The oldest macroscopic body fossils date back to the ancient Ediacaran period, which extended from 635 to 542 million years ago. The oldest known megafossil site is the Twitya Formation in the Mackenzie mountains of northwestern Canada, with fossils dated from 610 to 600 million years ago. The extreme age of these fossils is truly remarkable: to put things in perspective, for over a hundred years, it was thought that the Cambrian Period (starting 542 million years ago) contained the oldest multicellular fossils. It wasn’t until 1957 until it was realized that complex life could have been around for a few dozen million years prior to that.
The Twitya formation fossils are older than the second-oldest megafossil site by a factor of more than 5 million years. The next oldest is the Drook Formation from southeast Newfoundland, with estimated ages of 595 to 565 million years. The Twitya and Drook megafossils bear a resemblance to cnidarians – sea pens – quilted, bilateral frond-like animals which are similar to ferns in appearance. Known as Charnia, these animals have only been tentatively classified as related to cnidarians because cnidarians are the oldest known metazoans (multicellular organisms) and most paleontologists are at a loss to describe these enigmatic fossils.
Sometimes Charnia is cited as a “metazoan of cnidarian-grade complexity.” It is named after the Charnwood Forest of England, where it was first discovered. Charnia is both the most iconic megafossil of the Ediacaran period and the long-living – fossils of it have been dated to as recently as 520 million years ago, 20 million years past the Ediacaran-Cambrian boundary. So the duration of existence of Charnia was evidently as long as 90 million years.
Another megafossil found at the Twitya Formation is Nimbia occlusa, a simplistic circular impression sometimes excluded from the class of “Ediacaran fauna” due to its simplicity. Confusion is sometimes caused because it is debated whether such simple impressions are in fact independent animals or rather just colonies of unicellular organisms.
The Twitya Formation is especially unique among all fossil sites because it predates the Varangian-Marinoan glaciation, an ice age which occurred between 600 and 585 million years ago. This glaciation is frequently cited as a barrier that held back the formation of complex multicellular life, although we can tell from the Twitya Formation that this is not true. It is true that the Twitya Formation only includes relatively simplistic fossils, but they are indeed macroscopic and very likely multicellular as well.
Post-Twitya Formation fossils include a variety of segmented worms, fronds, disks, and immobile bags, as well as probable holdfasts and trace fossils. These are collectively known as the Ediacaran fauna and are one of the most mysterious categories of life to have ever lived. They are considered life’s first experiments in multicellularity.
