigger plankton animals mean that it starts to be rewarding to hunt them (remember ecological pyramid). These hunters were probably first macroscopic carnivores, which caused the end of Ediacaran’s “soft life”.
After Ediacaran great extinction (this is the first documented great extinction), one can observe the rise of very different creatures, small, skeletal Cambrian organisms. They appear insignificant diversity and represent many current phyla of animals. This is called “Cambrian revolution”, or “Cambrian explosion” (see below).