History of invertebrates: Part 1 Ediacaran epoch

The Ediacaran Epoch: Where the invertebrates around us today started

          The true origins of the early Metazoa (Another name for the Animalia kingdom) is still a mystery to us. What we do know now though is that by 600 million years ago (mya) marine invertebrate fauna had begun to make an appearance. This happened during the late Proterozoic Eon known as the Ediacaran Epoch which began at around 605 mya and lasted till around 570 - 540 mya. The Ediacaran fauna contains a lot of fossil records with the first evidence on the origins of modern phyla, however, we still do not know how these early records fit into the evolution of life. 

          The modern phyla that are represented in the Ediacaran fauna are Porifera, Cnidaria, Echiura, Mollusca, Onychophora, Echinodermata, a group of annelid-like organisms and quite possibly, Arthropods. However, a lot of the animals discovered from this time cannot be positively placed in categories with living taxa as they may be parts of phyla that were wiped out at the Proterozoic-Cambrian transition.

Figure 1. An artists impression of what the "Garden of Ediacara
would look like

Figure 2. A fossilised Dickinsonia showing its bilateral sym-
metry. The organism managed to reach a meter in length.

          The Ediacaran fossils have been found in more than 100 sites in the world with a large majority found at Ediacara in the Flinders Ranges of South Australia which is where the name is derived from. Most of these Edicaran organisms have been found to be shallow water species but they have also been found in deep water and continental slop communities. Most of the species found have been soft bodied organisms  with none have heavily shelled bodies. Many of these Edicaran organisms are described as being immobile discs, fronds, tubes, and quilted mattresses that stayed attached to the same place their entire life. The period of time when they existed is even known as the "Garden of Ediacara" because nearly all of the organisms filtered water for energy or used sunlight which meant that they did not attack each other.However, even though they were soft bodied, chitinous structures had developed to start forming the early jaw structures of annelid-like worms. Even with these structures though it is still hard to find Edicaran fossils because the soft bodied organisms do not fossilise well and many of the fossils are just imprints of the bodies found in sandstone beds and ashes. Many of the organisms found in the Ediacaran epoch lacked complex internal organ structures and their body plan followed radial symmetry, however, towards the end large animals with bilateral symmetry had started to appear and did have more complex internal organ structures (Such as the the segmented, sheet-like Dickinsonia seen in Figure 2).

          The Ediacaran epoch heralded the end of the Precambrian period and the Proterozoic eon. These periods were followed by the Cambrian period that is characterised by the great "explosion" of skeletonised life that evolved and brought about the fist mass extinction of most of the Ediacaran fauna. It is still a mystery though, as to why skeletonised animals appeared at this time and in such large quantities. Data on the Earths early atmosphere tells us that the atmosphere lacked oxygen that would be needed for animal life to evolve. This oxygen would have then been added to the atmosphere from the early Cyanobacteria (blue-green algae) that used photosynthesis to get the energy they needed. Some workers still just prefer the idea that early metazoan life was just soft bodied so isn't present in the fossil records due to them not fossilising properly. However, some metazoan meiofauna has been discovered in the Proterozoic strata in China which suggests that the organisms were tiny in the Ediacarna epoch and that by the Cambrian period they had just started to grow larger with the increase in atmospheric oxygen levels. This can also be debated though because large creatures did exist during the Ediacaran epoch such as the Dickinsonia. WHat we have seen though is that early chitinous jaws had started to form which would allow Precambrian organisms to start feeding on other animals and plants. We also know from modern studies that animals evoolve in an ecological arms race which means that the skeleton was the start of the ecological arms race that started as a defense against predators.

          The development of these skeletons did mark the end of the Proterozoic eon and marked the start of the Phonerozoic eon that started with the Cambrian period. The Cambrian period included most of the trophic levels of feeding that we see in our current marine communities, which included giant predatory arthropods, and was very different from the peaceful life of the Ediacaran epoch. This fascinating period is what we will be looking at in part 2 of this series.

Thanks for reading,
Heyze.